Bulk events performance experiments

[elexisvenator]

This is the result of a series of tests I have been doing on the performance of bulk operations with events.

The tests:

THe branch I am running these experiments on is master...elexisvenator:marten:bulk-event-experiments

I am testing the following scenarios:

1. Insert a single stream with 1000 events
2. Update a single stream with 1000 events
3. Insert 1000 streams with 1 event each
4. Update 1000 streams with 1 event each

For all of these tests, there are 2 inline projections - 1 that uses the event inserted, and 1 that doesn't. Additionally multi tenancy is on, the stream identity is a string, and quick append is enabled. With this configuration scenarios 3 and 4 are an exact match for my real-world use cases relating to bulk import/update.

From a benchmarking perspective, these tests are not at all scientific. I am using them to try to identify low hanging fruit for performance improvements, but as the obvious fixes are dealt with these tests will need to be turned into proper benchmarks. For this reason, no specific durations are mentioned below.

The findings

• As is fairly obvious, 1000 events in 1 stream is way more performant than 1 event in 1000 streams.
• Updates are slightly slower than inserts, but the difference is tiny compared to the previous points performance difference.
• Pretty sure the function mt_quick_append_events function will not compile if single-tenant mode is enabled. It doesn't affect these tests but it's something to consider.
• Inline projections will fetch the current aggregate to apply events to. One improvement is to skip this for ISingleStreamSlicer when it is known that the stream is a new stream. This is done with d2f8442 and is a significant performance boost to scenario 3, bringing its performance almost on par with scenario 1.
• When using quick append, revision numbers on inline projections are (very) broken. This is because:
  • Revision numbers are based on the version of the last handled event. When updating a stream with quick append, this number is always 1 (unless using optimistic appends or an expectedVersion is provided). This causes the inline projection's revision to be set to 1
  • If an inline projection's revision is less than what already exists in the database, then an insert silently fails 😱
  • My first thought to fix this (which I noted in in the commit linked above) is to infer the new version from the inline projection's current version. This should be possible as the information is already fetch, just not readily available in the current code. I have since realized that this will not work due to a) it requires the projection version to remain consistent with the stream, with no tolerance for drift, and b) the projection version already drifts by design whenever the new events in a stream are not ones processed by the projection (because the projection processing is skipped).
  • Really I think the only way this can be solved is by fetching the current streams version prior to applying events. I know this undermines a lot of the performance of quick append but i don't see a better way to do this ☹️

Potential improvements:

• A large bottleneck for scenario 4 is the need to get existing projection data. This is done as a single round trip per stream. It would be very easy to batch these together into a LoadMany call.
• The mt_quick_append_events is using loops to perform its updates. I am not 100% sure on the impact for postgres specifically but this definitely fires my mssql "cursors are bad" alarms. It should be possible to redesign this query into a single statement. (see next point)
• Here is a gist that takes the quick append function and extends it to work with both new and updated streams, while also processing many streams at once. This should be much more performant than many individual insert statements even on the same command batch (due to reduced queryplanner overhead, less index updates, and better optimisations available when running in bulk)
• Inserting projections can also be done in bulk
• At some point bulkcopy becomes better, I do not know when that is...
• The line streams.Where(x => Projection.AppliesTo(x.Events.Select(x => x.EventType))) seems to me to be very excessive in a scenario where there are hundreds to thousands of streams/events. Maybe the work tracker could precalculate this by keeping track of a list of event types as they are appended, that way a precheck could be done to see if a projection applies to any stream in the set at all. Useful especially when there are a lot of projections that are not relevant to what is being bulk loaded.

That's my thoughts from a day satisfying my morbid curiosity around performance

[jeremydmiller]

mt_quick_append_events has been tested with both conjoined and single tenant modes. I get that it doesn't vary on the tenant id parameter presence. It's effectively a copy/paste of the old <= V3 append behavior with >= V4 metadata added in

[jeremydmiller]

"Really I think the only way this can be solved is by fetching the current streams version prior to applying events. I know this undermines a lot of the performance of quick append but i don't see a better way to do this ☹️"

Like you said, that 100% defeats the purpose of the quick append. Are you saying this based on observed behavior, or by guessing looking at the code? I think the way that the revisions would be updated on a Store() (which is what a projection does), you're still good to go. If you're using an Inline projection, the projection has to fetch the current version of the doc anyway. If that doc is IRevisioned, you're still good to go.

[jeremydmiller]

I do like the idea of trying to "know" when the existing aggregate would have to be fetched anyway, then use that to get at the current revision and working normally with that.

"The mt_quick_append_events is using loops to perform its updates. I am not 100% sure on the impact for postgres specifically but this definitely fires my mssql "cursors are bad" alarms. It should be possible to redesign this query into a single statement. (see next point)"

Sorry that fires off your Spidey Sense about a completely different scenario in a completely different database engine, but I kept that because the alternative was going to require fetching the existing stream version in a round trip otherwise -- which 100% defeats the purpose of the whole quick append thing altogether. Making the DB work a little harder (dispute that w/o hard numbers) is much less than the back and forth chattiness.

[jeremydmiller]

Look at how the upsert functions are built with revisioning turned on. If you pass in a 0 version/revision, that function will auto-increment the mt_version column for you. On fetching an IRevisioned document, you'd get the latest value from the database overwritten by Marten upon fetching,. That was my theory for how to make the quick append work end to end even w/o knowing the version upfront

[elexisvenator]

"Really I think the only way this can be solved is by fetching the current streams version prior to applying events. I know this undermines a lot of the performance of quick append but i don't see a better way to do this ☹️"

Like you said, that 100% defeats the purpose of the quick append. Are you saying this based on observed behavior, or by guessing looking at the code? I think the way that the revisions would be updated on a Store() (which is what a projection does), you're still good to go. If you're using an Inline projection, the projection has to fetch the current version of the doc anyway. If that doc is IRevisioned, you're still good to go.

Saying this based on observed behaviour.
This test specifically will fail if the expected version is not passed in on line 81. By debugging what is sent to the db I could see that the updated aggregate is sent to the db but with a lower revision than what is in the db, so is discarded in the stored procedure.

Having a requirement of IRevisioned or [Version] on an inline projection is pretty reasonable I agree, unfortunately you cannot rely on the existing projections revision being equal to the stream's version (due to projection updates being skipped when the projection doesnt handle any events in the current changeset)

[elexisvenator]

Sorry that fires off your Spidey Sense about a completely different scenario in a completely different database engine, but I kept that because the alternative was going to require fetching the existing stream version in a round trip otherwise -- which 100% defeats the purpose of the whole quick append thing altogether. Making the DB work a little harder (dispute that w/o hard numbers) is much less than the back and forth chattiness.

Yeah, I get that my experience is only tangentially applicable here. And 100% agree that more round trips is not the solution. The alternative instead would be to replace the loop with a single statement, similar to the one in the linked gist. Would need a lot of careful measurement to determine if its better or not though.

[elexisvenator]

Another consideration would be to enforce Revisioning and SingleStream projections only when quick append is enabled. This won't make things faster, but it would make optimisations simpler as they wont have to account for other configurations.