Re: Historical rebalance
Roman, What is the advantage of your algorithm compared to previous one? Previous algorithm does almost the same, but without updating two separate counters, and looks simpler to me. Only one update is sufficient - at transaction commit. When transaction starts we just read currently active update counter (LWM), which is enough for us to know where to start from. Moreover, we do not need to learn any kind of WAL pointers and write additional WAL records. Please note that we are trying to solve more difficult problem - how to rebalance as less WAL as possible in case of long-running transactions. On Mon, Dec 3, 2018 at 2:29 PM Roman Kondakov wrote: > Vladimir, > > the difference between per-transaction basis and update-counters basis > is the fact that at the first update we don't know the actual update > counter of this update - we just count deltas on enlist phase. Actual > update counter of this update will be assigned on transaction commit. > But for per-transaction based the actual HWM is known for each > transaction from the very beginning and this value is the same for > primary and backups. Having this number it is very easy to find where > transaction begins on any node. > > > -- > Kind Regards > Roman Kondakov > > On 03.12.2018 13:46, Vladimir Ozerov wrote: > > Roman, > > > > We already track updates on per-transaction basis. The only difference is > > that instead of doing a single "increment(1)" for transaction we do > > "increment(X)" where X is number of updates in the given transaction. > > > > On Mon, Dec 3, 2018 at 1:16 PM Roman Kondakov > > > wrote: > > > >> Igor, Vladimir, Ivan, > >> > >> perhaps, we are focused too much on update counters. This feature was > >> designed for the continuous queries and it may not be suited well for > >> the historical rebalance. What if we would track updates on > >> per-transaction basis instead of per-update basis? Let's consider two > >> counters: low-water mark (LWM) and high-water mark (HWM) which should be > >> added to each partition. They have the following properties: > >> > >> * HWM - is a plane atomic counter. When Tx makes its first write on > >> primary node it does incrementAndGet for this counter and remembers > >> obtained value within its context. This counter can be considered as tx > >> id within current partition - transactions should maintain per-partition > >> map of their HWM ids. WAL pointer to the first record should remembered > >> in this map. Also this id should be recorded to WAL data records. > >> > >> When Tx sends updates to backups it sends Tx HWM too. When backup > >> receives this message from the primary node it takes HWM and do > >> setIfGreater on the local HWM counter. > >> > >> * LWM - is a plane atomic counter. When Tx terminates (either with > >> commit or rollback) it updates its local LWM in the same manner as > >> update counters do it using holes tracking. For example, if partition's > >> LWM = 10 now, and tx with id (HWM id) = 12 commits, we do not update > >> partition LWM until tx with id = 11 is committed. When id = 11 is > >> committed, LWM is set to 12. If we have LWM == N, this means that all > >> transactions with id <= N have been terminated for the current partition > >> and all data is already recorded in the local partition. > >> > >> Brief summary for both counters: HWM - means that partition has already > >> seen at least one update of transactions with id <= HWM. LWM means that > >> partition has all updates made by transactions wth id <= LWM. > >> > >> LWM is always <= HWM. > >> > >> On checkpoint we should store only these two counters in checkpoint > >> record. As optimization we can also store list of pending LWMs - ids > >> which haven't been merged to LWM because of the holes in sequence. > >> > >> Historical rebalance: > >> > >> 1. Demander knows its LWM - all updates before it has been applied. > >> Demander sends LWM to supplier. > >> > >> 2. Supplier finds the earliest checkpoint where HWM(supplier) <= LWM > >> (demander) > >> > >> 3. Supplier starts moving forward on WAL until it finds first data > >> record with HWM id = LWM (demander). From this point WAL can be > >> rebalanced to demander. > >> > >> In this approach updates and checkpoints on primary and backup can be > >> reordered in any way, but we can always find a proper point to read WAL > >> from. > >> > >> Let's consider a couple of examples. In this examples transaction > >> updates marked as w1(a) - transaction 1 updates key=a, c1 - transaction > >> 1 is committed, cp(1, 0) - checkpoint with HWM=1 and LWM=0. (HWM,LWM) - > >> current counters after operation. (HWM,LWM[hole1, hole2]) - counters > >> with holes in LWM. > >> > >> > >> 1. Simple case with no reordering: > >> > >> PRIMARY > >> > -w1(a)---cp(1,0)---w2(b)w1(c)--c1c2-cp(2,2) > >> (HWM,LWM)(1,0) (2,0)(2,0) (2,1) > (2,2) > >> | || |
Re: Historical rebalance
Vladimir, the difference between per-transaction basis and update-counters basis is the fact that at the first update we don't know the actual update counter of this update - we just count deltas on enlist phase. Actual update counter of this update will be assigned on transaction commit. But for per-transaction based the actual HWM is known for each transaction from the very beginning and this value is the same for primary and backups. Having this number it is very easy to find where transaction begins on any node. -- Kind Regards Roman Kondakov On 03.12.2018 13:46, Vladimir Ozerov wrote: Roman, We already track updates on per-transaction basis. The only difference is that instead of doing a single "increment(1)" for transaction we do "increment(X)" where X is number of updates in the given transaction. On Mon, Dec 3, 2018 at 1:16 PM Roman Kondakov wrote: Igor, Vladimir, Ivan, perhaps, we are focused too much on update counters. This feature was designed for the continuous queries and it may not be suited well for the historical rebalance. What if we would track updates on per-transaction basis instead of per-update basis? Let's consider two counters: low-water mark (LWM) and high-water mark (HWM) which should be added to each partition. They have the following properties: * HWM - is a plane atomic counter. When Tx makes its first write on primary node it does incrementAndGet for this counter and remembers obtained value within its context. This counter can be considered as tx id within current partition - transactions should maintain per-partition map of their HWM ids. WAL pointer to the first record should remembered in this map. Also this id should be recorded to WAL data records. When Tx sends updates to backups it sends Tx HWM too. When backup receives this message from the primary node it takes HWM and do setIfGreater on the local HWM counter. * LWM - is a plane atomic counter. When Tx terminates (either with commit or rollback) it updates its local LWM in the same manner as update counters do it using holes tracking. For example, if partition's LWM = 10 now, and tx with id (HWM id) = 12 commits, we do not update partition LWM until tx with id = 11 is committed. When id = 11 is committed, LWM is set to 12. If we have LWM == N, this means that all transactions with id <= N have been terminated for the current partition and all data is already recorded in the local partition. Brief summary for both counters: HWM - means that partition has already seen at least one update of transactions with id <= HWM. LWM means that partition has all updates made by transactions wth id <= LWM. LWM is always <= HWM. On checkpoint we should store only these two counters in checkpoint record. As optimization we can also store list of pending LWMs - ids which haven't been merged to LWM because of the holes in sequence. Historical rebalance: 1. Demander knows its LWM - all updates before it has been applied. Demander sends LWM to supplier. 2. Supplier finds the earliest checkpoint where HWM(supplier) <= LWM (demander) 3. Supplier starts moving forward on WAL until it finds first data record with HWM id = LWM (demander). From this point WAL can be rebalanced to demander. In this approach updates and checkpoints on primary and backup can be reordered in any way, but we can always find a proper point to read WAL from. Let's consider a couple of examples. In this examples transaction updates marked as w1(a) - transaction 1 updates key=a, c1 - transaction 1 is committed, cp(1, 0) - checkpoint with HWM=1 and LWM=0. (HWM,LWM) - current counters after operation. (HWM,LWM[hole1, hole2]) - counters with holes in LWM. 1. Simple case with no reordering: PRIMARY -w1(a)---cp(1,0)---w2(b)w1(c)--c1c2-cp(2,2) (HWM,LWM)(1,0) (2,0)(2,0) (2,1) (2,2) | || || BACKUP --w1(a)-w2(b)w1(c)---cp(2,0)c1c2-cp(2,2) (HWM,LWM)(1,0) (2,0)(2,0) (2,1) (2,2) In this case if backup failed before c1 it will receive all updates from the beginning (HWM=0). If it fails between c1 and c2, it will receive WAL from primary's cp(1,0), because tx with id=1 is fully processed on backup: HWM(supplier cp(1,0))=1 == LWM(demander)=1 if backup fails after c2, it will receive nothing because it has all updates HWM(supplier)=2 == LWM(demander)=2 2. Case with reordering PRIMARY -w1(a)---cp(1,0)---w2(b)--cp(2,0)--w1(c)--c1-c2---cp(2,2) (HWM,LWM)(1,0) (2,0) (2,0) (2,1) (2,2) \_ | | \ | \___ | | \__|___ \__|__ | | \ | \|
