SPIKE: Investigate how ElasticPress could gracefully fallback when ES times out or errors

[owaincuvelier]

Currently, if your ES cluster goes down, this can cause the entire request to fail or timeout.
Ideally, the request to ES would time out in isolation and fallback to MySQL.

This needs to consider both standard front end queries, admin queries, updates, etc.
Edges cases should be covered and documented.

Check with 10Up, on their plans/approach for resiliency.

Potentially produce a simple Proof of Concept.

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Acceptance Criteria

• A documented response from 10Up
• An outline of any potential approach
• A plan for implementation

TimeBox: 2 days

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For Altis Team Use

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• Has the acceptance criteria been met?
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  • Or are manual tests documented (at least on this ticket)?
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• Have the appropriate backport labels been added to the PR?

[mikelittle]

Hey team! Please add your planning poker estimate with Zenhub @jerico @kovshenin