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Pod JVM modify return

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Pod JVM modify return is a Kubernetes pod-level chaos fault that overrides the return value of a specific Java method in a JVM running in a target container for a configurable duration. The method runs as usual but returns the configured value instead of its real result. When the fault ends, the method returns its real result again immediately.

Use this fault to test how callers handle silently wrong data: a feature flag that always returns true, an authorization check that always returns false, a price lookup that returns a stale value, or a serialization helper that returns null.

Run your first experiment

If you have not configured the chaos infrastructure yet, go to Quickstart to install the chaos infrastructure and run an experiment end to end.

Use cases​

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

  • Null-safety coverage: When the method returns null instead of an object, does the caller crash with NullPointerException or handle it gracefully?
  • Boolean-flag flipping: Force a feature flag, permission check, or rate-limit decision to a specific value and verify downstream behavior.
  • Stale-data handling: Return an outdated value and confirm whether caches, validation, or staleness checks catch it.
  • Type-coercion bugs: Return an edge-case value (empty string, zero, MAX_VALUE) and look for unguarded parsing or arithmetic.
  • Defensive copying: If the method returns a mutable collection, a forced empty or shared instance can expose bugs in callers that assume per-call uniqueness.

Prerequisites​

  • Kubernetes version: 1.21 or later. Go to What's supported to confirm distribution support.
  • Target pod is Running: The Java application pod is in the Running state.
  • Java agent attach available: The Java process allows agent attach. Utilities such as ps, pgrep, and bash are present in the container, and the JVM is not built with a restricted runtime that strips attach modules.
  • Privileged pods allowed: The cluster lets you schedule privileged pods in the chaos namespace. GKE Autopilot supports this fault but requires the one-time setup in Chaos on GKE Autopilot; other locked-down distributions may need similar exemptions.
  • Container runtime access: The chaos pod can reach the container runtime socket on the target node (/run/containerd/containerd.sock, /var/run/docker.sock, or /var/run/crio/crio.sock).
  • Workload selector defined: The chaos experiment knows the target workload by kind, namespace, and either names or labels.
JVM chaos uses the Byteman agent

This fault attaches a Byteman agent to the target JVM over BYTEMAN_PORT. The port must be reachable from the chaos pod and must not be in use by the application.

Supported environments​

PlatformSupport status
Amazon EKSSupported
Azure AKSSupported
Google GKESupported
Red Hat OpenShiftSupported
RancherSupported
VMware TanzuSupported
Self-managed Kubernetes (CNCF-certified)Supported
GKE AutopilotSupported with Autopilot setup
EKS Fargate, ACI virtual nodesNot supported (no access to container runtime sockets)

Permissions required​

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

Resource (apiGroup)VerbsWhy it is needed
pods ("")get, list, create, delete, deletecollection, patch, updateDiscover target pods and run the chaos pod on the same node
pods/log ("")get, list, watchStream chaos pod logs for status and debugging
deployments, statefulsets, replicasets, daemonsets (apps)get, listResolve the target workload to the pods it owns
events ("")get, list, create, patch, updateRecord fault progress as Kubernetes events
jobs (batch)get, list, create, delete, deletecollectionRun the chaos job that drives the fault

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

Fault tunables​

Configure the following fault parameters when you add Pod JVM modify return to an experiment in Chaos Studio. Defaults are shown for reference.

Required parameters

TunableDescriptionDefault
CLASSFully qualified class name containing the target method (for example com.example.payments.PaymentService).(required)
METHODName of the method whose return value to override.(required)
RETURNValue to return from the method. Must be compatible with the method's declared return type. Use null for nullable references, a quoted string for String, or a numeric literal for primitives.(required)

Chaos parameters

TunableDescriptionDefault
TOTAL_CHAOS_DURATIONDuration of the fault in seconds.60

JVM

TunableDescriptionDefault
BYTEMAN_PORTPort on which the Byteman agent listens inside the container. Must not conflict with any port already in use.9091
JAVA_HOMEAbsolute path to the Java installation inside the container. Empty auto-detects from PATH.""