Re: Historical rebalance
Roman, We already track updates on per-transaction basis. The only difference is that instead of doing a single "increment(1)" for transaction we do "increment(X)" where X is number of updates in the given transaction. On Mon, Dec 3, 2018 at 1:16 PM Roman Kondakov wrote: > Igor, Vladimir, Ivan, > > perhaps, we are focused too much on update counters. This feature was > designed for the continuous queries and it may not be suited well for > the historical rebalance. What if we would track updates on > per-transaction basis instead of per-update basis? Let's consider two > counters: low-water mark (LWM) and high-water mark (HWM) which should be > added to each partition. They have the following properties: > > * HWM - is a plane atomic counter. When Tx makes its first write on > primary node it does incrementAndGet for this counter and remembers > obtained value within its context. This counter can be considered as tx > id within current partition - transactions should maintain per-partition > map of their HWM ids. WAL pointer to the first record should remembered > in this map. Also this id should be recorded to WAL data records. > > When Tx sends updates to backups it sends Tx HWM too. When backup > receives this message from the primary node it takes HWM and do > setIfGreater on the local HWM counter. > > * LWM - is a plane atomic counter. When Tx terminates (either with > commit or rollback) it updates its local LWM in the same manner as > update counters do it using holes tracking. For example, if partition's > LWM = 10 now, and tx with id (HWM id) = 12 commits, we do not update > partition LWM until tx with id = 11 is committed. When id = 11 is > committed, LWM is set to 12. If we have LWM == N, this means that all > transactions with id <= N have been terminated for the current partition > and all data is already recorded in the local partition. > > Brief summary for both counters: HWM - means that partition has already > seen at least one update of transactions with id <= HWM. LWM means that > partition has all updates made by transactions wth id <= LWM. > > LWM is always <= HWM. > > On checkpoint we should store only these two counters in checkpoint > record. As optimization we can also store list of pending LWMs - ids > which haven't been merged to LWM because of the holes in sequence. > > Historical rebalance: > > 1. Demander knows its LWM - all updates before it has been applied. > Demander sends LWM to supplier. > > 2. Supplier finds the earliest checkpoint where HWM(supplier) <= LWM > (demander) > > 3. Supplier starts moving forward on WAL until it finds first data > record with HWM id = LWM (demander). From this point WAL can be > rebalanced to demander. > > In this approach updates and checkpoints on primary and backup can be > reordered in any way, but we can always find a proper point to read WAL > from. > > Let's consider a couple of examples. In this examples transaction > updates marked as w1(a) - transaction 1 updates key=a, c1 - transaction > 1 is committed, cp(1, 0) - checkpoint with HWM=1 and LWM=0. (HWM,LWM) - > current counters after operation. (HWM,LWM[hole1, hole2]) - counters > with holes in LWM. > > > 1. Simple case with no reordering: > > PRIMARY > -w1(a)---cp(1,0)---w2(b)w1(c)--c1c2-cp(2,2) > (HWM,LWM)(1,0) (2,0)(2,0) (2,1) (2,2) >| || || > BACKUP > --w1(a)-w2(b)w1(c)---cp(2,0)c1c2-cp(2,2) > (HWM,LWM)(1,0) (2,0)(2,0) (2,1) (2,2) > > > In this case if backup failed before c1 it will receive all updates from > the beginning (HWM=0). > If it fails between c1 and c2, it will receive WAL from primary's cp(1,0), > because tx with id=1 is fully processed on backup: HWM(supplier cp(1,0))=1 > == LWM(demander)=1 > if backup fails after c2, it will receive nothing because it has all > updates HWM(supplier)=2 == LWM(demander)=2 > > > > 2. Case with reordering > > PRIMARY > -w1(a)---cp(1,0)---w2(b)--cp(2,0)--w1(c)--c1-c2---cp(2,2) > (HWM,LWM)(1,0) (2,0) (2,0) > (2,1) (2,2) > \_ | | > \ | >\___ | | > \__|___ >\__|__ | > | \ > | \| > |\ > BACKUP > -w2(b)---w1(a)cp(2,0)---w1(c)c2---c1-cp(2,2) > (HWM,LWM) (2,0) (2,0) (2,0) > (2,0[2]) (2,2) > > > Note here we have a hole on backup when tx2 has committed earlier than tx1 > and LWM wasn't changed at this moment. > > In last case if backup is failed before c1, the entire WAL will be > supplied because LWM=0 until this moment. > If
Re: Historical rebalance
Igor, Vladimir, Ivan, perhaps, we are focused too much on update counters. This feature was designed for the continuous queries and it may not be suited well for the historical rebalance. What if we would track updates on per-transaction basis instead of per-update basis? Let's consider two counters: low-water mark (LWM) and high-water mark (HWM) which should be added to each partition. They have the following properties: * HWM - is a plane atomic counter. When Tx makes its first write on primary node it does incrementAndGet for this counter and remembers obtained value within its context. This counter can be considered as tx id within current partition - transactions should maintain per-partition map of their HWM ids. WAL pointer to the first record should remembered in this map. Also this id should be recorded to WAL data records. When Tx sends updates to backups it sends Tx HWM too. When backup receives this message from the primary node it takes HWM and do setIfGreater on the local HWM counter. * LWM - is a plane atomic counter. When Tx terminates (either with commit or rollback) it updates its local LWM in the same manner as update counters do it using holes tracking. For example, if partition's LWM = 10 now, and tx with id (HWM id) = 12 commits, we do not update partition LWM until tx with id = 11 is committed. When id = 11 is committed, LWM is set to 12. If we have LWM == N, this means that all transactions with id <= N have been terminated for the current partition and all data is already recorded in the local partition. Brief summary for both counters: HWM - means that partition has already seen at least one update of transactions with id <= HWM. LWM means that partition has all updates made by transactions wth id <= LWM. LWM is always <= HWM. On checkpoint we should store only these two counters in checkpoint record. As optimization we can also store list of pending LWMs - ids which haven't been merged to LWM because of the holes in sequence. Historical rebalance: 1. Demander knows its LWM - all updates before it has been applied. Demander sends LWM to supplier. 