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Add Gitlab::Redis::BufferedCounter to migrate buffered counters

Sylvester Chin requested to merge sc1-buffered-counter-redis-migration-helper into master

What does this MR do and why?

This MR creates a new Redis class for BufferedCounter which uses redis.shared_state.yml or SharedState's config file.

This stops the use of MultiStore for buffered counter. Since buffered counter is write-heavy and prone to data race, MultiStore is not an ideal mode of migration.

See gitlab-com/gl-infra/production#17188 (comment 1687915727)

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Before After

How to set up and validate locally

  1. Open a rails console and verify that the Redis::BufferedCounter uses SharedState's instrumentation class.
pry(main)> Gitlab::Redis::BufferedCounter.params
=> {:instrumentation_class=>Gitlab::Instrumentation::Redis::SharedState, :path=>"/Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket", :db=>0}
  1. It is a single Redis connection (non-multistore)
[1] pry(main)> Gitlab::Redis::BufferedCounter.with{|c|c}
=> #<Redis client v4.8.0 for unix:///Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket/0>
[2] pry(main)> Gitlab::Redis::SharedState.with{|c|c}
  Feature::FlipperGate Pluck (0.4ms)  SELECT "feature_gates"."key", "feature_gates"."value" FROM "feature_gates" WHERE "feature_gates"."feature_key" = 'use_primary_store_as_default_for_shared_state' /*application:console,db_config_name:main,console_hostname:SylvestersMBP2.localdomain,console_username:sylvesterchin,line:/lib/feature.rb:314:in `block in current_feature_value'*/
=> #<Gitlab::Redis::MultiStore:0x0000000137c9c7b8
 @instance_name="SharedState",
 @primary_store=#<Redis client v4.8.0 for unix:///Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket/0>,
 @same_redis_store=true,
 @secondary_store=#<Redis client v4.8.0 for unix:///Users/sylvesterchin/work/gitlab-development-kit/redis/redis.socket/0>>

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Edited by Sylvester Chin

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