I've got several concerns around this repair process. - The first thing I notice is that we're talking about repairing the entire table across the entire cluster all in one go. It's been a *long* time since I tried to do a full repair of an entire table without using sub-ranges. Is anyone here even doing that with clusters of non trivial size? How long does a full repair of a 100 node cluster with 5TB / node take even in the best case scenario?
- Even in a scenario where sub-range repair is supported, you'd have to scan *every* sstable on the base table in order to construct the a merkle tree, as we don't know in advance which SSTables contain the ranges that the MV will. That means a subrange repair would have to do a *ton* of IO. Anyone who's mis-configured a sub-range incremental repair to use too many ranges will probably be familiar with how long it can take to anti-compact a bunch of SSTables. With MV sub-range repair, we'd have even more overhead, because we'd have to read in every SSTable, every time. If we do 10 subranges, we'll do 10x the IO of a normal repair. I don't think this is practical. - Merkle trees make sense when you're comparing tables with the same partition key, but I don't think they do when you're transforming a base table to a view. When there's a mis-match, what's transferred? We have a range of data in the MV, but now we have to go find that from the base table. That means the merkle tree needs to not just track the hashes and ranges, but the original keys it was transformed from, in order to go find all of the matching partitions in that mis-matched range. Either that or we end up rescanning the entire dataset in order to find the mismatches. Jon On Tue, May 13, 2025 at 10:29 AM Runtian Liu <curly...@gmail.com> wrote: > > Looking at the details of the CEP it seems to describe Paxos as > PaxosV1, but PaxosV2 works slightly differently (it can read during the > prepare phase). I assume that supporting Paxos means supporting both V1 and > V2 for materialized views? > We are going to support Paxos V2. The CEP is not clear on that, we add > this to clarify that. > > It looks like the online portion is now fairly well understood. For the > offline repair part, I see two main concerns: one around the scalability of > the proposed approach, and another regarding how it handles tombstones. > > Scalability: > I have added a section > <https://cwiki.apache.org/confluence/display/CASSANDRA/CEP-48%3A+First-Class+Materialized+View+Support#CEP48:FirstClassMaterializedViewSupport-MVRepairVSFullRepairwithanExample> > in the CEP with an example to compare full repair and the proposed MV > repair, the overall scalability should not be a problem. > > Consider a dataset with tokens from 1 to 4 and a cluster of 4 nodes, where > each node owns one token. The base table uses (pk, ck) as its primary key, > while the materialized view (MV) uses (ck, pk) as its primary key. Both > tables include a value column v, which allows us to correlate rows between > them. The dataset consists of 16 records, distributed as follows: > > *Base table* > (pk, ck, v) > (1, 1, 1), (1, 2, 2), (1, 3, 3), (1, 4, 4) // N1 > (2, 1, 5), (2, 2, 6), (2, 3, 7), (2, 4, 8) // N2 > (3, 1, 9), (3, 2, 10), (3, 3, 11), (3, 4, 12) // N3 > (4, 1, 13), (4, 2, 14), (4, 3, 15), (4, 4, 16) // N4 > > *Materialized view* > (ck, pk, v) > (1, 1, 1), (1, 2, 5), (1, 3, 9), (1, 4, 13) // N1 > (2, 1, 2), (2, 2, 6), (2, 3, 10), (2, 4, 14) // N2 > (3, 1, 3), (3, 2, 7), (3, 3, 11), (3, 4, 15) // N3 > (4, 1, 4), (4, 2, 8), (4, 3, 12), (4, 4, 16) // N4 > > The chart below compares one round of full repair with one round of MV > repair. As shown, both scan the same total number of rows. However, MV > repair has higher time complexity because its Merkle tree processes each > row more intensively. To avoid all nodes scanning the entire table > simultaneously, MV repair should use a snapshot-based approach, similar to > normal repair with the --sequential option. Time complexity increase > compare to full repair can be found in the "Complexity and Memory > Management" section. > > n: number of rows > > d: depth of one Merkle tree for MV repair > > d': depth of one Merkle tree for full repair > > r: number of split ranges > > Assuming one leaf node covers same amount of rows, 2^d' = (2^d) * r. > > We can see that the space complexity is the same, while MV repair has > higher time complexity. However, this should not pose a significant issue > in production, as the Merkle tree depth and the number of split ranges are > typically not large. > > 1 Round Merkle Tree Building Complexity > Full Repair > MV Repair > Time complexity O(n) O(n*d*log(r)) > Space complexity O((2^d')*r) O((2^d)*r^2) = O((2^d')*r) > > Tombstone: > > The current proposal focuses on rebuilding the MV for a granular token > range where a mismatch is detected, rather than rebuilding the entire MV > token range. Since the MV is treated as a regular table, standard full or > incremental repair processes should still apply to both the base and MV > tables to keep their replicas in sync. > > Regarding tombstones, if we introduce special tombstone types or handling > mechanisms for the MV table, we may be able to support tombstone > synchronization between the base table and the MV. I plan to spend more > time exploring whether we can introduce changes to the base table that > enable this synchronization. > > > > On Mon, May 12, 2025 at 11:35 AM Jaydeep Chovatia < > chovatia.jayd...