Vladimir, This looks like a complete overhaul of our transactional behavior and I do not think we are at a liberty to make such drastic changes. Can you please explain why the current behavior would not map to the SQL transactions?
D. On Mon, Sep 25, 2017 at 12:09 PM, Vladimir Ozerov <voze...@gridgain.com> wrote: > Folks, > > Sorry for late reply. I had a chat with several Ignite veterans today. We > tried to design transactional SQL for Ignite. One of our questions was how > to align SQL transactions with current Ignite transactions. And we failed. > And then we came to conclusion that current transaction API is unusable. We > have 6 pairs of modes from API standpoint and 4 real modes. This is very > counterintuitive and cannot be mapped to any transactional framework our > users are familiar with. > > So we thought how new tx API might looks like, and here is the draft. > > 1) Define new enum *TransactionIsolationLevel *(to avoid clashes with > current enum) with three standard modes - READ_COMMITTED, REPEATABLE_READ, > SERIALIZABLE. > 2) Define new enum *TransactionHint* - READ_ONLY, OPTIMISTIC_LOCKING > 3) No more OPTIMISTIC and PESSIMISTIC. Seriously. > 4) Reads never acuire locks > 5) Writes always acquire locks > 6) *IgniteCache.withReadForUpdate()* will return special facade which will > obtain locks on reads. This is analogue of SELECT FOR UPDATE in SQL. > 7) *TransactionHint.READ_ONLY* - forces transaction to throw an exception > on any update > 8) *TransactionHint.OPTIMISTIC_LOCKING* - turns transaction into our > current deadlock-free OPTIMISTIC/SERIALIZABLE mode. Applicable only to > SERIALIZABLE isolation level. > 9) Define new API methods: > - IgniteTransactions.txStart(TransactionIsolationLevel isolation) > - IgniteTransactions.txStart(TransactionIsolationLevel isolation, > TransactionHint... hints) > 10) Deprecate old TX start methods > > As a result we will have simple, clean and extensible API. Which can be > explained to users in 5 minutes, instead of current half an hour. And which > is perfectly aligned with upcoming transactional SQL. > > Thoughts? > > > On Thu, Sep 7, 2017 at 6:48 AM, Dmitriy Setrakyan <dsetrak...@apache.org> > wrote: > > > Vova, > > > > Thanks for doing the research. The changes you are suggesting are a bit > too > > bold, so let's discuss them in some more detail... > > > > On Wed, Sep 6, 2017 at 4:51 AM, Vladimir Ozerov <voze...@gridgain.com> > > wrote: > > > > > Igniters, > > > > > > We are moving towards DBMS system. None of them has a notion of > > > OPTIMISTIC/PESSIMISTIC transactions. Instead, they work as follows: > > > 1) Reads (SELECT) do not acquire exclusive row locks > > > 2) Exclusive lock on read could be forced explicitly (SELECT ... FOR > > > UPDATE) > > > 3) Writes do acuire explicit row locks > > > 4) Locks are always acquired immediately once statement is executed > > > 5) The strictest concurrency level - typically SERIALIZABLE - rely on > > > so-called *range locks* (or *predicate locks*) to track dependencies > > > between transactions. Some vendors throw an exception in case of > > conflict - > > > these are ones where snapshot-based MVCC is used - PostgreSQL, Oracle. > > > Others do aggressive locking - ones where two-phase locking algorithm > is > > > used - SQL Server, MySQL. > > > > > > As you see, there is no concept of PESSIMISTIC/OPTIMISTIC modes. > Instead, > > > all updates are "PESSIMISTIC", reads are "OPTIMISTIC" but could become > > > "PESSIMISTIC" if requested explicitly, and for snapshot-based vendors > (we > > > are going in this direction) read-write conflicts are resolved in > manner > > > somewhat similar to our OPTIMISTIC/SERIALIZABLE. > > > > > > That said, I would propose to think on how transactions could look like > > in > > > future Ignite versions (say, 3.0). My rough vision: > > > > > > 1) No OPTIMISTIC mode at all - too counterintuitive and complex. It's > > only > > > advantage is deadlock-freedom when combined with SERIALIZABLE. If we > have > > > good deadlock detector and nice administrative capabilities, this would > > not > > > be a problem for us. > > > > > > Hm... The advantage of Optimistic Serialiazable mode is actually > lock-free > > transactions. The deadlock is impossible in this case. I doubt any > deadlock > > detector would match the performance advantage we get from lock-free > > transactions. > > > > > > > > > > 2) Behavior of reads could be controlled through "with" facade: > > > V val1 = cache.get(key1); // Shared lock or no lock > > > V val2 = cache.withForUpdate().get(key2); // Exclusive lock > > > > > > > Don't like the API. We are not trying to abandon the data grid use-case > or > > API, we are trying to add the database use case. > > > > > > > 3) REPEATABLE_READ - throw exception in case of write-write conflict > > > > > > > Well, I would like to preserve the PESSIMISTIC mode. I find it more > > convenient than the "withForUpdate" API. It almost seems like you are > > trying to force the pendulum too far in the opposite direction. > > > > > > > 4) SERIALIZABLE - throw exception in case of write-write and write-read > > > confilct (this is how our OPTIMISTIC/SERIALZABLE works now, but it > > doesn't > > > support predicates) > > > > > > > So, no change here? Good :) > > > > > > > 5) Add READ_ONLY isolation mode where updates will not be allowed at > all. > > > Such transacrtons would be able to bypass some Ignite internals to > > achieve > > > greater performance, what could be valuable for mostly-read use cases > > (e.g. > > > OLAP). > > > > > > > Love the idea. We have already seen many use cases that could benefit > from > > it. > > How hard is it to implement? > > > > > > > > > > Thoughts? > > > > > > Vladimir. > > > > > >
