In the case of bitrot / losing an SSTable, wouldn't a normal repair (just the MV against the other nodes) resolve the issue?
On Wed, May 14, 2025 at 11:27 AM Blake Eggleston <bl...@ultrablake.com> wrote: > Mutation tracking is definitely an approach you could take for MVs. > Mutation reconciliation could be extended to ensure all changes have been > replicated to the views. When a base table received a mutation w/ an id it > would generate a view update. If you block marking a given mutation id as > reconciled until it’s been fully replicated to the base table and its view > updates have been fully replicated to the views, then all view updates will > eventually be applied as part of the log reconciliation process. > > A mutation tracking implementation would also allow you to be more > flexible with the types of consistency levels you can work with, allowing > users to do things like use LOCAL_QUORUM without leaving themselves open to > introducing view inconsistencies. > > That would more or less eliminate the need for any MV repair in normal > usage, but wouldn't address how to repair issues caused by bugs or data > loss, though you may be able to do something with comparing the latest > mutation ids for the base tables and its view ranges. > > On Wed, May 14, 2025, at 10:19 AM, Paulo Motta wrote: > > I don't see mutation tracking [1] mentioned in this thread or in the > CEP-48 description. Not sure this would fit into the scope of this > initial CEP, but I have a feeling that mutation tracking could be > potentially helpful to reconcile base tables and views ? > > For example, when both base and view updates are acknowledged then this > could be somehow persisted in the view sstables mutation tracking > summary[2] or similar metadata ? Then these updates would be skipped during > view repair, considerably reducing the amount of work needed, since only > un-acknowledged views updates would need to be reconciled. > > [1] - > https://cwiki.apache.org/confluence/display/CASSANDRA/CEP-45%3A+Mutation+Tracking| > <https://cwiki.apache.org/confluence/display/CASSANDRA/CEP-45%3A+Mutation+Tracking%7C> > [2] - https://issues.apache.org/jira/browse/CASSANDRA-20336 > > On Wed, May 14, 2025 at 12:59 PM Paulo Motta <pauloricard...@gmail.com> > wrote: > > > - 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? > > I haven't checked the CEP yet so I may be missing out something but I > think this effort doesn't need to be conflated with dense node support, to > make this more approachable. I think prospective users would be OK with > overprovisioning to make this feasible if needed. We could perhaps have > size guardrails that limit the maximum table size per node when MVs are > enabled. Ideally we should make it work for dense nodes if possible, but > this shouldn't be a reason not to support the feature if it can be made to > work reasonably with more resources. > > I think the main issue with the current MV is about correctness, and the > ultimate goal of the CEP must be to provide correctness guarantees, even if > it has an inevitable performance hit. I think that the performance of the > repair process is definitely an important consideration and it would be > helpful to have some benchmarks to have an idea of how long this repair > process would take for lightweight and denser tables. > > On Wed, May 14, 2025 at 7:28 AM Jon Haddad <j...@rustyrazorblade.com> > wrote: > > 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. > > > > >
