Coolify can replace an application container without first stopping the running one, but overlapping containers are not the same as guaranteed zero downtime. For supported non-Compose applications, Coolify starts the replacement, waits for its readiness check when one exists, and removes the old container only after the replacement is considered ready. Reliable, interruption-free deploys also require a truthful health check, graceful shutdown, compatible releases, and persistent data outside the container.
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What a Coolify rolling update actually does
A rolling update is a replacement sequence for an individual application. The currently serving container remains in place while Coolify prepares and starts a new container from the updated source or image. If the new container becomes ready, Coolify stops and removes the old one. If the new container fails its readiness check, Coolify can remove the failed replacement and leave the healthy current container running.
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This path applies to eligible Nixpacks, Railpack, Static, Dockerfile, and Docker Image applications when Coolify can run both containers at the same time. It is not a blanket promise that every deployment will be interruption-free. Coolify’s own documentation says that rolling updates do not guarantee zero downtime and that a passing container health check alone does not prove a zero-downtime transition.
The replacement sequence, step by step
- Build or pull the new release. Coolify creates the image and generated configuration, or pulls the requested image, before changing the running service.
- Start the replacement container. The existing container continues serving while the new one initializes, provided the deployment meets the overlap conditions described below.
- Wait for readiness when a health check exists. Coolify uses a check configured in the dashboard or inherited from the image. The replacement must pass it before the old container is retired.
- Abort an unhealthy replacement. If the new container fails its check, Coolify can remove it and preserve the currently healthy container.
- Retire the old container. After the replacement is considered ready, Coolify sends the old container its stop signal, waits through the configured grace period, and removes it.
When no health check is configured, a process that starts successfully is treated as ready. That can be much earlier than real readiness: the application may still be running migrations, warming a cache, connecting to a database, loading models, or waiting for another service. For production deployments, “the process is running” should not be your readiness definition.
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Which application types use this path?
| Application type | Coolify update behavior | What controls readiness |
|---|---|---|
| Nixpacks | Coolify’s application-level replacement can overlap old and new containers when no exclusivity setting blocks it. | Dashboard health check, if configured; otherwise successful process start. |
| Railpack | Same application-level rolling sequence when the containers can coexist. | Dashboard health check or image-provided check. |
| Static | Uses the same supported application replacement mechanism. | Configured or image-provided health check. |
| Dockerfile | Coolify starts a new image and can wait on the image’s HEALTHCHECK or a dashboard check. |
Docker health status or dashboard check. |
| Docker Image | Runs the replacement image and removes the old container after readiness. | Health check supplied by the image or configured in Coolify. |
| Docker Compose | Does not use Coolify’s application-level sequence; Coolify runs docker compose up and Compose reconciles the services. |
Compose service definition and application behavior. |
| Docker Swarm target | Coolify uses docker stack deploy; Swarm controls replica replacement and routing. |
Swarm service settings, health checks, and application behavior. |
For Compose and Swarm, inspect the service definition rather than assuming the application-level rules above apply. Replica counts, update order, routing, and health-check settings belong to those orchestrators.
Design a health check that means “ready for traffic”
Coolify treats a health check as a readiness signal for Docker and Coolify Proxy. With Traefik in the proxy path, an unhealthy container can be removed from routing. If all available containers are unhealthy, users may receive 404 Not Found or No available server. A check that merely returns HTTP 200 from a shallow status page can therefore create a false sense of safety.
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Use an endpoint that succeeds only after the application can handle the requests that matter. It should verify the minimum dependencies needed to serve normal traffic, such as a database connection or required configuration, without performing an expensive user operation. Keep liveness and readiness separate when possible: a process can be alive while still unable to serve requests.
Configure the dashboard check
- Open the application in Coolify.
- Go to the health-check settings and enable the HTTP check.
- Set the internal listening port used by the process, not a host-published port.
- Enter the readiness path, such as
/healthor another endpoint that reflects actual startup completion. - Save the configuration and deploy a test release.
The check runs inside the application container. The final image must therefore contain curl or wget, depending on the command Coolify uses. A check aimed at a public hostname or an external port can fail even when the service is healthy because container networking is different from a browser request.
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Use an image health check for Dockerfile deployments
A Dockerfile can carry its own health check into the image. Coolify’s documentation gives this pattern as an example:
HEALTHCHECK --interval=30s --timeout=5s --start-period=20s --retries=3
CMD curl --fail http://localhost:3000/health || exit 1The 30-second interval, 5-second timeout, 20-second start period, and three retries are documentation examples, not universal requirements. Adjust them to the application’s startup time and failure characteristics. The image must include curl (or change the command to a tool that is actually installed), and port 3000 must be the container’s listening port in this example.
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- Return a failing status until migrations, dependency connections, and essential initialization are complete.
- Keep the endpoint fast enough to finish within the timeout.
- Log the reason for a failed readiness check so deployment logs identify the dependency or configuration problem.
- Do not make readiness depend on a one-time warm cache if the application can safely serve uncached requests.
Graceful shutdown is part of zero-downtime behavior
When Coolify retires the old container, the application must handle the termination signal correctly. It should stop accepting new work, allow in-flight requests to finish or be safely cancelled, close listeners, and exit. If the process ignores the signal or a wrapper fails to forward it, the container may be force-stopped and requests or jobs can be lost.
- Open the application and choose Advanced > Operations.
- Set Stop Grace Period to the longest time the application legitimately needs after receiving its termination signal.
- Deploy a test release while sending traffic and running representative background work.
- Confirm that the process exits cleanly before the grace period expires.
