Read replica support for Postgres and MySQL datastores #1878

josephschorr · 2024-04-25T20:16:15Z

No description provided.

benny-yamagata · 2024-05-02T02:58:07Z

Just to clarify, this update will allow spice to use multiple reader nodes for postgres and mysql? So if I have 3 readers I would be able to pass in the entries for all of them to be used?

josephschorr · 2024-05-02T03:50:23Z

@benny-yamagata Yes, the replica URI parameter is a list of URIs and the system will round robin between them

vroldanbet

The implementation works with the assumption individual hosts will be listed, but folks most of the time would use replicas behind a load-balancer in order to be able to scale read traffic when needed. I don't think the implementation will work for that more common use case because the datastore snapshot reader does not use a single transaction.

internal/datastore/mysql/datastore.go

vroldanbet · 2024-05-02T10:54:13Z

internal/datastore/mysql/datastore.go

+	index uint32,
+	options ...Option,
+) (datastore.ReadOnlyDatastore, error) {
+	ds, err := newMySQLDatastore(ctx, url, int(index), options...)


I searched online if there is any URI query parameter we can add to enforce read only, but didn't find any. Users should make sure the credentials provided for the replica have read-only permissions.

Not really necessary; the datastore itself ensures it is read only

does it install the read only datastore proxy on the replica?

internal/datastore/mysql/datastore_test.go

internal/datastore/postgres/postgres.go

internal/datastore/proxy/replicated.go

vroldanbet · 2024-05-02T11:53:13Z

internal/datastore/proxy/replicated.go

+	return rr.chosenReader.LookupNamespacesWithNames(ctx, nsNames)
+}
+
+func (rr *replicatedReader) chooseSource(ctx context.Context) error {


I think returning the chosen reader in chooseSource would better encapsulate this logic.

internal/datastore/proxy/replicated.go

josephschorr · 2024-05-02T15:05:00Z

I don't think the implementation will work for that more common use case because the datastore snapshot reader does not use a single transaction.

It uses a single connection, which means it should stay connected to the same replica

internal/datastore/postgres/postgres.go

ecordell · 2024-05-02T17:00:03Z

It uses a single connection, which means it should stay connected to the same replica

I possibly just missed it, but it looked like you were using a pgxpool.Conn for the read replicas which cycles actual connections out from under itself.

josephschorr · 2024-05-02T17:00:48Z

It uses a single connection, which means it should stay connected to the same replica

I possibly just missed it, but it looked like you were using a pgxpool.Conn for the read replicas which cycles actual connections out from under itself.

Yeah, I traced it and it does use the pool. We'll have to do something else

vroldanbet

👍🏻 on the overall strategy, this should work. My main concerns are:

exposing strict via the CLI, I don't see a reason we should do that
cached checked revision proxy may race
missing tests on strictReaderQueryFuncs, clarification how in progress surfaces in a replica
clarification on Watch API behaviour, do we have a test? I understand it requires no extra work, it would just lag behind the primary
inconsistent refactor of common datastore flags

pkg/cmd/datastore/datastore.go

vroldanbet · 2024-06-26T06:40:41Z

pkg/cmd/datastore/datastore.go

 	var legacyConnPool ConnPoolConfig
 	RegisterConnPoolFlagsWithPrefix(flagSet, "datastore-conn", DefaultReadConnPool(), &legacyConnPool)
 	deprecateUnifiedConnFlags(flagSet)
 	RegisterConnPoolFlagsWithPrefix(flagSet, "datastore-conn-pool-read", &legacyConnPool, &opts.ReadConnPool)
 	RegisterConnPoolFlagsWithPrefix(flagSet, "datastore-conn-pool-write", DefaultWriteConnPool(), &opts.WriteConnPool)
+	RegisterConnPoolFlagsWithPrefix(flagSet, "datastore-read-replica-conn-pool", DefaultReadConnPool(), &opts.ReadReplicaConnPool)


Naming pattern differs from existing conn pools. Should be datastore-conn-pool-read-replica.

I wanted all the read replica flags to start with datastore-read-replica to be explicit

I can tell you that as someone usually tinkering with these flags, having to remember 2 different naming patterns is not going to make my life easier

Its a tradeoff, but I still think having all the read replica flags sharing a common prefix makes more sense. Perhaps we should ask?

what is the tradeoff?