2. Supplier finds the earliest checkpoint where HWM(supplier) <= LWM (demander) 3. Supplier starts moving forward on WAL until it finds first data record with HWM id = LWM (demander). From this point WAL can be rebalanced to demander. In this approach updates and checkpoints on primary and backup can be reordered in any way, but we can always find a proper point to read WAL from. Let's consider a couple of examples. In this examples transaction updates marked as w1(a) - transaction 1 updates key=a, c1 - transaction 1 is committed, cp(1, 0) - checkpoint with HWM=1 and LWM=0. (HWM,LWM) - current counters after operation. (HWM,LWM[hole1, hole2]) - counters with holes in LWM. 1. Simple case with no reordering: PRIMARY -w1(a)---cp(1,0)---w2(b)w1(c)--c1c2-cp(2,2) (HWM,LWM)(1,0) (2,0)(2,0) (2,1) (2,2) | || || BACKUP --w1(a)-w2(b)w1(c)---cp(2,0)c1c2-cp(2,2) (HWM,LWM)(1,0) (2,0)(2,0) (2,1) (2,2) In this case if backup failed before c1 it will receive all updates from the beginning (HWM=0). If it fails between c1 and c2, it will receive WAL from primary's cp(1,0), because tx with id=1 is fully processed on backup: HWM(supplier cp(1,0))=1 == LWM(demander)=1 if backup fails after c2, it will receive nothing because it has all updates HWM(supplier)=2 == LWM(demander)=2 2. Case with reordering PRIMARY -w1(a)---cp(1,0)---w2(b)--cp(2,0)--w1(c)--c1-c2---cp(2,2) (HWM,LWM)(1,0) (2,0) (2,0) (2,1) (2,2) \_ | | \ | \___ | | \__|___ \__|__ | | \ | \| | \ BACKUP -w2(b)---w1(a)cp(2,0)---w1(c)c2---c1-cp(2,2) (HWM,LWM) (2,0) (2,0) (2,0) (2,0[2]) (2,2) Note here we have a hole on backup when tx2 has committed earlier than tx1 and LWM wasn't changed at this moment. In last case if backup is failed before c1, the entire WAL will be supplied because LWM=0 until this moment. If backup fails after c1 - there is nothing to rebalance, because HWM(supplier)=2 == LWM(demander)=2 What do you think? -- Kind Regards Roman Kondakov On 30.11.2018 2:01, Seliverstov Igor wrote: Vladimir, Look at my example: One active transaction (Tx1 which does opX ops) while another tx (Tx2 which does opX' ops) is finishes with uc4:
Re: Historical rebalance
Vladimir, Look at my example: One active transaction (Tx1 which does opX ops) while another tx (Tx2 which does opX' ops) is finishes with uc4: uc1--op1op2---uc2--op1'uc3--uc4---op3X- Node1 uc1op1uc2op2uc3--op3--uc4cp1 Tx1 - ^ | | | | -Node2 ^-- | | | uc1-uc2-uc3op1'uc4cp1 Tx2 - state on Node2: tx1 -> op3 -> uc2 cp1 [current=uc4, backpointer=uc2] Here op2 was acknowledged by op3, op3 was applied before op1' (linearized by WAL). All nodes having uc4 must have op1' because uc4 cannot be get earlier than prepare stage while prepare stage happens after all updates so *op1' happens before uc4* regardless Tx2 was committed or rolled back. This means *op2 happens before uc4* (uc4 cannot be earlier op2 on any node because on Node2 op2 was already finished (acknowledged by op3) when op1' happens) That was my idea which easy to proof. You used a different approach, but yes, It has to work. чт, 29 нояб. 2018 г. в 22:19, Vladimir Ozerov : > "If more recent WAL records will contain *ALL* updates of the transaction" > -> "More recent WAL records will contain *ALL* updates of the transaction" > > On Thu, Nov 29, 2018 at 10:15 PM Vladimir Ozerov > wrote: > > > Igor, > > > > Yes, I tried to draw different configurations, and it really seems to > > work, despite of being very hard to proof due to non-inituitive HB edges. > > So let me try to spell the algorithm once again to make sure that we are > on > > the same page here. > > > > 1) There are two nodes - primary (P) and backup (B) > > 2) There are three type of events: small transactions which possibly > > increments update counter (ucX), one long active transaction which is > split > > into multiple operations (opX), and checkpoints (cpX) > > 3) Every node always has current update counter. When transaction commits > > it may or may not shift this counter further depending on whether there > are > > holes behind. But we have a strict rule that it always grow. Higher > > coutners synchrnoizes with smaller. Possible cases: > > uc1uc2uc3 > > uc1uc3--- // uc2 missing due to reorder, but is is ok > > > > 4) Operations within a single transaction is always applied sequentially, > > and hence also have HB edge: > > op1op2op3 > > > > 5) When transaction operation happens, we save in memory current update > > counter available at this moment. I.e. we have a map from transaction ID > to > > update counter which was relevant by the time last *completed* operation > > *started*. This is very important thing - we remember the counter when > > operation starts, but update the map only when it finishes. This is > needed > > for situation when update counter is bumber in the middle of a long > > operation. > > uc1op1op2uc2uc3op3 > > | || > >uc1uc1 uc3 > > > > state: tx1 -> op3 -> uc3 > > > > 6) Whenever checkpoint occurs, we save two counters with: "current" and > > "backpointer". The latter is the smallest update counter associated with > > active transactions. If there are no active transactions, current update > > counter is used. > > > > Example 1: no active transactions. > > uc1cp1 > > ^ | > > > > > > state: cp1 [current=uc1, backpointer=uc1] > > > > Example 2: one active transaction: > > --- > > | | > > uc1op1uc2op2op3uc3cp1 > >^ | > >-- > > > > state: tx1 -> op3 -> uc2 > >cp1 [current=uc3, backpointer=uc2] > > > > 7) Historical rebalance: > > 7.1) Demander finds latest checkpoint, get it's backpointer and sends it > > to supplier. > > 7.2) Supplier finds earliest checkpoint where [supplier(current) <= > > demander(backpointer)] > > 7.3) Supplier reads checkpoint backpointer and finds associated WAL > > record. This is where we start. > > > > So in terms of WAL we have: supplier[uc_backpointer <- cp(uc_current <= > > demanter_uc_backpointer)] <- demander[uc_backpointer <- cp(last)] > > > > Now the most important - why it works :-) > > 1) Transaction opeartions are sequential, so at the time of crash nodes > > are *at most one operation ahead *each other > > 2) Demander goes to the past and finds update counter which was current > at > > the time of last TX completed operation > > 3) Supplier goes
Re: Historical rebalance
Igor, Yes, I tried to draw different configurations, and it really seems to work, despite of being very hard to proof due to non-inituitive HB edges. So let me try to spell the algorithm once again to make sure that we are on the same page here. 1) There are two nodes - primary (P) and backup (B) 2) There are three type of events: small transactions which possibly increments update counter (ucX), one long active transaction which is split into multiple operations (opX), and checkpoints (cpX) 3) Every node always has current update counter. When transaction commits it may or may not shift this counter further depending on whether there are holes behind. But we have a strict rule that it always grow. Higher coutners synchrnoizes with smaller. Possible cases: uc1uc2uc3 uc1uc3--- // uc2 missing due to reorder, but is is ok 4) Operations within a single transaction is always applied sequentially, and hence also have HB edge: op1op2op3 5) When transaction operation happens, we save in memory current update counter available at this moment. I.e. we have a map from transaction ID to update counter which was relevant by the time last *completed* operation *started*. This is very important thing - we remember the counter when operation starts, but update the map only when it finishes. This is needed for situation when update counter is bumber in the middle of a long operation. uc1op1op2uc2uc3op3 | || uc1uc1 uc3 state: tx1 -> op3 -> uc3 6) Whenever checkpoint occurs, we save two counters with: "current" and "backpointer". The latter is the smallest update counter associated with active transactions. If there are no active transactions, current update counter is used. Example 1: no active transactions. uc1cp1 ^ | state: cp1 [current=uc1, backpointer=uc1] Example 2: one active transaction: --- | | uc1op1uc2op2op3uc3cp1 ^ | -- state: tx1 -> op3 -> uc2 cp1 [current=uc3, backpointer=uc2] 7) Historical rebalance: 7.1) Demander finds latest checkpoint, get it's backpointer and sends it to supplier. 