MySQL in Docker

First Launch and Storage

The official image provisions accounts and databases through environment variables on first boot:

docker run -d --name mysql \
  -e MYSQL_ROOT_PASSWORD=rootpass \
  -e MYSQL_DATABASE=app \
  -e MYSQL_USER=app \
  -e MYSQL_PASSWORD=apppass \
  -v mysqlData:/var/lib/mysql \
  mysql:8

State lives in /var/lib/mysql, so the volume is as mandatory as with Postgres. Health checks use mysqladmin ping -h 127.0.0.1 (it waits for the temporary server used during init to finish). Note that MySQL 8 defaults to caching_sha2_password; older clients that only speak mysql_native_password fail to authenticate until you configure the user's plugin.

Primary-Replica Replication

Binlog-based replication needs three pieces: enabled binary logging (default in 8.0), a replication account on the primary, and the replica pointed at it:

-- primary
CREATE USER 'repl'@'%' IDENTIFIED BY 'replpass';
GRANT REPLICATION SLAVE ON *.* TO 'repl'@'%';

-- replica
CHANGE REPLICATION SOURCE TO
  SOURCE_HOST='primary', SOURCE_USER='repl',
  SOURCE_PASSWORD='replpass', SOURCE_AUTO_POSITION=1;
START REPLICA;

SOURCE_AUTO_POSITION=1 lets GTID coordinates resume automatically after restarts. Verify with SHOW REPLICA STATUS\G — both Replica_IO_Running and Replica_SQL_Running must be Yes. Each container needs its own server-id.

Backups and Pitfalls

For InnoDB, a transactionally consistent logical backup:

docker exec mysql mysqldump -u root -p --single-transaction \
  --routines --triggers app > app.sql

--single-transaction takes a consistent snapshot without locking writes; skip it for MyISAM tables, which need --lock-all-tables. Restore with docker exec -i mysql mysql -u root -p app < app.sql.

Pitfalls. Init SQL in /docker-entrypoint-initdb.d runs only on an empty datadir. Test restores — an untested dump is a hope, not a backup. Prefer replica-based backups to keep load off the primary.

Interview: describe the replication handshake, name the two status fields you monitor, and explain why --single-transaction is safe for InnoDB.