Support optional health check queries in MultiActiveRecordCheck

[mdkent]

Summary

Easymon::MultiActiveRecordCheck previously determined database health solely via connection.active?. This adds support for an optional health check query that returns a single truthy/falsey value (e.g. 1/0 from MySQL), useful for richer checks like read-only state or replication lag.

Two ways to supply a query:

Default for all connections via the query: option:

check = Easymon::MultiActiveRecordCheck.new(query: "SELECT @@read_only") {
  { "Primary": ActiveRecord::Base.connection }
}

Per connection, by pairing a connection with its query (takes precedence over the default):

check = Easymon::MultiActiveRecordCheck.new {
  {
    "Primary": [ActiveRecord::Base.connection, "SELECT @@read_only"],
    "PrimaryReplica": [Easymon::PrimaryReplica.connection, "SELECT TIMESTAMPDIFF(MICROSECOND, MAX(ts), NOW(6)) / 1000000 > 1 FROM percona.heartbeat"],
    "OtherReplica": Easymon::OtherReplica.connection # plain connection.active?
  }
}

Query results are cast with ActiveModel::Type::Boolean, so MySQL's 1/0 map to Up/Down and an empty result (nil) is treated as Down. Any raise during the query still rescues to Down, and existing callers without queries are unchanged.

Testing

• Added unit tests covering the default query, per-connection queries, precedence, false results, and raised errors
• bin/rails test test/unit/checks/multi_active_record_check_test.rb — 9 runs, 0 failures

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds support for running an optional per-connection (or default) SQL health-check query in Easymon::MultiActiveRecordCheck, allowing richer DB health semantics than connection.active? alone (e.g., read-only state checks).

Changes:

• Add query: support to MultiActiveRecordCheck, including per-connection query overrides and boolean casting of query results.
• Extend unit tests to cover query-based checks, precedence rules, falsey results, and raised errors.

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Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
lib/easymon/checks/multi_active_record_check.rb	Adds optional query-based health checking (default and per-connection) and updates inline docs.
test/unit/checks/multi_active_record_check_test.rb	Adds unit tests for query-based behavior and precedence.

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