ActiveRecord logging: include current DB role and shard when using Multiple Databases

rubyonrails-core
1

Summary

When using Rails Multiple Databases, the default ActiveRecord SQL log output does not indicate which database role(writing or reading) and shard(shard_one and shard_two) is currently in use . This makes it difficult to verify routing behavior and diagnose issues in environments with writer/replica setup.

Current behavior

Given:

class ApplicationRecord < ActiveRecord::Base
  connects_to database: { writing: :primary, reading: :primary_replica }
end

and:

ActiveRecord::Base.connected_to(role: :writing) { User.count }

the default log looks like:

User Count (0.3ms)  SELECT COUNT(*) FROM `users`

No role/connection information is shown.

Problem

In multi-DB applications, developers need to quickly confirm whether a query ran against writer or reader. Without role visibility in logs, it is hard to:

  • validate read/write splitting
  • debug unexpected replica reads/writes
  • investigate consistency/replication-lag related behavior

Proposal

Include the current connection role and shard in SQL log lines when Multiple Databases is enabled.

Example:

[db:writing] User Count (db time 0.3ms, cpu time 0.0866ms)  SELECT COUNT(*) FROM `users`
[db:reading,shard:shard_two] User Count (db time 0.5ms, cpu time 0.1154ms)  SELECT COUNT(*) FROM `users`

At minimum, adding role labels like [writing] / [reading] would significantly improve observability.

Expected benefits

  • Better debugging and operational visibility for multi-DB apps
  • Easier verification of connected_to and automatic role switching behavior
  • Reduced ambiguity in development and production logs

Compatibility

This could be introduced in a backward-compatible way, for example:

  • enabled automatically only when multiple DB roles are configured, or
  • controlled by a config flag (opt-in/opt-out)