Service workers: when requests never reach the proxy

Core idea

A service worker can intercept fetch and return a cached or synthetic Response without a network request. Missing proxy logs can therefore be expected, and changing the proxy will not alter a locally selected response.

Turn Service workers: when requests never reach the proxy into a reproducible scenario with inputs, expected state, total timeout, concurrency limit, and stop condition. The proxy is one dependency; page, browser, and test-data failures must remain distinguishable from channel failures.

What the primary source establishes

Separate Cache API and offline responses from network traffic while controlling lifecycle only on an owned test origin.

The primary source, W3C Service Workers, MDN Service Worker API, MDN Cache API, WHATWG Fetch, RFC 9111 — HTTP Caching, defines the technical baseline but not every client and provider configuration. Read the normative behavior with its version and then verify your implementation. Treat anything beyond the source as a product feature that needs separate confirmation.

Controlled lab

Cache a v1 marker, go offline, and require only cache or synthetic routes to work. Deploy v2, keep one old tab, and open another. Correlate controller and proxy logs, then delete one entry to produce a predictable cache miss.

Step-by-step verification

Create network-only, cache-first, and synthetic-offline routes in an owned PWA. Record worker version, controller state, and response source, then compare normal reload, bypass, and a clean profile.

Start with one authorized URL and one proxy. Verify the exit IP, then add the target action and wait for its explicit result. Store a request ID and stage, never credentials. Add regional matrices and bounded parallelism only after single runs are stable.

Retry only proven safe reads. Respect Retry-After and back off after 429 or network bursts. Purchases, credential changes, and renewals need idempotency plus reconciliation before any repeat. A timeout does not prove failure because the external system may have completed the mutation.

• Create three strategies
• Test offline
• Compare old and new tabs
• Delete one cache entry

Evidence to retain

Record page URL, worker version, lifecycle and controller state, route strategy, cache key, response marker, online state, and proxy-request presence. Exclude cookies and user responses.

Log the scenario, stage, start and finish, result code, attempt count, and correlation ID. Attach sanitized HAR or screenshots only to failures. Keep a batch summary separate from detailed rows so one failure cannot disappear among successes.

Define report columns and time format before the run. A result without context becomes a guess: the address, cache state, and changed condition are unknown. Record controlled failures as well as successes so the check proves that it can distinguish states.

Interpreting the result

Worker updates move through install, waiting, and activate; an old tab may retain the prior controller. Clearing all browser storage first destroys the evidence needed to explain the behavior.

A service worker is limited to its scope and secure context. DevTools options can alter caching, so record the inspection mode.

One successful run confirms only one client, route, and moment. Repeat while changing one variable and state the limits. When observation conflicts with documentation, rule out cache, client version, and intermediaries before creating a reproducible support case.

Worked decision process

Model Service workers: when requests never reach the proxy with queued, running, succeeded, terminally failed, and uncertain states. An external timeout is uncertain because the provider may have completed the mutation. Reconcile with a read before allowing any repeat.

Limit the whole queue, each domain, each account, and retries. Add schedule jitter, honor Retry-After, and back off after bursts. A larger IP pool does not remove origin limits or infrastructure cost.

Store scenario ID, attempt, stage, timestamps, safe result code, and source task. Show stuck and uncertain work separately. Recover with one control task before releasing the bulk queue.

Common mistakes

Long sleeps hide races and immediate retries amplify incidents. Do not evade 429 by rotating addresses or run state-changing tests for one account concurrently. Wait for conditions, bound queues, isolate accounts, and use explicit terminal states.

Stop when errors rise, a source returns a limit, the task would require bypassing protection, or secrets enter logs. Save sanitized diagnostics and correct the cause first. More concurrency or another IP can hide the fault and add load without improving evidence.

Rollout and maintenance criteria

Define the decision boundary before rollout: which observation permits continuation, which requires review, and which stops the workflow. Record acceptable error ratio, maximum wait, and the owner of every exception so a temporary failure cannot silently become permanent configuration.

Review real load, cost, and quality after the first week. Schedule a small control after client, proxy-service, or network changes. Archive outdated instructions with their replacement date and reason so operators do not follow conflicting configurations.

Operational checklist

Turn the successful experiment into a short procedure covering owner, safe configuration, limits, and stop conditions. Every run needs a terminal status. After browser, library, or network changes, run a small control before the main queue.

• Success and stop are defined
• Exit IP is verified
• Waits observe events
• Retries are bounded
• Mutations are idempotent
• Artifacts contain no secrets