Coolify documents a default Stop Grace Period of 30 seconds and accepts values from 1 to 3600 seconds. A longer value only grants time; it cannot make an application graceful if the application does not implement signal handling. Choose a value based on observed request and job duration rather than using the maximum automatically.
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When Coolify cannot overlap the containers
Some deployment settings make simultaneous containers impossible or ambiguous. In these cases Coolify stops the current container before starting the replacement, so a short interruption is expected.
| Condition | Why overlap is disabled | What to inspect |
|---|---|---|
| Host-published port | Both containers would try to bind the same host port. | Port mappings and whether the proxy can route through the container network instead. |
| Consistent container name | The new container cannot be created while the old one owns the fixed name. | Deployment naming options. |
| Custom container name | A user-specified name creates the same naming conflict. | Remove the custom name if unique deployment names are acceptable. |
| Custom IP or IPv6 address | The requested address cannot be assigned to two overlapping containers. | Network and address settings. |
| Pull-request preview deployment | The preview deployment path does not use the normal overlap behavior. | Whether the deployment is a preview rather than the main application. |
If Coolify reports that rolling updates are unsupported, read the deployment log and check these settings first. An optional generated Container Name Prefix can include a deployment-specific timestamp; that prefix does not itself disable rolling updates.
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For a Compose application, Coolify invokes docker compose up. Compose reconciles the services described in the file, so update order, replica behavior, health checks, and dependencies are determined by that configuration rather than by Coolify’s single-application replacement sequence. A Compose file that declares one instance and no safe update strategy should not be expected to behave like an overlapping two-container deployment.
For a Swarm target, Coolify invokes docker stack deploy. Swarm handles service replicas, replacement order, and routing. Configure the service’s update policy and health behavior there, then validate the resulting traffic path. In both cases, an orchestrator can start a new task while an old one remains, but the application still has to tolerate that overlap.
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Make the application safe to run in two versions
During a rolling deployment, old and new code can execute at the same time. Treat that interval as a compatibility contract.
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- Database schema: Use an expand-and-contract approach. Add structures in a backward-compatible step before deploying code that requires them; remove obsolete structures only after every old instance is gone.
- APIs and serialized data: Keep responses, messages, and stored formats readable by both releases during the overlap window.
- Queues and workers: Make jobs idempotent and ensure two worker versions can process the same queue without corrupting state.
- Schedulers: Prevent duplicate cron or scheduled work, or make the task safe when two schedulers run briefly.
- Sessions and caches: Use shared, compatible storage when requests can land on either version. Do not rely on process-local memory for data that must survive a switch.
- Files and uploads: Store them in a persistent volume or managed storage mounted by the replacement, not only in the container’s writable layer.
Containers are replaceable. Data written only to a container filesystem disappears when that container is removed. Databases, uploads, and other durable state belong in managed services, persistent volumes, or mounted files that are present in the new container definition.
Validate a deployment instead of assuming zero downtime
- Prepare readiness. Configure a check that fails during startup and passes only when a critical request can succeed.
- Test shutdown. Verify signal handling and that representative requests and jobs complete within the configured grace period.
- Deploy a compatible change. Use a release whose database and API changes support both versions during the test.
- Generate continuous traffic. Repeatedly request a critical endpoint throughout the deployment, including while the replacement is starting and the old container is stopping.
- Read the deployment log. Look for messages equivalent to “Rolling update started,” “New container is healthy,” and “Rolling update completed.”
- Check application work. Verify database writes, queued jobs, sessions, uploads, and other state after the switch.
Container overlap proves that Coolify used the replacement path; it does not prove that users experienced a successful transition. The validation must include real requests and stateful work.
Troubleshoot common interruptions
The old container stops before the new one starts
Inspect the deployment log for a host port mapping, consistent or custom container name, custom IP or IPv6 address, or pull-request preview deployment. Remove the exclusivity condition where practical, then redeploy. If the condition is required by your network design, plan for a stop-then-start interruption or move routing to an arrangement that permits distinct container identities.
The replacement starts but is removed
Open the replacement’s health status and logs. Confirm that the check binary exists in the final image, the check uses the internal port and path, and startup can finish before the interval, timeout, start period, and retry budget expire. Test the command from inside an equivalent container so DNS, credentials, and dependency failures are visible.
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Requests fail after the replacement is marked healthy
The readiness check is probably too weak. Make it represent the critical request path, then inspect proxy routing and graceful-shutdown logs. A container can return a simple 200 response while its real application routes, database pool, or background workers are still unusable.
A source build fails
Coolify’s deployment history records the source revision, image build or pull, generated configuration, and container actions. A failed source build does not replace the running application. Failures after a successful build depend on the selected replacement path and health-check configuration, so distinguish build errors from startup, readiness, routing, and shutdown errors in the log.
A practical go/no-go checklist
- The application type supports Coolify’s application-level rolling replacement, or its Compose/Swarm update policy has been configured separately.
- Old and new containers can coexist: no conflicting host port, fixed name, custom address, or preview-only deployment path.
- The readiness check runs inside the image, uses the internal port, and stays failing until real traffic can succeed.
- The image contains the required check command.
- The process forwards and handles its termination signal.
- Stop Grace Period covers the longest legitimate in-flight request or job.
- Schema, API, queue, cache, session, scheduler, and file changes are compatible during overlap.
- Durable data is stored outside the replaceable writable layer.
- A traffic test and post-deploy state check have passed.
When all of these conditions hold, Coolify’s rolling replacement can make a deployment appear continuous to users. If any condition fails, container overlap alone is not evidence of zero downtime.
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