Targeting

TunableDescriptionDefault
TARGET_PODSComma-separated list of pod names to target. Empty selects from the workload's pods using POD_AFFECTED_PERCENTAGE.""
TARGET_CONTAINERContainer in the pod running the JVM. Empty targets the first container in the pod spec.""
NODE_LABELLabel selector to filter target pods by the node they run on. Empty disables node-based filtering.""
POD_AFFECTED_PERCENTAGEPercentage of the workload's pods to target. 0 means one pod.0
SEQUENCEWhen multiple pods are targeted, inject parallel (all at once) or serial (one after another).parallel

Runtime and helper

TunableDescriptionDefault
CONTAINER_RUNTIMEContainer runtime on the target nodes. One of containerd, docker, crio.containerd
SOCKET_PATHPath to the container runtime socket on the target node. Set to match CONTAINER_RUNTIME./run/containerd/containerd.sock
RAMP_TIMEWait period in seconds before and after the fault. Go to ramp time to read how it is applied.0

Common pod selection tunables (TARGET_WORKLOAD_KIND, TARGET_WORKLOAD_NAMESPACE, TARGET_WORKLOAD_NAMES, TARGET_WORKLOAD_LABELS) are documented in common pod fault tunables. Tunables that apply to every fault are documented in common tunables for all faults.

Match the declared return type

The RETURN value must be assignable to the method's declared return type. For boxed primitives, prefer the unboxed literal (for example true for boolean, 0 for int). For object types, null is always valid.

Configure for your container runtime​

Set CONTAINER_RUNTIME and SOCKET_PATH to match the runtime on the target node:

CONTAINER_RUNTIMESOCKET_PATH
containerd (default)/run/containerd/containerd.sock
docker/var/run/docker.sock
crio/var/run/crio/crio.sock

Fault execution in brief​

Attaches a Java agent to the target JVM and instruments CLASS.METHOD so that each invocation runs as usual but returns the value in RETURN to the caller, for TOTAL_CHAOS_DURATION seconds.

Expected behavior during fault execution​

  • Every call to the named method returns the configured value. Side effects of the real method execution (writes, network calls, state mutations) still happen.
  • Direct callers see the overridden value and may branch differently, throw NullPointerException, or pass the wrong value downstream.
  • Tracing systems see the method span complete successfully; only the return value is unusual.
  • Self-consistency invariants (if (getX() == lookUpX()) ...) may fail silently.
When the fault ends

The method returns its real result again immediately.

Signals to watch​

Attach resilience probes to assert each layer:

  • Caller error rate: Use an HTTP probe against endpoints that consume the method's return value to detect downstream failures.
  • Method invariants: Use a Prometheus probe on application metrics that should be sensitive to the overridden value.
  • Application logs: Use a command probe to grep for unexpected NullPointerException or coercion errors.

Verify the fault execution effect​

While the experiment is running, confirm the overridden return:

  1. Exercise the method from a client.

    kubectl run -n <namespace> tester --image=nicolaka/netshoot --rm -it -- \
      curl -s http://<service>:<port>/<endpoint-that-uses-the-return-value>

    The response should reflect the overridden value.

  2. Compare against baseline.

    Capture the same endpoint's response before the fault starts and after it ends; only the fault window should show the overridden behavior.

Recovery and cleanup​

  • End of duration: The method returns its real result again automatically.
  • Abort the experiment: Stopping the experiment from Chaos Studio triggers the same cleanup path.
  • Cached overridden value: If the caller cached the overridden return, the application may keep using the wrong value until the cache refreshes; clear the cache or restart the pod if needed.

Limitations​

  • Serverless Kubernetes (EKS Fargate, ACI virtual nodes): These platforms do not expose container runtime sockets and reject the privileged access the fault needs. GKE Autopilot is supported once the one-time setup in Chaos on GKE Autopilot is in place.
  • Windows containers: This fault is supported on Linux pods only.
  • Non-JVM processes: This fault targets a Java process.
  • Inlined methods: Methods inlined by the JIT compiler may not be interceptable.
  • Method overloads: Only the method name is matched. If multiple overloads with different return types share a name, only those whose return type is compatible with RETURN work as expected.

Troubleshooting​

Pod JVM modify return experiment stays Pending or never starts in Harness Chaos Engineering

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

Return value not overridden during pod-jvm-modify-return

The most common causes are: CLASS or METHOD does not match; RETURN is not assignable to the method's declared return type; the method is JIT-inlined; or the wrong container is targeted. Re-run with a simple known-compatible RETURN such as null (for objects) or 0 (for ints) to confirm the path is working.

Connection to container runtime fails for pod-jvm-modify-return in Harness Chaos Engineering

The default SOCKET_PATH is /run/containerd/containerd.sock. For Docker, set CONTAINER_RUNTIME=docker and SOCKET_PATH=/var/run/docker.sock. For CRI-O, set CONTAINER_RUNTIME=crio and SOCKET_PATH=/var/run/crio/crio.sock.