Remove executionLog to reduce memory consumption

[rudolf]

Summary

Saves at least 400MB of heap when doing a migration of 100k saved objects (because of GC it's hard to measure exactly, but while this migration previously required --max_old_space_size=1000 it now passes with --max_old_space_size=600)

When I wrote the initial implementation I was quite worried we'd get some weird failure edge case that we can't reproduce. So I added the executionLog so that if a migration fails we could log a lot of detail (i.e. the complete state and all action responses). However, we haven't really needed these detailed logs and we can still get this information by asking users to run migrations with debug logging enabled. So at the expense of losing some detail in our logs migrations have a much better chance of succeeding when there's a huge amount of saved objects, or many large saved objects.

Checklist

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• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
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Risk Matrix

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Before closing this PR, invite QA, stakeholders, and other developers to identify risks that should be tested prior to the change/feature release.

When forming the risk matrix, consider some of the following examples and how they may potentially impact the change:

Risk	Probability	Severity	Mitigation/Notes
Multiple Spaces—unexpected behavior in non-default Kibana Space.	Low	High	Integration tests will verify that all features are still supported in non-default Kibana Space and when user switches between spaces.
Multiple nodes—Elasticsearch polling might have race conditions when multiple Kibana nodes are polling for the same tasks.	High	Low	Tasks are idempotent, so executing them multiple times will not result in logical error, but will degrade performance. To test for this case we add plenty of unit tests around this logic and document manual testing procedure.
Code should gracefully handle cases when feature X or plugin Y are disabled.	Medium	High	Unit tests will verify that any feature flag or plugin combination still results in our service operational.
See more potential risk examples

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[rudolf]

@elasticmachine merge upstream

[pgayvallet]

Looking at the code, it seems to be the case, but just to be sure:

1. When the migration logger's level is DEBUG, we're still outputting the same information we previously stored in the executionLogs, right?

2. In case of failure, the full cause is still outputted in the logs even when not in DEBUG level?

[rudolf]

1. Yes
2. We still output the full fatal message + traces etc. The difference is, before we would also log the execution log including the full state and responses to each action. Because this was a meta field, it wasn't visible by default with the legacy logger config, you would have to switch to NP JSON logging to actually see this. So in practice for most on-prem and cloud users we won't loose any details.

[rudolf]

I missed that we only logged the state transition meta information from the executionLog. So I'm adding debug logging when there's a state transition so that we can inspect the complete state after every step when debugging is enabled.

[rudolf]

@elasticmachine merge upstream

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 993f9c0
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Metrics [docs]

✅ unchanged

History

• 💚 Build #153127 succeeded 052bf5f
• 💔 Build #152755 failed 31abf46
• 💚 Build #152681 succeeded ce86dcf

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

[pgayvallet]

Can't we just

this.outputStream.end(() => {   
    resolve();
});
this.outputStream = undefined;

?

I think the handler will never be executed synchronously?

[pgayvallet]

This will only be visible when using the JSON layout (in addition to the debug level I mean), but I guess this can't be helped.

[kibanamachine]

💚 Backport successful

Status	Branch	Result
✅	7.15
✅	7.x

The backport PRs will be merged automatically after passing CI.

[kibanamachine]

Looks like this PR has backport PRs but they still haven't been merged. Please merge them ASAP to keep the branches relatively in sync.