Restart Policies and Inspection

Restart policies

By default a container that stops or crashes stays stopped. Restart policies tell Docker when to restart it automatically.

Policy	Behaviour
no	Never restart (default).
always	Always restart, even on manual docker stop. Starts on Docker daemon boot.
unless-stopped	Restart automatically, but not if you stopped it manually. Starts on daemon boot unless it was stopped.
on-failure[:N]	Restart only when the exit code is non-zero. Optional max retry count.

# Recommended for long-running services
docker run -d --restart unless-stopped nginx:alpine

# Restart up to 5 times on failure
docker run -d --restart on-failure:5 myworker:latest

Inspecting a container

docker inspect returns the full JSON configuration of a container — network settings, mounts, environment variables, restart policy, and more:

docker inspect webserver

To pull out a single field use --format with Go template syntax:

# Get the container's IP address
docker inspect --format '{{range .NetworkSettings.Networks}}{{.IPAddress}}{{end}}' webserver

# Get the restart policy
docker inspect --format '{{.HostConfig.RestartPolicy.Name}}' webserver

Live resource monitoring with docker stats

docker stats shows a live, updating table of CPU, memory, network I/O, and disk I/O for running containers:

# All running containers
docker stats

# Specific container, one snapshot (no live update)
docker stats --no-stream webserver

CONTAINER ID   NAME        CPU %  MEM USAGE / LIMIT   MEM %   NET I/O       BLOCK I/O
a1b2c3d4e5f6   webserver   0.1%   5.2MiB / 7.62GiB    0.07%   1.2kB / 648B  0B / 0B

Resource limits

Containers can consume all available memory and CPU if left unconstrained. Set limits at run time:

docker run -d \
  --name limited \
  --memory 256m \
  --cpus 0.5 \
  nginx:alpine

• --memory 256m — hard memory cap (container is OOM-killed if it exceeds this)
• --cpus 0.5 — allow the container to use at most half a CPU core

Verify the limits are applied:

docker inspect --format '{{.HostConfig.Memory}}' limited
# 268435456  (256 * 1024 * 1024 bytes)

# 1. Run nginx with a restart policy
docker run -d --name webserver --restart unless-stopped nginx:alpine

# 2. Inspect the full config (JSON)
docker inspect webserver | head -60

# 3. Extract just the restart policy
docker inspect --format '{{.HostConfig.RestartPolicy.Name}}' webserver

# 4. Live stats (one snapshot)
docker stats --no-stream webserver

# 5. Run a memory-limited container
docker run -d --name limited --memory 128m --cpus 0.25 nginx:alpine

# 6. Confirm the memory limit
docker inspect --format '{{.HostConfig.Memory}}' limited

# 7. Clean up
docker stop webserver limited && docker rm webserver limited

Restart policies, inspect, and resource limits ↗

🐳 Use --restart unless-stopped for long-running services

unless-stopped is the recommended restart policy for services like web servers, databases, and background workers. The container survives host reboots and unexpected crashes automatically, but respects a deliberate docker stop — so you can take it offline for maintenance without it fighting you.

Which restart policy starts the container automatically after a host reboot but respects a manual 'docker stop'?

always on-failure unless-stopped no

What does 'docker stats --no-stream' do?

Disables network stats collection Prints a single snapshot of resource usage and exits Shows stats only for stopped containers Outputs stats to a file instead of the terminal

What happens when a container exceeds its --memory limit?

Docker pauses the container until memory is available Docker logs a warning but takes no action The container's process is OOM-killed by the kernel Docker automatically increases the limit

Which command shows the full JSON configuration of a running container?

docker info docker top docker inspect docker stats

Submit

Mark as complete