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Reconfigure DB load balancing connection on code reload

Thong Kuah requested to merge fix_reloading_database_load_balance_connection into master

What does this MR do and why?

In development, when code is reloaded, we lose the connection class attribute. So we need to setup a to_prepare block to reconfigure DB load balancing.

Also add logging for when DB load balancing setup runs.

Note: whether code reloads depends on config.cache_classes, and config.reload_classes_only_on_change. In production, config.cache_classes is true, so no code reloading happens there.

Changelog: fixed

Related issue: #356349 (closed)

To replicate:

  1. Setup multiple databases
  2. Run the following in the Rails console:
[1] pry(main)> Ci::Pipeline.count
   (2.0ms)  SELECT COUNT(*) FROM "ci_pipelines" /*application:console,db_config_name:ci,line:(pry):1:in `__pry__'*/
=> 263
[2] pry(main)> reload!
Reloading...
[3] pry(main)> Ci::Pipeline.count
   (1.0ms)  SELECT COUNT(*) FROM "ci_pipelines" /*application:console,db_config_name:ci,line:(pry):3:in `__pry__'*/
=> 263

Before this fix, the db_config_name will change to main.

An alternative replication method is #356353 (comment 902255686)

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Edited by Thong Kuah

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