7.2) Supplier finds earliest checkpoint where [supplier(current) <= demander(backpointer)] 7.3) Supplier reads checkpoint backpointer and finds associated WAL record. This is where we start. So in terms of WAL we have: supplier[uc_backpointer <- cp(uc_current <= demanter_uc_backpointer)] <- demander[uc_backpointer <- cp(last)] Now the most important - why it works :-) 1) Transaction opeartions are sequential, so at the time of crash nodes are *at most one operation ahead *each other 2) Demander goes to the past and finds update counter which was current at the time of last TX completed operation 3) Supplier goes to the closest checkpoint in the past where this update counter either doesn't exist or just appeared 4) Transaction cannot be committed on supplier at this checkpoint, as it would violate UC happens-before rule 5) Tranasction may have not started yet on supplier at this point. If more recent WAL records will contain *ALL* updates of the transaction 6) Transaction may exist on supplier at this checkpoint. Thanks to p.1 we must skip at most one operation. Jump back through supplier's checkpoint backpointer is guaranteed to do this. Igor, do we have the same understanding here? Vladimir. On Thu, Nov 29, 2018 at 2:47 PM Seliverstov Igor wrote: > Ivan, > > different transactions may be applied in different order on backup nodes. > That's why we need an active tx set > and some sorting by their update times. The idea is to identify a point in > time which starting from we may lost some updates. > This point: >1) is the last acknowledged by all backups (including possible further > demander) update on timeline; >2) have a specific update counter (aka back-counter) which we going to > start iteration from. > > After additional thinking on, I've identified a rule: > > There is two fences: > 1) update counter (UC) - this means that all updates, with less UC than > applied one, was applied on a node, having this UC. > 2) update in scope of TX - all updates are applied one by one > sequentially, this means that the fact of update guaranties the previous > update (statement) was finished on all TX participants. > > Сombining them, we can say the next: > > All updates, that was acknowledged at the time the last update of tx, which > updated UC, applied, are guaranteed to be presented on a node having such > UC > > We can use this rule to find an iterator start pointer. > > ср, 28 нояб. 2018 г. в 20:26, Павлухин Иван : > > > Guys, > > > > Another one idea. We can introduce additional update counter which is > > incremented by MVCC
Re: Historical rebalance
"If more recent WAL records will contain *ALL* updates of the transaction" -> "More recent WAL records will contain *ALL* updates of the transaction" On Thu, Nov 29, 2018 at 10:15 PM Vladimir Ozerov wrote: > Igor, > > Yes, I tried to draw different configurations, and it really seems to > work, despite of being very hard to proof due to non-inituitive HB edges. > So let me try to spell the algorithm once again to make sure that we are on > the same page here. > > 1) There are two nodes - primary (P) and backup (B) > 2) There are three type of events: small transactions which possibly > increments update counter (ucX), one long active transaction which is split > into multiple operations (opX), and checkpoints (cpX) > 3) Every node always has current update counter. When transaction commits > it may or may not shift this counter further depending on whether there are > holes behind. But we have a strict rule that it always grow. Higher > coutners synchrnoizes with smaller. Possible cases: > uc1uc2uc3 > uc1uc3--- // uc2 missing due to reorder, but is is ok > > 4) Operations within a single transaction is always applied sequentially, > and hence also have HB edge: > op1op2op3 > > 5) When transaction operation happens, we save in memory current update > counter available at this moment. I.e. we have a map from transaction ID to > update counter which was relevant by the time last *completed* operation > *started*. This is very important thing - we remember the counter when > operation starts, but update the map only when it finishes. This is needed > for situation when update counter is bumber in the middle of a long > operation. > uc1op1op2uc2uc3op3 > | || >uc1uc1 uc3 > > state: tx1 -> op3 -> uc3 > > 6) Whenever checkpoint occurs, we save two counters with: "current" and > "backpointer". The latter is the smallest update counter associated with > active transactions. If there are no active transactions, current update > counter is used. > > Example 1: no active transactions. > uc1cp1 > ^ | > > > state: cp1 [current=uc1, backpointer=uc1] > > Example 2: one active transaction: > --- > | | > uc1op1uc2op2op3uc3cp1 >^ | >-- > > state: tx1 -> op3 -> uc2 >cp1 [current=uc3, backpointer=uc2] > > 7) Historical rebalance: > 7.1) Demander finds latest checkpoint, get it's backpointer and sends it > to supplier. > 7.2) Supplier finds earliest checkpoint where [supplier(current) <= > demander(backpointer)] > 7.3) Supplier reads checkpoint backpointer and finds associated WAL > record. This is where we start. > > So in terms of WAL we have: supplier[uc_backpointer <- cp(uc_current <= > demanter_uc_backpointer)] <- demander[uc_backpointer <- cp(last)] > > Now the most important - why it works :-) > 1) Transaction opeartions are sequential, so at the time of crash nodes > are *at most one operation ahead *each other > 2) Demander goes to the past and finds update counter which was current at > the time of last TX completed operation > 3) Supplier goes to the closest checkpoint in the past where this update > counter either doesn't exist or just appeared > 4) Transaction cannot be committed on supplier at this checkpoint, as it > would violate UC happens-before rule > 5) Tranasction may have not started yet on supplier at this point. If more > recent WAL records will contain *ALL* updates of the transaction > 6) Transaction may exist on supplier at this checkpoint. Thanks to p.1 we > must skip at most one operation. Jump back through supplier's checkpoint > backpointer is guaranteed to do this. > > Igor, do we have the same understanding here? > > Vladimir. > > On Thu, Nov 29, 2018 at 2:47 PM Seliverstov Igor > wrote: > >> Ivan, >> >> different transactions may be applied in different order on backup nodes. >> That's why we need an active tx set >> and some sorting by their update times. The idea is to identify a point in >> time which starting from we may lost some updates. >> This point: >>1) is the last acknowledged by all backups (including possible further >> demander) update on timeline; >>2) have a specific update counter (aka back-counter) which we going to >> start iteration from. >> >> After additional thinking on, I've identified a rule: >> >> There is two fences: >> 1) update counter (UC) - this means that all updates, with less UC than >> applied one, was applied on a node, having this UC. >> 2) update in scope of TX - all updates are applied one by one >> sequentially, this means that the fact of update guaranties the previous >> update (statement) was finished on all TX participants. >> >> Сombining them, we can say the next:
Re: Historical rebalance
Ivan, different transactions may be applied in different order on backup nodes. That's why we need an active tx set and some sorting by their update times. The idea is to identify a point in time which starting from we may lost some updates. This point: 1) is the last acknowledged by all backups (including possible further demander) update on timeline; 2) have a specific update counter (aka back-counter) which we going to start iteration from. After additional thinking on, I've identified a rule: There is two fences: 1) update counter (UC) - this means that all updates, with less UC than applied one, was applied on a node, having this UC. 2) update in scope of TX - all updates are applied one by one sequentially, this means that the fact of update guaranties the previous update (statement) was finished on all TX participants. Сombining them, we can say the next: All updates, that was acknowledged at the time the last update of tx, which updated UC, applied, are guaranteed to be presented on a node having such UC We can use this rule to find an iterator start pointer. ср, 28 нояб. 2018 г. в 20:26, Павлухин Иван : > Guys, > > Another one idea. We can introduce additional update counter which is > incremented by MVCC transactions right after executing operation (like > is done for classic transactions). And we can use that counter for > searching needed WAL records. Can it did the trick? > > P.S. Mentally I am trying to separate facilities providing > transactions