@gmail.com> wrote: > >> >Like something doesn't add up here because if it always includes the >> base table's primary key columns that means >> >> The requirement for materialized views (MVs) to include the base table's >> primary key appears to be primarily a syntactic constraint specific to >> Apache Cassandra. For instance, in DynamoDB, the DDL for defining a Global >> Secondary Index does not mandate inclusion of the base table's primary key. >> This suggests that the syntax requirement in Cassandra could potentially be >> relaxed in the future (outside the scope of this CEP). As Benedict noted, >> the base table's primary key is optional when querying a materialized view. >> >> Jaydeep >> >> On Mon, May 12, 2025 at 10:45 AM Jon Haddad <j...@rustyrazorblade.com> >> wrote: >> >>> >>> > Or compaction hasn’t made a mistake, or cell merge reconciliation >>> hasn’t made a mistake, or volume bitrot hasn’t caused you to lose a file. >>> > Repair isnt’ just about “have all transaction commits landed”. It’s >>> “is the data correct N days after it’s written”. >>> >>> Don't forget about restoring from a backup. >>> >>> Is there a way we could do some sort of hybrid compaction + incremental >>> repair? Maybe have the MV verify it's view while it's compacting, and when >>> it's done, mark the view's SSTable as repaired? Then the repair process >>> would only need to do a MV to MV repair. >>> >>> Jon >>> >>> >>> On Mon, May 12, 2025 at 9:37 AM Benedict Elliott Smith < >>> bened...@apache.org> wrote: >>> >>>> Like something doesn't add up here because if it always includes the >>>> base table's primary key columns that means they could be storage attached >>>> by just forbidding additional columns and there doesn't seem to be much >>>> utility in including additional columns in the primary key? >>>> >>>> >>>> You can re-order the keys, and they only need to be a part of the >>>> primary key not the partition key. I think you can specify an arbitrary >>>> order to the keys also, so you can change the effective sort order. So, the >>>> basic idea is you stipulate something like PRIMARY KEY ((v1),(ck1,pk1)). >>>> >>>> This is basically a global index, with the restriction on single >>>> columns as keys only because we cannot cheaply read-before-write for >>>> eventually consistent operations. This restriction can easily be relaxed >>>> for Paxos and Accord based implementations, which can also safely include >>>> additional keys. >>>> >>>> That said, I am not at all sure why they are called materialised views >>>> if we don’t support including any other data besides the lookup column and >>>> the primary key. We should really rename them once they work, both to make >>>> some sense and to break with the historical baggage. >>>> >>>> I think this can be represented as a tombstone which can always be >>>> fetched from the base table on read or maybe some other arrangement? I >>>> agree it can't feasibly be represented as an enumeration of the deletions >>>> at least not synchronously and doing it async has its own problems. >>>> >>>> >>>> If the base table must be read on read of an index/view, then I think >>>> this proposal is approximately linearizable for the view as well (though, I >>>> do not at all warrant this statement). You still need to propagate this >>>> eventually so that the views can cleanup. This also makes reads 2RT on >>>> read, which is rather costly. >>>> >>>> On 12 May 2025, at 16:10, Ariel Weisberg <ar...