Health probes in Azure Container Apps

  • Article

Azure Container Apps health probes allow the Container Apps runtime to regularly inspect the status of your container apps.

You can set up probes using either TCP or HTTP(S) exclusively.

Container Apps supports the following probes:

Probe Description
Startup Checks if your application has successfully started. This check is separate from the liveness probe and executes during the initial startup phase of your application.
Liveness Checks if your application is still running and responsive.
Readiness Checks to see if a replica is ready to handle incoming requests.

For a full list of the probe specification supported in Azure Container Apps, refer to Azure REST API specs.

HTTP probes

HTTP probes allow you to implement custom logic to check the status of application dependencies before reporting a healthy status.

Configure your health probe endpoints to respond with an HTTP status code greater than or equal to 200 and less than 400 to indicate success. Any other response code outside this range indicates a failure.

The following example demonstrates how to implement a liveness endpoint in JavaScript.

const express = require('express');
const app = express();

app.get('/liveness', (req, res) => {
  let isSystemStable = false;

  // check for database availability
  // check filesystem structure
  //  etc.

  // set isSystemStable to true if all checks pass

  if (isSystemStable) {
    res.status(200); // Success
  } else {
    res.status(503); // Service unavailable
  }
})

TCP probes

TCP probes wait to establish a connection with the server to indicate success. The probe fails if it can't establish a connection to your application.

Restrictions

  • You can only add one of each probe type per container.
  • exec probes aren't supported.
  • Port values must be integers; named ports aren't supported.
  • gRPC isn't supported.

Examples

The following code listing shows how you can define health probes for your containers.

The ... placeholders denote omitted code. Refer to Container Apps ARM template API specification for full ARM template details.

{
  ...
  "containers":[
    {
      "image":"nginx",
      "name":"web",
      "probes": [
        {
          "type": "liveness",
          "httpGet": {
            "path": "/health",
            "port": 8080,
            "httpHeaders": [
              {
                "name": "Custom-Header",
                "value": "liveness probe"
              }]
          },
          "initialDelaySeconds": 7,
          "periodSeconds": 3
        },
        {
          "type": "readiness",
          "tcpSocket": {
            "port": 8081
          },
          "initialDelaySeconds": 10,
          "periodSeconds": 3
        },
        {
          "type": "startup",
          "httpGet": {
            "path": "/startup",
            "port": 8080,
            "httpHeaders": [
              {
                "name": "Custom-Header",
                "value": "startup probe"
              }]
          },
          "initialDelaySeconds": 3,
          "periodSeconds": 3
        }]
    }]
  ...
}

The optional failureThreshold setting defines the number of attempts Container Apps tries to execute the probe if execution fails. Attempts that exceed the failureThreshold amount cause different results for each probe type.

Default configuration

If ingress is enabled, the following default probes are automatically added to the main app container if none is defined for each type.

Probe type Default values
Startup Protocol: TCP
Port: ingress target port
Timeout: 3 seconds
Period: 1 second
Initial delay: 1 second
Success threshold: 1
Failure threshold: 240
Readiness Protocol: TCP
Port: ingress target port
Timeout: 5 seconds
Period: 5 seconds
Initial delay: 3 seconds
Success threshold: 1
Failure threshold: 48
Liveness Protocol: TCP
Port: ingress target port

If your app takes an extended amount of time to start (which is common in Java) you often need to customize the probes so your container doesn't crash.

The following example demonstrates how to configure the liveness and readiness probes in order to extend the startup times.

"probes": [
       {
        "type": "liveness",
        "failureThreshold": 3,
        "periodSeconds": 10,
        "successThreshold": 1,
        "tcpSocket": {
          "port": 80
        },
        "timeoutSeconds": 1
       },
       {
         "type": "readiness",
         "failureThreshold": 48,
         "initialDelaySeconds": 3,
         "periodSeconds": 5,
         "successThreshold": 1,
         "tcpSocket": {
           "port": 80
          },
          "timeoutSeconds": 5
       }]

Next steps

Application logging