and durability. And it seems to me that those facilities > are in different dimensions. > ср, 28 нояб. 2018 г. в 16:26, Павлухин Иван : > > > > Sorry, if it was stated that a SINGLE transaction updates are applied > > in a same order on all replicas then I have no questions so far. I > > thought about reordering updates coming from different transactions. > > > I have not got why we can assume that reordering is not possible. What > > have I missed? > > ср, 28 нояб. 2018 г. в 13:26, Павлухин Иван : > > > > > > Hi, > > > > > > Regarding Vladimir's new idea. > > > > We assume that transaction can be represented as a set of > independent operations, which are applied in the same order on both primary > and backup nodes. > > > I have not got why we can assume that reordering is not possible. What > > > have I missed? > > > вт, 27 нояб. 2018 г. в 14:42, Seliverstov Igor : > > > > > > > > Vladimir, > > > > > > > > I think I got your point, > > > > > > > > It should work if we do the next: > > > > introduce two structures: active list (txs) and candidate list > (updCntr -> > > > > txn pairs) > > > > > > > > Track active txs, mapping them to actual update counter at update > time. > > > > On each next update put update counter, associated with previous > update, > > > > into a candidates list possibly overwrite existing value (checking > txn) > > > > On tx finish remove tx from active list only if appropriate update > counter > > > > (associated with finished tx) is applied. > > > > On update counter update set the minimal update counter from the > candidates > > > > list as a back-counter, clear the candidate list and remove an > associated > > > > tx from the active list if present. > > > > Use back-counter instead of actual update counter in demand message. > > > > > > > > вт, 27 нояб. 2018 г. в 12:56, Seliverstov Igor >: > > > > > > > > > Ivan, > > > > > > > > > > 1) The list is saved on each checkpoint, wholly (all transactions > in > > > > > active state at checkpoint begin). > > > > > We need whole the list to get oldest transaction because after > > > > > the previous oldest tx finishes, we need to get the following one. > > > > > > > > > > 2) I guess there is a description of how persistent storage works > and how > > > > > it restores [1] > > > > > > > > > > Vladimir, > > > > > > > > > > the whole list of what we going to store on checkpoint (updated): > > > > > 1) Partition counter low watermark (LWM) > > > > > 2) WAL pointer of earliest active transaction write to partition > at the > > > > > time the checkpoint have started > > > > > 3) List of prepared txs with acquired partition counters (which > were > > > > > acquired but not applied yet) > > > > > > > > > > This way we don't need any additional info in demand message. > Start point > > > > > can be easily determined using stored WAL "back-pointer". > > > > > > > > > > [1] > > > > > > https://cwiki.apache.org/confluence/display/IGNITE/Ignite+Persistent+Store+-+under+the+hood#IgnitePersistentStore-underthehood-LocalRecoveryProcess > > > > > > > > > > > > > > > вт, 27 нояб. 2018 г. в 11:19, Vladimir Ozerov < > voze...@gridgain.com>: > > > > > > > > > >> Igor, > > > > >> > > > > >> Could you please elaborate - what is the whole set of information > we are > > > > >> going to save at checkpoint time? From what I understand this > should be: > > > > >> 1) List of active transactions with WAL pointers of their first > writes > > > > >> 2) List of prepared transactions with
Re: Historical rebalance
Guys, Another one idea. We can introduce additional update counter which is incremented by MVCC transactions right after executing operation (like is done for classic transactions). And we can use that counter for searching needed WAL records. Can it did the trick? P.S. Mentally I am trying to separate facilities providing transactions and durability. And it seems to me that those facilities are in different dimensions. ср, 28 нояб. 2018 г. в 16:26, Павлухин Иван : > > Sorry, if it was stated that a SINGLE transaction updates are applied > in a same order on all replicas then I have no questions so far. I > thought about reordering updates coming from different transactions. > > I have not got why we can assume that reordering is not possible. What > have I missed? > ср, 28 нояб. 2018 г. в 13:26, Павлухин Иван : > > > > Hi, > > > > Regarding Vladimir's new idea. > > > We assume that transaction can be represented as a set of independent > > > operations, which are applied in the same order on both primary and > > > backup nodes. > > I have not got why we can assume that reordering is not possible. What > > have I missed? > > вт, 27 нояб. 2018 г. в 14:42, Seliverstov Igor : > > > > > > Vladimir, > > > > > > I think I got your point, > > > > > > It should work if we do the next: > > > introduce two structures: active list (txs) and candidate list (updCntr -> > > > txn pairs) > > > > > > Track active txs, mapping them to actual update counter at update time. > > > On each next update put update counter, associated with previous update, > > > into a candidates list possibly overwrite existing value (checking txn) > > > On tx finish remove tx from active list only if appropriate update counter > > > (associated with finished tx) is applied. > > > On update counter update set the minimal update counter from the > > > candidates > > > list as a back-counter, clear the candidate list and remove an associated > > > tx from the active list if present. > > > Use back-counter instead of actual update counter in demand message. > > > > > > вт, 27 нояб. 2018 г. в 12:56, Seliverstov Igor : > > > > > > > Ivan, > > > > > > > > 1) The list is saved on each checkpoint, wholly (all transactions in > > > > active state at checkpoint begin). > > > > We need whole the list to get oldest transaction because after > > > > the previous oldest tx finishes, we need to get the following one. > > > > > > > > 2) I guess there is a description of how persistent storage works and > > > > how > > > > it restores [1] > > > > > > > > Vladimir, > > > > > > > > the whole list of what we going to store on checkpoint (updated): > > > > 1) Partition counter low watermark (LWM) > > > > 2) WAL pointer of earliest active transaction write to partition at the > > > > time the checkpoint have started > > > > 3) List of prepared txs with acquired partition counters (which were > > > > acquired but not applied yet) > > > > > > > > This way we don't need any additional info in demand message. Start > > > > point > > > > can be easily determined using stored WAL "back-pointer". > > > > > > > > [1] > > > > https://cwiki.apache.org/confluence/display/IGNITE/Ignite+Persistent+Store+-+under+the+hood#IgnitePersistentStore-underthehood-LocalRecoveryProcess > > > > > > > > > > > > вт, 27 нояб. 2018 г. в 11:19, Vladimir Ozerov : > > > > > > > >> Igor, > > > >> > > > >> Could you please elaborate - what is the whole set of information we > > > >> are > > > >> going to save at checkpoint time? From what I understand this should > > > >> be: > > > >> 1) List of active transactions with WAL pointers of their first writes > > > >> 2) List of prepared transactions with their update counters > > > >> 3) Partition counter low watermark (LWM) - the smallest partition > > > >> counter > > > >> before which there are no prepared transactions. > > > >> > > > >> And the we send to supplier node a message: "Give me all updates > > > >> starting > > > >> from that LWM plus data for that transactions which were active when I > > > >> failed". > > > >> > > > >> Am I right? > > > >> > > > >> On Fri, Nov 23, 2018 at 11:22 AM Seliverstov Igor > > > >> > > > >> wrote: > > > >> > > > >> > Hi Igniters, > > > >> > > > > >> > Currently I’m working on possible approaches how to implement > > > >> > historical > > > >> > rebalance (delta rebalance using WAL iterator) over MVCC caches. > > > >> > > > > >> > The main difficulty is that MVCC writes changes on tx active phase > > > >> > while > > > >> > partition update version, aka update counter, is being applied on tx > > > >> > finish. This means we cannot start iteration over WAL right from the > > > >> > pointer where the update counter updated, but should include updates, > > > >> which > > > >> > the transaction that updated the counter did. > > > >> > > > > >> > These updates may be much earlier than the point where the update > > > >> counter > > > >> > was updated, so we have to be able to identify the point where
