How should a containerised Node service handle logging, and what happens to those logs by default?

Write logs to stdout and stderr, not to files inside the container.

Why this works:

• Docker captures both streams.
• docker logs reads them.
• The platform ships them to a central log system.

This follows the Twelve-Factor idea: treat logs as a stream of events. Write structured JSON lines, because they are easier to search.

What happens by default? Docker uses the json-file logging driver. It stores logs on the host and does not rotate them (rotate means delete or archive old logs). So a container that logs a lot can fill the disk.

Fixes:

• Set rotation with log options like max-size and max-file. You can set them per container or in the daemon config.
• Or use the local driver, which rotates by default.

Logs written to files inside the container are invisible to docker logs. They are also lost when the container is removed.