Its harder discover using autocomplete: right now I can do --datastore-read-replica- and tab to see all the read replica flags

@jzelinskie can you share your thoughts on the flag UX here?

A few questions/thoughts that could help us understand the usage here:

When would you ever want to configure read replica pool to be not the same as the read pool?

Do we think we could use a heuristic inside SpiceDB to come up with these values rather than even exposing them at all?

When would you ever want to configure read replica pool to be not the same as the read pool?

Always: you can't use the read pool with replicas

Do we think we could use a heuristic inside SpiceDB to come up with these values rather than even exposing them at all?

Incredibly unlikely... We need to know that a replica is a replica

Always: you can't use the read pool with replicas

Configure them differently, not use the same pool.
For example, if I'm using a RDS with a read replica, the read replica is likely the same size as my primary likely with the same max connections.

Incredibly unlikely... We need to know that a replica is a replica

I'm not advocating altering the system to be unaware of read replicas. I'm advocating for the system being smart enough to discover the right size of connection pools because users are very likely to pick non-ideal values themselves

pkg/cmd/datastore/datastore.go

vroldanbet · 2024-06-26T06:49:42Z

pkg/cmd/datastore/datastore.go

+		mysql.RevisionQuantization(opts.RevisionQuantization),
+		mysql.MaxRevisionStalenessPercent(opts.MaxRevisionStalenessPercent),
+		mysql.TablePrefix(opts.TablePrefix),
+		mysql.WatchBufferLength(opts.WatchBufferLength),
+		mysql.WatchBufferWriteTimeout(opts.WatchBufferWriteTimeout),
+		mysql.WithEnablePrometheusStats(opts.EnableDatastoreMetrics),
+		mysql.MaxRetries(uint8(opts.MaxRetries)),
+		mysql.OverrideLockWaitTimeout(1),
+		mysql.CredentialsProviderName(opts.ReadReplicaCredentialsProviderName),


I don't see it done for MySQL

jschorr edit since I can't reply: this is done

internal/datastore/proxy/replicated.go

internal/datastore/postgres/strictreader.go

josephschorr · 2024-06-26T16:39:43Z

Updated

pkg/cmd/datastore/datastore.go

This is accomplished by adding a datastore proxy that selects a read replica for each SnapshotReader, in a round robin fashion, and ensures that the revision being requested is available on the replica. This extra check does add some latency to the overall operation, but it should provide for the safest approach for using read replicas Fixes authzed#1321 Fixes authzed#1320

…d of using read replicas

This will allow replicas behind load balancers to be supported (just in Postgres for now)

github-actions bot added area/CLI Affects the command line area/datastore Affects the storage system area/tooling Affects the dev or user toolchain (e.g. tests, ci, build tools) labels Apr 25, 2024

josephschorr force-pushed the read-replica-support branch from 1d5612b to aa51f2a Compare April 25, 2024 20:25

josephschorr marked this pull request as ready for review April 26, 2024 18:22

josephschorr requested review from vroldanbet and a team as code owners April 26, 2024 18:22

vroldanbet requested changes May 2, 2024

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josephschorr marked this pull request as draft May 2, 2024 17:00

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josephschorr reopened this Jun 21, 2024

josephschorr force-pushed the read-replica-support branch from aa51f2a to de54d54 Compare June 23, 2024 00:38

josephschorr marked this pull request as ready for review June 24, 2024 19:08

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josephschorr force-pushed the read-replica-support branch from de54d54 to 5a5dd26 Compare June 26, 2024 16:39

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jzelinskie reviewed Jun 28, 2024

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pkg/cmd/datastore/datastore.go Outdated Show resolved Hide resolved

josephschorr added 3 commits June 28, 2024 17:01

Add caching on read replica CheckRevision calls to reduce the overhea…

a088b39

…d of using read replicas

Add an additional mode to replica support that uses a strict read mode

da79df9

This will allow replicas behind load balancers to be supported (just in Postgres for now)

josephschorr force-pushed the read-replica-support branch from 3d4516e to da79df9 Compare June 28, 2024 21:26

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Read replica support for Postgres and MySQL datastores #1878

Read replica support for Postgres and MySQL datastores #1878

josephschorr commented Apr 25, 2024

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josephschorr commented Jun 26, 2024

Read replica support for Postgres and MySQL datastores #1878

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Read replica support for Postgres and MySQL datastores #1878

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