Re: Historical rebalance
Sorry, if it was stated that a SINGLE transaction updates are applied in a same order on all replicas then I have no questions so far. I thought about reordering updates coming from different transactions. > I have not got why we can assume that reordering is not possible. What have I missed? ср, 28 нояб. 2018 г. в 13:26, Павлухин Иван : > > Hi, > > Regarding Vladimir's new idea. > > We assume that transaction can be represented as a set of independent > > operations, which are applied in the same order on both primary and backup > > nodes. > I have not got why we can assume that reordering is not possible. What > have I missed? > вт, 27 нояб. 2018 г. в 14:42, Seliverstov Igor : > > > > Vladimir, > > > > I think I got your point, > > > > It should work if we do the next: > > introduce two structures: active list (txs) and candidate list (updCntr -> > > txn pairs) > > > > Track active txs, mapping them to actual update counter at update time. > > On each next update put update counter, associated with previous update, > > into a candidates list possibly overwrite existing value (checking txn) > > On tx finish remove tx from active list only if appropriate update counter > > (associated with finished tx) is applied. > > On update counter update set the minimal update counter from the candidates > > list as a back-counter, clear the candidate list and remove an associated > > tx from the active list if present. > > Use back-counter instead of actual update counter in demand message. > > > > вт, 27 нояб. 2018 г. в 12:56, Seliverstov Igor : > > > > > Ivan, > > > > > > 1) The list is saved on each checkpoint, wholly (all transactions in > > > active state at checkpoint begin). > > > We need whole the list to get oldest transaction because after > > > the previous oldest tx finishes, we need to get the following one. > > > > > > 2) I guess there is a description of how persistent storage works and how > > > it restores [1] > > > > > > Vladimir, > > > > > > the whole list of what we going to store on checkpoint (updated): > > > 1) Partition counter low watermark (LWM) > > > 2) WAL pointer of earliest active transaction write to partition at the > > > time the checkpoint have started > > > 3) List of prepared txs with acquired partition counters (which were > > > acquired but not applied yet) > > > > > > This way we don't need any additional info in demand message. Start point > > > can be easily determined using stored WAL "back-pointer". > > > > > > [1] > > > https://cwiki.apache.org/confluence/display/IGNITE/Ignite+Persistent+Store+-+under+the+hood#IgnitePersistentStore-underthehood-LocalRecoveryProcess > > > > > > > > > вт, 27 нояб. 2018 г. в 11:19, Vladimir Ozerov : > > > > > >> Igor, > > >> > > >> Could you please elaborate - what is the whole set of information we are > > >> going to save at checkpoint time? From what I understand this should be: > > >> 1) List of active transactions with WAL pointers of their first writes > > >> 2) List of prepared transactions with their update counters > > >> 3) Partition counter low watermark (LWM) - the smallest partition counter > > >> before which there are no prepared transactions. > > >> > > >> And the we send to supplier node a message: "Give me all updates starting > > >> from that LWM plus data for that transactions which were active when I > > >> failed". > > >> > > >> Am I right? > > >> > > >> On Fri, Nov 23, 2018 at 11:22 AM Seliverstov Igor > > >> wrote: > > >> > > >> > Hi Igniters, > > >> > > > >> > Currently I’m working on possible approaches how to implement > > >> > historical > > >> > rebalance (delta rebalance using WAL iterator) over MVCC caches. > > >> > > > >> > The main difficulty is that MVCC writes changes on tx active phase > > >> > while > > >> > partition update version, aka update counter, is being applied on tx > > >> > finish. This means we cannot start iteration over WAL right from the > > >> > pointer where the update counter updated, but should include updates, > > >> which > > >> > the transaction that updated the counter did. > > >> > > > >> > These updates may be much earlier than the point where the update > > >> counter > > >> > was updated, so we have to be able to identify the point where the > > >> > first > > >> > update happened. > > >> > > > >> > The proposed approach includes: > > >> > > > >> > 1) preserve list of active txs, sorted by the time of their first > > >> > update > > >> > (using WAL ptr of first WAL record in tx) > > >> > > > >> > 2) persist this list on each checkpoint (together with TxLog for > > >> example) > > >> > > > >> > 4) send whole active tx list (transactions which were in active state > > >> > at > > >> > the time the node was crushed, empty list in case of graceful node > > >> stop) as > > >> > a part of partition demand message. > > >> > > > >> > 4) find a checkpoint where the earliest tx exists in persisted txs and > > >> use > > >> > saved WAL ptr as a start point or apply current approach
Re: Historical rebalance
Hi, Regarding Vladimir's new idea. > We assume that transaction can be represented as a set of independent > operations, which are applied in the same order on both primary and backup > nodes. I have not got why we can assume that reordering is not possible. What have I missed? вт, 27 нояб. 2018 г. в 14:42, Seliverstov Igor : > > Vladimir, > > I think I got your point, > > It should work if we do the next: > introduce two structures: active list (txs) and candidate list (updCntr -> > txn pairs) > > Track active txs, mapping them to actual update counter at update time. > On each next update put update counter, associated with previous update, > into a candidates list possibly overwrite existing value (checking txn) > On tx finish remove tx from active list only if appropriate update counter > (associated with finished tx) is applied. > On update counter update set the minimal update counter from the candidates > list as a back-counter, clear the candidate list and remove an associated > tx from the active list if present. > Use back-counter instead of actual update counter in demand message. > > вт, 27 нояб. 2018 г. в 12:56, Seliverstov Igor : > > > Ivan, > > > > 1) The list is saved on each checkpoint, wholly (all transactions in > > active state at checkpoint begin). > > We need whole the list to get oldest transaction because after > > the previous oldest tx finishes, we need to get the following one. > > > > 2) I guess there is a description of how persistent storage works and how > > it restores [1] > > > > Vladimir, > > > > the whole list of what we going to store on checkpoint (updated): > > 1) Partition counter low watermark (LWM) > > 2) WAL pointer of earliest active transaction write to partition at the > > time the checkpoint have started > > 3) List of prepared txs with acquired partition counters (which were > > acquired but not applied yet) > > > > This way we don't need any additional info in demand message. Start point > > can be easily determined using stored WAL "back-pointer". > > > > [1] > > https://cwiki.apache.org/confluence/display/IGNITE/Ignite+Persistent+Store+-+under+the+hood#IgnitePersistentStore-underthehood-LocalRecoveryProcess > > > > > > вт, 27 нояб. 2018 г. в 11:19, Vladimir Ozerov : > > > >> Igor, > >> > >> Could you please elaborate - what is the whole set of information we are > >> going to save at checkpoint time? From what I understand this should be: > >> 1) List of active transactions with WAL pointers of their first writes > >> 2) List of prepared transactions with their update counters > >> 3) Partition counter low watermark (LWM) - the smallest partition counter > >> before which there are no prepared transactions. > >> > >> And the we send to supplier node a message: "Give me all updates starting > >> from that LWM plus data for that transactions which were active when I > >> failed". > >> > >> Am