Designing Failure Cases with a Debugging Proxy
Choose HTTP edits, delays and interrupted exchanges according to the client behavior a proxy-based test needs to exercise.
A debugging proxy is useful when a client needs to encounter a response the normal server rarely produces. Define the client behavior first, then select the intervention that produces that condition.
“The request failed” is too broad a test case. A valid HTTP error response, a response that never arrives and a connection interrupted halfway through a download ask different things of the application.
| Case | Behavior to examine |
|---|---|
| A complete HTTP error response | Whether the client reads the status and applies the intended error policy |
| A delayed response | Whether the user can cancel and the timeout applies at the intended stage |
| An unexpected header or body field | Whether parsing and validation fail clearly |
| An interrupted exchange | Whether incomplete data remains distinguishable from a finished result |
Use an intervention at the right layer
Charles provides breakpoints that pause a request or response before forwarding it. The intercepted message can be examined and edited, allowed through or aborted. Its Rewrite tool applies repeatable changes to matching headers, URLs or bodies. Throttling controls bandwidth and added latency.
These controls can exercise an HTTP client’s behavior, but a bandwidth limit is not a precise model of random IP packet loss. A proxy can buffer data, reuse connections and open a separate upstream connection. Choose a network impairment tool when the required condition is specifically below HTTP.
Keep each case interpretable
Match the intended host and path narrowly. Begin with a normal exchange, enable one change and record what was changed. If a retry reaches a second endpoint, decide whether the same rule should affect that attempt. An unconditional rewrite can otherwise turn every recovery path into the original failure.
For an upload or payment-like operation, a lost response does not prove the server rejected the request. Include that ambiguity in the retry case rather than asserting that every transport failure is safe to repeat.
Inspecting or editing HTTPS content requires the client to participate in the debugging proxy’s TLS inspection setup. A proxy that only carries CONNECT sees encrypted application bytes. Use an isolated test configuration and restore its routing and trust setup afterward so later observations describe the normal client.