11. Logs, Health, and Resources
Inspect an application's process, logs, and resource use.
Start with Evidence
For the container you are investigating, replace CONTAINER_NAME with its name:
docker ps -a
docker logs --tail 100 CONTAINER_NAME
docker inspect CONTAINER_NAME
docker stats --no-stream CONTAINER_NAME
docker top CONTAINER_NAME
Logs show application output written to stdout and stderr. Inspect shows configuration and state. Stats provides a point-in-time view of resource use.
Running vs Healthy
Running means the main process is alive. A health check tests a command that represents application health. An unhealthy status alone does not cause standalone Docker to restart the container automatically.
A restart policy responds to container exits according to its policy. It is not a replacement for fixing configuration errors or implementing readiness.
Resource Limits
docker run --rm --memory 64m --cpus 0.5 busybox:1.37 echo "resource-limited example"
The command demonstrates limit syntax; its small workload does not stress the limits. Size real limits using observed application behavior.
Practice
Compare the evidence needed for a stopped container, a running but unreachable server, and a memory-constrained application.
Quick Interview Answer
Diagnose state, logs, configuration, and resources together. Running, healthy, and reachable describe different properties.
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