I right? > >> > >> On Fri, Nov 23, 2018 at 11:22 AM Seliverstov Igor > >> wrote: > >> > >> > Hi Igniters, > >> > > >> > Currently I’m working on possible approaches how to implement historical > >> > rebalance (delta rebalance using WAL iterator) over MVCC caches. > >> > > >> > The main difficulty is that MVCC writes changes on tx active phase while > >> > partition update version, aka update counter, is being applied on tx > >> > finish. This means we cannot start iteration over WAL right from the > >> > pointer where the update counter updated, but should include updates, > >> which > >> > the transaction that updated the counter did. > >> > > >> > These updates may be much earlier than the point where the update > >> counter > >> > was updated, so we have to be able to identify the point where the first > >> > update happened. > >> > > >> > The proposed approach includes: > >> > > >> > 1) preserve list of active txs, sorted by the time of their first update > >> > (using WAL ptr of first WAL record in tx) > >> > > >> > 2) persist this list on each checkpoint (together with TxLog for > >> example) > >> > > >> > 4) send whole active tx list (transactions which were in active state at > >> > the time the node was crushed, empty list in case of graceful node > >> stop) as > >> > a part of partition demand message. > >> > > >> > 4) find a checkpoint where the earliest tx exists in persisted txs and > >> use > >> > saved WAL ptr as a start point or apply current approach in case the > >> active > >> > tx list (sent on previous step) is empty > >> > > >> > 5) start iteration. > >> > > >> > Your thoughts? > >> > > >> > Regards, > >> > Igor > >> > > -- Best regards, Ivan Pavlukhin
Re: Historical rebalance
Vladimir, I think I got your point, It should work if we do the next: introduce two structures: active list (txs) and candidate list (updCntr -> txn pairs) Track active txs, mapping them to actual update counter at update time. On each next update put update counter, associated with previous update, into a candidates list possibly overwrite existing value (checking txn) On tx finish remove tx from active list only if appropriate update counter (associated with finished tx) is applied. On update counter update set the minimal update counter from the candidates list as a back-counter, clear the candidate list and remove an associated tx from the active list if present. Use back-counter instead of actual update counter in demand message. вт, 27 нояб. 2018 г. в 12:56, Seliverstov Igor : > Ivan, > > 1) The list is saved on each checkpoint, wholly (all transactions in > active state at checkpoint begin). > We need whole the list to get oldest transaction because after > the previous oldest tx finishes, we need to get the following one. > > 2) I guess there is a description of how persistent storage works and how > it restores [1] > > Vladimir, > > the whole list of what we going to store on checkpoint (updated): > 1) Partition counter low watermark (LWM) > 2) WAL pointer of earliest active transaction write to partition at the > time the checkpoint have started > 3) List of prepared txs with acquired partition counters (which were > acquired but not applied yet) > > This way we don't need any additional info in demand message. Start point > can be easily determined using stored WAL "back-pointer". > > [1] > https://cwiki.apache.org/confluence/display/IGNITE/Ignite+Persistent+Store+-+under+the+hood#IgnitePersistentStore-underthehood-LocalRecoveryProcess > > > вт, 27 нояб. 2018 г. в 11:19, Vladimir Ozerov : > >> Igor, >> >> Could you please elaborate - what is the whole set of information we are >> going to save at checkpoint time? From what I understand this should be: >> 1) List of active transactions with WAL pointers of their first writes >> 2) List of prepared transactions with their update counters >> 3) Partition counter low watermark (LWM) - the smallest partition counter >> before which there are no prepared transactions. >> >> And the we send to supplier node a message: "Give me all updates starting >> from that LWM plus data for that transactions which were active when I >> failed". >> >> Am I right? >> >> On Fri, Nov 23, 2018 at 11:22 AM Seliverstov Igor >> wrote: >> >> > Hi Igniters, >> > >> > Currently I’m working on possible approaches how to implement historical >> > rebalance (delta rebalance using WAL iterator) over MVCC caches. >> > >> > The main difficulty is that MVCC writes changes on tx active phase while >> > partition update version, aka update counter, is being applied on tx >> > finish. This means we cannot start iteration over WAL right from the >> > pointer where the update counter updated, but should include updates, >> which >> > the transaction that updated the counter did. >> > >> > These updates may be much earlier than the point where the update >> counter >> > was updated, so we have to be able to identify the point where the first >> > update happened. >> > >> > The proposed approach includes: >> > >> > 1) preserve list of active txs, sorted by the time of their first update >> > (using WAL ptr of first WAL record in tx) >> > >> > 2) persist this list on each checkpoint (together with TxLog for >> example) >> > >> > 4) send whole active tx list (transactions which were in active state at >> > the time the node was crushed, empty list in case of graceful node >> stop) as >> > a part of partition demand message. >> > >> > 4) find a checkpoint where the earliest tx exists in persisted txs and >> use >> > saved WAL ptr as a start point or apply current approach in case the >> active >> > tx list (sent on previous step) is empty >> > >> > 5) start iteration. >> > >> > Your thoughts? >> > >> > Regards, >> > Igor >> >
Re: Historical rebalance
Ivan, 1) The list is saved on each checkpoint, wholly (all transactions in active state at checkpoint begin). We need whole the list to get oldest transaction because after the previous oldest tx finishes, we need to get the following one. 2) I guess there is a description of how persistent storage works and how it restores [1] Vladimir, the whole list of what we going to store on checkpoint (updated): 1) Partition counter low watermark (LWM) 2) WAL pointer of earliest active transaction write to partition at the time the checkpoint have started 3) List of prepared txs with acquired partition counters (which were acquired but not applied yet) This way we don't need any additional info in demand message. Start point can be easily determined using stored WAL "back-pointer". [1] https://cwiki.apache.org/confluence/display/IGNITE/Ignite+Persistent+Store+-+under+the+hood#IgnitePersistentStore-underthehood-LocalRecoveryProcess вт, 27 нояб. 2018 г. в 11:19, Vladimir Ozerov : > Igor, > > Could you please elaborate - what is the whole set of information we are > going to save at checkpoint time? From what I understand this should be: > 1) List of active transactions with WAL pointers of their first writes > 2) List of prepared transactions with their update counters > 3) Partition counter low watermark (LWM) - the smallest partition counter > before which there are no prepared transactions. > > And the we send to supplier node a message: "Give me all updates starting > from that LWM plus data for that transactions which were active when I > failed". > > Am I right? > > On Fri, Nov 23, 2018 at 11:22 AM Seliverstov Igor > wrote: > > > Hi Igniters, > > > > Currently I’m working on possible approaches how to implement historical > > rebalance (delta rebalance using WAL iterator) over MVCC caches. > > > > The main difficulty is that MVCC writes changes on tx active phase while > > partition update version, aka update counter, is being applied on tx > > finish. This means we cannot start iteration over WAL right from the > > pointer where the update counter updated, but should include updates, > which > > the transaction that updated the counter did. > > > > These updates may be much earlier than the point where the update counter > > was updated, so we have to be able to identify the point where the first > > update happened. > > > > The proposed approach includes: > > > > 1) preserve list of active txs, sorted by the time of their first update > > (using WAL ptr of first WAL record in tx) > > > > 2) persist this list on each checkpoint (together with TxLog for example) > > > > 4) send whole active tx list (transactions which were in active state at > > the time the node was crushed, empty list in case of graceful node stop) > as > > a part of partition demand message. > > > > 4) find a checkpoint where the earliest tx exists in persisted txs and > use > > saved WAL ptr as a start point or apply current approach in case the > active > > tx list (sent on previous step) is empty > > > > 5) start iteration. > > > > Your thoughts? > > > > Regards, > > Igor >
