Container health checks in Docker - Step-by-Step Execution

Process Flow - Container health checks

Start Container

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Run Health Check Command

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Check Command Exit Code

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Mark Healthy

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Repeat Health Check Periodically

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Container Status Updated

The container runs a health check command repeatedly. If the command succeeds, the container is healthy; if it fails, it is unhealthy.

Execution Sample

Docker

HEALTHCHECK --interval=5s --timeout=2s \
  CMD curl -f http://localhost/ || exit 1

This Dockerfile instruction sets a health check that tries to curl the container's localhost every 5 seconds and fails if curl returns an error.

Process Table

Check Number	Health Check Command	Exit Code	Health Status	Action
1	curl -f http://localhost/	0	Healthy	Container marked healthy
2	curl -f http://localhost/	0	Healthy	Container remains healthy
3	curl -f http://localhost/	1	Unhealthy	Container marked unhealthy
4	curl -f http://localhost/	1	Unhealthy	Container remains unhealthy
5	curl -f http://localhost/	0	Healthy	Container marked healthy

💡 Health checks continue periodically; this trace shows status changes over 5 checks.

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5
Exit Code	N/A	0	0	1	1	0
Health Status	Starting	Healthy	Healthy	Unhealthy	Unhealthy	Healthy

Key Moments - 2 Insights

Why does the container become unhealthy at check 3 even though it was healthy before?

Because the health check command returned exit code 1 at check 3, indicating failure, so the container status changed to unhealthy as shown in execution_table row 3.

Does the container stay unhealthy forever after one failure?

No, the container status updates on each health check. At check 5, the command succeeded again, so the container became healthy again (execution_table row 5).

Visual Quiz - 3 Questions

Look at the execution table, what is the health status after the second health check?

A Healthy

B Unhealthy

C Starting

D Unknown

💡 Hint

Check the 'Health Status' column in row 2 of the execution_table.

At which check number does the container first become unhealthy?

A 1

B 3

C 2

D 4

💡 Hint

Look for the first 'Unhealthy' status in the execution_table.

If the health check command always returns exit code 0, what will happen to the container status?

A It will alternate between healthy and unhealthy

B It will become unhealthy eventually

C It will remain healthy

D It will stop running health checks

💡 Hint

Refer to the variable_tracker showing health status changes based on exit codes.

Concept Snapshot

Docker container health checks run a command periodically.
If the command succeeds (exit code 0), container is healthy.
If it fails (non-zero exit), container is unhealthy.
Use HEALTHCHECK in Dockerfile to define the command and timing.
Docker updates container status based on these results.

Full Transcript

Container health checks in Docker work by running a command inside the container repeatedly. Each time the command runs, Docker checks its exit code. If the exit code is zero, Docker marks the container as healthy. If the exit code is not zero, Docker marks it as unhealthy. This status updates every time the health check runs. For example, a health check command might be 'curl -f http://localhost/'. If curl succeeds, the container is healthy; if curl fails, the container is unhealthy. The health check runs every few seconds as configured. The container status can change back and forth depending on the command results. This helps Docker and users know if the container is working properly.