> For the complete documentation index, see [llms.txt](https://developer.harness.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://developer.harness.io/resilience-testing/chaos-engineering/faults/chaos-fault-categories/kubernetes/pod/pod-application-function-error.md).

# Pod application function error

Pod application function error is a Kubernetes pod-level chaos fault that causes a specific function in an instrumented application to raise a configurable error for a configurable duration. Only the named function is affected; other application code paths run normally. When the fault ends, the function returns to its normal behavior immediately.

Use this fault to validate how callers and dependents behave when a specific business function starts failing: a third-party SDK that throws on a code path, a wrapper that emits a custom exception, or a side-effecting operation that suddenly returns an error.

{% hint style="info" %}
**RUN YOUR FIRST EXPERIMENT**

If you have not configured the chaos infrastructure yet, go to [Quickstart](/resilience-testing/chaos-engineering/new-to-chaos-engineering/quickstart.md) to install the chaos infrastructure and run an experiment end to end.
{% endhint %}

***

### Use cases <a href="#use-cases" id="use-cases"></a>

Run this fault when you want to answer concrete questions like:

* **Third-party SDK failure simulation:** Inject errors into a wrapper around a payment or auth SDK and confirm callers fall back or queue retries.
* **Core-function failure modes:** When a critical business function raises an unexpected error, does the request fail closed, or does partial work leak through?
* **Retry and fallback validation:** Do retry budgets respect maximum attempts, or do retries amplify load on downstream services?
* **Circuit breaker behavior:** Does a circuit breaker around the failing function open after the configured failure threshold and short-circuit subsequent calls?
* **Observability coverage:** Does the failure surface in dashboards, traces, and alerts as expected, or does it slip past monitoring?

***

### Prerequisites <a href="#prerequisites" id="prerequisites"></a>

* **Kubernetes version:** 1.21 or later. Go to [What's supported](/resilience-testing/chaos-engineering/new-to-chaos-engineering/whats-supported.md) to confirm distribution support.
* **Target pod is Running:** The application pod is in the `Running` state.
* **Application is instrumented:** The application registers a name and exposes the target function to the chaos infrastructure. Without instrumentation, the chaos pod cannot reach the function.
* **Function is identifiable:** The function to fail is reachable by the name set in `TARGET_APPLICATION_FUNCTION`.
* **Workload selector defined:** The chaos experiment knows the target application by name.

***

### Supported environments <a href="#supported-environments" id="supported-environments"></a>

| Platform                                 | Support status                                   |
| ---------------------------------------- | ------------------------------------------------ |
| Amazon EKS                               | Supported                                        |
| Azure AKS                                | Supported                                        |
| Google GKE                               | Supported                                        |
| Red Hat OpenShift                        | Supported                                        |
| Rancher                                  | Supported                                        |
| VMware Tanzu                             | Supported                                        |
| Self-managed Kubernetes (CNCF-certified) | Supported                                        |
| GKE Autopilot                            | Supported (no privileged access required)        |
| EKS Fargate, ACI virtual nodes           | Supported (no container runtime socket required) |

***

### Permissions required <a href="#permissions-required" id="permissions-required"></a>

The fault runs under the chaos infrastructure's service account.

| Resource (`apiGroup`)                                               | Verbs                                                                    | Why it is needed                                |
| ------------------------------------------------------------------- | ------------------------------------------------------------------------ | ----------------------------------------------- |
| `pods` (`""`)                                                       | `get`, `list`, `create`, `delete`, `deletecollection`, `patch`, `update` | Discover the target pod and run the chaos pod   |
| `pods/log` (`""`)                                                   | `get`, `list`, `watch`                                                   | Stream chaos pod logs for status and debugging  |
| `deployments`, `statefulsets`, `replicasets`, `daemonsets` (`apps`) | `get`, `list`                                                            | Resolve the target workload to the pods it owns |
| `events` (`""`)                                                     | `get`, `list`, `create`, `patch`, `update`                               | Record fault progress as Kubernetes events      |
| `jobs` (`batch`)                                                    | `get`, `list`, `create`, `delete`, `deletecollection`                    | Run the chaos job that drives the fault         |

The default Harness chaos infrastructure service account already includes these permissions.

***

### Fault tunables <a href="#fault-tunables" id="fault-tunables"></a>

Configure the following fault parameters when you add Pod application function error to an experiment in Chaos Studio. Defaults are shown for reference.

**Required parameters**

| Tunable                       | Description                                                                  | Default    |
| ----------------------------- | ---------------------------------------------------------------------------- | ---------- |
| `TARGET_APPLICATION_NAME`     | Name of the target application as registered with the chaos infrastructure.  | (required) |
| `TARGET_APPLICATION_FUNCTION` | Name of the function inside the target application to inject the error into. | (required) |

**Chaos parameters**

| Tunable                | Description                                                                                                                                                                                                      | Default |
| ---------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------- |
| `MESSAGE`              | Error message attached to the injected failure. Empty uses a default message.                                                                                                                                    | `""`    |
| `TOTAL_CHAOS_DURATION` | Duration of the fault in seconds.                                                                                                                                                                                | `60`    |
| `RAMP_TIME`            | Wait period in seconds before and after the fault. Go to [ramp time](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/common-tunables-for-all-faults.md#ramp-time) to read how it is applied. | `0`     |

Tunables that apply to every fault are documented in [common tunables for all faults](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/common-tunables-for-all-faults.md).

