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Move feature flag list into process cache

Stan Hu requested to merge sh-feature-flag-persisted-process-cache into master

When we switched from a single-threaded application server (Unicorn) to a multithreaded one (Puma), we did not realize that Puma often reaps threads after a request is done and recreates them later. This makes the thread-local cache ineffective, as the cache does not store anything beyond the lifetime of the thread.

Since ActiveSupport::Cache::MemoryStore is thread-safe, we should be able to switch the L1 cache for the feature flag list to use this to reduce load on Redis.

Since read and write access is synchronized, this does have the side effect of adding contention when feature flags are accessed.

We made a similar change in !26935 (merged), and this seems to be working fine.

Discovered in https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/9414

Edited by Yorick Peterse

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