Mysql2::Error::ConnectionError in CI

The mysql2 adapter could not connect to the MySQL server. The service may not be ready, or the host/port/credentials are wrong. Connecting early to a starting database service is a common transient failure.

What this error means

Tests fail at connect time with Mysql2::Error::ConnectionError (can't connect, access denied, or unknown host). It can be intermittent while the MySQL service warms up.

Mysql2::Error::ConnectionError:
       Can't connect to MySQL server on 'localhost' (111 "Connection refused")

Common causes

Service still starting

MySQL takes longer to become ready than Postgres; tests connected before it accepted connections.

Host/port mismatch

Config points at the wrong host (127.0.0.1 vs the service alias) or a non-default port.

Credential or auth-plugin mismatch

User/password do not match the service, or the auth plugin (caching_sha2_password) is incompatible with the client.

How to fix it

Wait for MySQL and align config

Gate tests on MySQL readiness and point config at the right host/credentials.

until mysqladmin ping -h 127.0.0.1 --silent; do sleep 1; done
# DATABASE_URL: mysql2://root:root@127.0.0.1:3306/myapp_test

Match the auth plugin

1. Use 127.0.0.1 rather than localhost to force TCP instead of a missing socket.
2. Ensure the client supports the server auth plugin, or start MySQL with mysql_native_password.
3. Set credentials via env to match the service.

How to prevent it

• Add a MySQL readiness gate before the test step.
• Use 127.0.0.1 and an explicit port to avoid socket issues.
• On self-healing managed runners (Latchkey), a transient failure to reach a slow-to-start MySQL service is auto-retried instead of failing the run.

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