{% hint style="info" %}
**USE A RECOGNIZABLE ERROR MESSAGE**

Set `MESSAGE` to a unique string so you can grep logs and traces to confirm the injected failure during analysis.
{% endhint %}

***

### Fault execution in brief <a href="#fault-execution-in-brief" id="fault-execution-in-brief"></a>

Signals the instrumented application to make the function named in `TARGET_APPLICATION_FUNCTION` raise an error containing `MESSAGE` for `TOTAL_CHAOS_DURATION` seconds.

***

### Expected behavior during fault execution <a href="#expected-behavior-during-fault-execution" id="expected-behavior-during-fault-execution"></a>

* Calls to the named function fail with the configured error message. Other functions in the same application run normally.
* Direct callers of the function surface the failure as exceptions, error responses, or queued retries depending on the language and framework.
* Downstream services may see reduced or absent traffic if the failing function fronted upstream calls.
* Error dashboards and traces should show the injected error message alongside any cascading failures.

{% hint style="info" %}
**WHEN THE FAULT ENDS**

The function returns to its normal behavior immediately. In-flight calls that already raised the error stay failed; new calls succeed as before.
{% endhint %}

#### Signals to watch <a href="#signals-to-watch" id="signals-to-watch"></a>

Attach [resilience probes](/resilience-testing/chaos-engineering/use-chaos-engineering/probes.md) to assert each layer:

* **Application error rate:** Use an [HTTP probe](/resilience-testing/chaos-engineering/use-chaos-engineering/probes/http-probe.md) against endpoints that exercise the function to detect 4xx/5xx spikes.
* **Function-level metrics:** Use a [Prometheus probe](/resilience-testing/chaos-engineering/use-chaos-engineering/probes/apm-probes.md) on the function's error counter or success rate to confirm the failure injection.
* **Application logs:** Use a [command probe](/resilience-testing/chaos-testing/probes/command-probe.md) to grep container logs for the configured `MESSAGE`.

***

### Verify the fault execution effect <a href="#verify-the-fault-execution-effect" id="verify-the-fault-execution-effect"></a>

While the experiment is running, confirm the function is failing:

1. **Exercise the function from a client.**

   ```bash
   kubectl run -n <namespace> tester --image=nicolaka/netshoot --rm -it -- \
     curl -s http://<service>:<port>/<endpoint-that-calls-the-function>
   ```

   The response should reflect the failure, either as an HTTP error or an error payload.
2. **Confirm the failure surfaces in logs.**

   ```bash
   kubectl logs -n <namespace> <target-pod> --tail=200 | grep "<MESSAGE>"
   ```

   The configured `MESSAGE` should appear with each failed invocation.

***

### Recovery and cleanup <a href="#recovery-and-cleanup" id="recovery-and-cleanup"></a>

* **End of duration:** The function returns to its normal behavior automatically.
* **Abort the experiment:** Stopping the experiment from Chaos Studio triggers the same cleanup path.
* **Stuck state:** If the application caches the failure (for example by tripping a circuit breaker that does not reset on its own), restart the pod to clear the cached state.

***

### Limitations <a href="#limitations" id="limitations"></a>

* **Instrumentation required:** The fault only affects applications that have registered themselves and their functions with the chaos infrastructure. Uninstrumented applications cannot be targeted.
* **Function-name granularity:** Only one function at a time is targeted. Use multiple experiments in sequence for multi-function scenarios.
* **No payload control:** This fault raises an error from the function; it does not modify return values or arguments. Use Pod JVM modify return for JVM-specific return-value modification.

***

### Troubleshooting <a href="#troubleshooting" id="troubleshooting"></a>

<details>

<summary>Pod application function error experiment stays Pending or never starts in Harness Chaos Engineering</summary>

Inspect the chaos pods in the experiment namespace with kubectl describe pod -n . The most common causes are taints on the target node that the chaos pods do not tolerate, insufficient resources, or a PodSecurity admission policy. Add the required tolerations to the experiment or adjust the namespace's Pod Security level.

</details>

<details>

<summary>No errors observed during pod-application-function-error</summary>

The most common causes are: TARGET\_APPLICATION\_NAME does not match the registered application name; TARGET\_APPLICATION\_FUNCTION does not match a registered function; the application image does not include the chaos instrumentation; or the call path under test never invokes the named function. Verify registration by listing instrumented applications and confirm by exercising the function with a known client.

</details>

<details>

<summary>Function appears to fail intermittently after pod-application-function-error ends</summary>

Check whether the application has a circuit breaker or cooldown window that keeps the function in a degraded state after the injection ends. Adjust the circuit breaker reset interval or restart the pod to clear it.

</details>

***

### Related faults <a href="#related-faults" id="related-faults"></a>

* [Pod application function latency](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/kubernetes/pod/pod-application-function-latency.md): Inject latency into a function instead of failing it.
* [Pod JVM method exception](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/kubernetes/pod/pod-jvm-method-exception.md): JVM-specific method-level exception injection.
* [Pod JVM modify return](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/kubernetes/pod/pod-jvm-modify-return.md): Modify the return value of a JVM method instead of raising an exception.
* [Common pod fault tunables](/resilience-testing/chaos-engineering/faults/chaos-fault-categories/kubernetes/pod/common-tunables-for-pod-faults.md): Shared environment variables for selecting target pods and workloads.