Re: Historical rebalance
Just thought of this a bit more. I we will look for start of long-running transaction in WAL we may go back too far to the past only to get few entries. What if we consider slightly different approach? We assume that transaction can be represented as a set of independent operations, which are applied in the same order on both primary and backup nodes. Then we can do the following: 1) When next operation is finished, we assign transaction LWM of the last checkpoint. I.e. we maintain a map [Txn -> last_op_LWM]. 2) If "last_op_LWM" of transaction is not changed between two subsequent checkpoints, we assign it to special value "UP_TO_DATE". Now at the time of checkpoint we get minimal value among current partition LWM and active transaction LWMs, ignoring "UP_TO_DATE" values. Resulting value is the final partition counter which we will request from supplier node. We save it to checkpoint record. When WAL on demander is unwound from this value, then it is guaranteed to contain all missing data of demanders's active transactions. I.e. instead of tracking the whole active transaction, we track part of transaction which is possibly missing on a node. Will that work? On Tue, Nov 27, 2018 at 11:19 AM Vladimir Ozerov wrote: > Igor, > > Could you please elaborate - what is the whole set of information we are > going to save at checkpoint time? From what I understand this should be: > 1) List of active transactions with WAL pointers of their first writes > 2) List of prepared transactions with their update counters > 3) Partition counter low watermark (LWM) - the smallest partition counter > before which there are no prepared transactions. > > And the we send to supplier node a message: "Give me all updates starting > from that LWM plus data for that transactions which were active when I > failed". > > Am I right? > > On Fri, Nov 23, 2018 at 11:22 AM Seliverstov Igor > wrote: > >> Hi Igniters, >> >> Currently I’m working on possible approaches how to implement historical >> rebalance (delta rebalance using WAL iterator) over MVCC caches. >> >> The main difficulty is that MVCC writes changes on tx active phase while >> partition update version, aka update counter, is being applied on tx >> finish. This means we cannot start iteration over WAL right from the >> pointer where the update counter updated, but should include updates, which >> the transaction that updated the counter did. >> >> These updates may be much earlier than the point where the update counter >> was updated, so we have to be able to identify the point where the first >> update happened. >> >> The proposed approach includes: >> >> 1) preserve list of active txs, sorted by the time of their first update >> (using WAL ptr of first WAL record in tx) >> >> 2) persist this list on each checkpoint (together with TxLog for example) >> >> 4) send whole active tx list (transactions which were in active state at >> the time the node was crushed, empty list in case of graceful node stop) as >> a part of partition demand message. >> >> 4) find a checkpoint where the earliest tx exists in persisted txs and >> use saved WAL ptr as a start point or apply current approach in case the >> active tx list (sent on previous step) is empty >> >> 5) start iteration. >> >> Your thoughts? >> >> Regards, >> Igor > >
Re: Historical rebalance
Igor, Could you please elaborate - what is the whole set of information we are going to save at checkpoint time? From what I understand this should be: 1) List of active transactions with WAL pointers of their first writes 2) List of prepared transactions with their update counters 3) Partition counter low watermark (LWM) - the smallest partition counter before which there are no prepared transactions. And the we send to supplier node a message: "Give me all updates starting from that LWM plus data for that transactions which were active when I failed". Am I right? On Fri, Nov 23, 2018 at 11:22 AM Seliverstov Igor wrote: > Hi Igniters, > > Currently I’m working on possible approaches how to implement historical > rebalance (delta rebalance using WAL iterator) over MVCC caches. > > The main difficulty is that MVCC writes changes on tx active phase while > partition update version, aka update counter, is being applied on tx > finish. This means we cannot start iteration over WAL right from the > pointer where the update counter updated, but should include updates, which > the transaction that updated the counter did. > > These updates may be much earlier than the point where the update counter > was updated, so we have to be able to identify the point where the first > update happened. > > The proposed approach includes: > > 1) preserve list of active txs, sorted by the time of their first update > (using WAL ptr of first WAL record in tx) > > 2) persist this list on each checkpoint (together with TxLog for example) > > 4) send whole active tx list (transactions which were in active state at > the time the node was crushed, empty list in case of graceful node stop) as > a part of partition demand message. > > 4) find a checkpoint where the earliest tx exists in persisted txs and use > saved WAL ptr as a start point or apply current approach in case the active > tx list (sent on previous step) is empty > > 5) start iteration. > > Your thoughts? > > Regards, > Igor
Re: Historical rebalance
Igor, Could you please clarify some points? > 1) preserve list of active txs, sorted by the time of their first update > (using WAL ptr of first WAL record in tx) Is this list maintained per transaction or per checkpoint (or per something else)? Why can't we track only oldest active transaction instead of whole active list? > 4) find a checkpoint where the earliest tx exists in persisted txs and use > saved WAL ptr as a start point or apply current approach in case the active > tx list (sent on previous step) is empty What is the base storage state on a demanding node to which we are applying WAL records to? I mean the state before applying WAL records. Do we apply all records simply one by one or filter out some of them? пт, 23 нояб. 2018 г. в 11:22, Seliverstov Igor : > > Hi Igniters, > > Currently I’m working on possible approaches how to implement historical > rebalance (delta rebalance using WAL iterator) over MVCC caches. > > The main difficulty is that MVCC writes changes on tx active phase while > partition update version, aka update counter, is being applied on tx finish. > This means we cannot start iteration over WAL right from the pointer where > the update counter updated, but should include updates, which the transaction > that updated the counter did. > > These updates may be much earlier than the point where the update counter was > updated, so we have to be able to identify the point where the first update > happened. > > The proposed approach includes: > > 1) preserve list of active txs, sorted by the time of their first update > (using WAL ptr of first WAL record in tx) > > 2) persist this list on each checkpoint (together with TxLog for example) > > 4) send whole active tx list (transactions which were in active state at the > time the node was crushed, empty list in case of graceful node stop) as a > part of partition demand message. > > 4) find a checkpoint where the earliest tx exists in persisted txs and use > saved WAL ptr as a start point or apply current approach in case the active > tx list (sent on previous step) is empty > > 5) start iteration. > > Your thoughts? > > Regards, > Igor -- Best regards, Ivan Pavlukhin