@weisberg.ws> wrote: >>>> >>>> Hi, >>>> >>>> I think it's worth taking a step back and looking at the current MV >>>> restrictions which are pretty onerous. >>>> >>>> A view must have a primary key and that primary key must conform to the >>>> following restrictions: >>>> >>>> - it must contain all the primary key columns of the base table. >>>> This ensures that every row of the view correspond to exactly one row of >>>> the base table. >>>> - it can only contain a single column that is not a primary key >>>> column in the base table. >>>> >>>> At that point what exactly is the value in including anything except >>>> the original primary key in the MV's primary key columns unless you are >>>> using an ordered partitioner so you can iterate based on the leading >>>> primary key columns? >>>> >>>> Like something doesn't add up here because if it always includes the >>>> base table's primary key columns that means they could be storage attached >>>> by just forbidding additional columns and there doesn't seem to be much >>>> utility in including additional columns in the primary key? >>>> >>>> I'm not that clear on how much better it is to look something up in the >>>> MV vs just looking at the base table or some non-materialized view of it. >>>> How exactly are these MVs supposed to be used and what value do they >>>> provide? >>>> >>>> Jeff Jirsa wrote: >>>> >>>> There’s 2 things in this proposal that give me a lot of pause. >>>> >>>> >>>> Runtian Liu pointed out that the CEP is sort of divided into two parts. >>>> The first is the online part which is making reads/writes to MVs safer and >>>> more reliable using a transaction system. The second is offline which is >>>> repair. >>>> >>>> The story for the online portion I think is quite strong and worth >>>> considering on its own merits. >>>> >>>> The offline portion (repair) sounds a little less feasible to run in >>>> production, but I also think that MVs without any mechanism for checking >>>> their consistency are not viable to run in production. So it's kind of pay >>>> for what you use in terms of the feature? >>>> >>>> It's definitely worth thinking through if there is a way to fix one >>>> side of this equation so it works better. >>>> >>>> David Capwell wrote: >>>> >>>> As far as I can tell, being based off Accord means you don’t need to >>>> care about repair, as Accord will manage the consistency for you; you can’t >>>> get out of sync. >>>> >>>> I think a baseline requirement in C* for something to be in production >>>> is to be able to run preview repair and validate that the transaction >>>> system or any other part of Cassandra hasn't made a mistake. Divergence can >>>> have many sources including Accord. >>>> >>>> Runtian Liu wrote: >>>> >>>> For the example David mentioned, LWT cannot support. Since LWTs operate >>>> on a single token, we’ll need to restrict base-table updates to one >>>> partition—and ideally one row—at a time. A current MV base-table command >>>> can delete an entire partition, but doing so might touch hundreds of MV >>>> partitions, making consistency guarantees impossible. >>>> >>>> I think this can be represented as a tombstone which can always be >>>> fetched from the base table on read or maybe some other arrangement? I >>>> agree it can't feasibly be represented as an enumeration of the deletions >>>> at least not synchronously and doing it async has its own problems. >>>> >>>> Ariel >>>> >>>> On Fri, May 9, 2025, at 4:03 PM, Jeff Jirsa wrote: >>>> >>>> >>>> >>>> On May 9, 2025, at 12:59 PM, Ariel Weisberg <ar...@weisberg.ws> wrote: >>>> >>>> >>>> I am *big* fan of getting repair really working with MVs. It does seem >>>> problematic that the number of merkle trees will be equal to the number of >>>> ranges in the cluster and repair of MVs would become an all node >>>> operation. How would down nodes be handled and how many nodes would >>>> simultaneously working to validate a given base table range at once? How >>>> many base table ranges could simultaneously be repairing MVs? >>>> >>>> If a row containing a column that creates an MV partition is deleted, >>>> and the MV isn't updated, then how does the merkle tree approach propagate >>>> the deletion to the MV? The CEP says that anti-compaction would remove >>>> extra rows, but I am not clear on how that works. When is anti-compaction >>>> performed in the repair process and what is/isn't included in the outputs? >>>> >>>> >>>> >>>> I thought about these two points last night after I sent my email. >>>> >>>> There’s 2 things in this proposal that give me a lot of pause. >>>> >>>> One is the lack of tombstones / deletions in the merle trees, which >>>> makes properly dealing with writes/deletes/inconsistency very hard (afaict) >>>> >>>> The second is the reality that repairing a single partition in the base >>>> table may repair all hosts/ranges in the MV table, and vice versa. >>>> Basically scanning either base or MV is effectively scanning the whole >>>> cluster (modulo what you can avoid in the clean/dirty repaired sets). This >>>> makes me really, really concerned with how it scales, and how likely it is >>>> to be able to schedule automatically without blowing up. >>>> >>>> The paxos vs accord comments so far are interesting in that I think >>>> both could be made to work, but I am very concerned about how the merkle >>>> tree comparisons are likely to work with wide partitions leading to massive >>>> fanout in ranges. >>>> >>>> >>>> >>>> >>>>
