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Allow percentage rollout of load balancer of decomposed database

Kamil Trzciński (Back 2025-01-01) requested to merge feature-flag-rollout into master

What does this MR do and why?

This introduces a GITLAB_USE_MODEL_LOAD_BALANCING and use_model_load_balancing to enable percentage rollout of used connection when accessing data.

The usage pattern is to redirect 1% of new connections using CI DB to use a Load Balancer replicas (and presumably later primary connection).

This is achieved by injecting a FF evaluation logic as part of LB setup to re-define how connection is fetched.

This is not the nicest implementation, but this appears to be the only sane inject way of this functionality.

Related to:

Behavior

It introduces a ENV (which takes precedence) and FF (which is easy to model 1% rollout). The FF is sticky to the current request. If request store is not active this fallbacks to being disabled unless explicitly defined with ENV variable.

GITLAB USE_MODEL LOAD_BALANCING GITLAB LOAD_BALANCING REUSE_PRIMARY ci Request Store active? use_model load_balancing FF ActiveRecord Base primary ActiveRecord Base replica Ci ApplicationRecord connection Ci ApplicationRecord replica
TRUE undefined ignored ignored main primary main replica ci primary ci replica
TRUE main ignored ignored main primary main replica main primary ci replica
FALSE undefined ignored ignored main primary main replica main primary main replica
FALSE main ignored ignored main primary main replica main primary main replica
undefined undefined FALSE FALSE main primary main replica main primary main replica
undefined undefined FALSE TRUE main primary main replica main primary main replica
undefined undefined TRUE FALSE main primary main replica main primary main replica
undefined undefined TRUE TRUE main primary main replica ci primary ci replica
undefined main FALSE FALSE main primary main replica main primary main replica
undefined main FALSE TRUE main primary main replica main primary main replica
undefined main TRUE FALSE main primary main replica main primary main replica
undefined main TRUE TRUE main primary main replica main primary ci replica

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 Kamil Trzciński (Back 2025-01-01)

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