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A Docker container can keep running even when its application has stopped responding. Add a Dockerfile HEALTHCHECK that tests the service’s real behavior: Docker records whether the probe succeeds, but an unhealthy status does not, by itself, restart the container. For Compose startup ordering, pair the check with depends_on: condition: service_healthy.
Why “running” does not necessarily mean “working”
Docker can report a container as running because its main process is still alive, while the application inside is unable to serve requests. A process may be stuck, waiting on a failed dependency, or otherwise not responding as expected. Process state alone does not establish that the service is ready or functioning.
A health check adds a separate signal. Docker runs a command inside the container and uses its exit status to set the health state: 0 means healthy, 1 means unhealthy, and 2 is reserved. Docker describes the instruction as a way to test whether a container is still working in its Dockerfile reference.
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A probe should verify meaningful behavior, not merely that a process exists. Docker’s reference gives this HTTP example:
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HEALTHCHECK --interval=5m --timeout=3s
CMD curl -f http://localhost/ || exit 1This is a syntax example, not a universal configuration. Confirm that curl is installed in the image, that the URL reaches the intended local service, and that a failed response produces a nonzero exit code. A check that cannot run, or that tests an irrelevant endpoint, can give misleading results. Docker accepts shell-form and exec-array-form commands; if a Dockerfile contains multiple HEALTHCHECK instructions, only the last one takes effect.
Use a probe that matches the service. Docker’s examples include an HTTP request for a web service, redis-cli ping for Redis, and pg_isready for PostgreSQL. The command and any required utilities must be available inside the image.
Set the detection timing deliberately
Docker’s Dockerfile reference lists the following defaults. They are starting points, not recommended values for every application:
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| Setting | Docker default | What it controls |
|---|---|---|
interval |
30s |
How often Docker runs a check after the previous check completes. |
timeout |
30s |
How long a probe may run before Docker stops it with SIGKILL. |
start_period |
0s |
An initial allowance for the service to start. |
start_interval |
5s |
The check interval during the start period. Requires Docker Engine 25.0 or later. |
retries |
3 |
How many consecutive failures Docker tolerates before marking the container unhealthy. |
The first probe runs after the configured interval; subsequent probes run at the configured interval after the preceding probe completes. During start_period, Docker uses start_interval. Failures within the start period do not count toward retries until a probe succeeds; once there has been a success, subsequent consecutive failures count. This lets a slow-starting service initialize without immediately accumulating failures, while still allowing later problems to change its state.
Choose values in light of the service’s startup time, the duration of a useful probe, how quickly you need to detect a failure, and how much tolerance you want for temporary faults. A shorter interval can detect trouble sooner but runs checks more often; more retries allow more consecutive failures before the unhealthy state appears. Docker’s documentation lists numerical defaults and examples, not performance measurements or universal tuning guidance. See the HEALTHCHECK reference for timing behavior and version notes.
Use health checks to gate Compose startup
Compose’s short-form depends_on starts dependencies before dependents, but does not wait for a dependency to become healthy. To make a web service wait for a database’s health check to pass, use long-form depends_on with condition: service_healthy. Docker documents this pattern in its startup and shutdown order guide:
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services:
web:
build: .
depends_on:
db:
condition: service_healthy
db:
image: postgres:18
healthcheck:
test: ["CMD-SHELL", "pg_isready -U $${POSTGRES_USER} -d $${POSTGRES_DB}"]
interval: 10s
retries: 5
start_period: 30s
timeout: 10sWith this configuration, Compose waits for the database health check to pass before creating the dependent web service. The PostgreSQL timing values shown are from Docker’s documented example, not a prescription for every database deployment. The doubled dollar signs defer variable expansion to the container shell. Compose also allows a service definition to override a health check inherited from its image: test may be a string (equivalent to CMD-SHELL) or a list beginning with CMD, CMD-SHELL, or NONE. Use NONE or disable: true to disable an inherited check. See the Compose services reference.
Docker’s Compose Quickstart also demonstrates a Redis probe using redis-cli ping, with a five-second interval, three-second timeout, five retries, and a ten-second start period. Those values are an example configuration; choose settings that fit the service you are checking.
What an unhealthy status does—and does not do
A health check reports state; it is not a restart policy. Docker’s health-check documentation describes changing the health status and emitting a health_status event, while restart policies concern container exit behavior. Do not assume that a container marked unhealthy will restart automatically. If you need another system to react to unhealthy status, verify that system’s behavior and configure it separately.
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Compose startup gating is also distinct from recovery after startup: condition: service_healthy controls when Compose creates a dependent service. It does not make an unhealthy dependency restart. In long-form Compose dependencies, restart: true applies to explicit Compose-controlled dependency updates or restarts, not automated container-runtime restarts, as the startup-order documentation explains.
Inspect a failing check
Docker stores a limited amount of probe output in the health status: the first 4096 bytes currently. When a probe fails, inspect that output to distinguish an application failure from a broken check—for example, a missing executable, a bad URL, or an incorrect command. A change in health status also generates a health_status event, which can help identify when the state changed. See the Dockerfile reference for the recorded-output and event details.
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Check compatibility before relying on timing options
Docker Engine 25.0 or later is required for Dockerfile start_interval. The Compose services reference marks health-check duration support—including interval, timeout, start_period, and start_interval—as introduced in Compose 2.20.2. Verify the Engine and Compose versions in the environment where the configuration will run; the compatibility information is in the Dockerfile reference and Compose services reference.
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