Freshness Hidden by Browser Cache

Keep a returned sample time distinct from current server time and measure when a cached telemetry value becomes stale.

The Data Storage guide asks learners to retain the earlier event time separately from later arrival and review evidence. On iotclass.org a service worker in this lab folder plays the server; nothing leaves your browser., your practice guide

The Data Storage guide asks learners to retain the earlier event time separately from later arrival and review evidence. On iotclass.org a service worker in this lab folder plays the server; nothing leaves your browser.
Predict the reading, then compare it with the measurement.

Browser (Chromium DevTools)

Phone browser

Keep a returned sample time distinct from current server time and measure when a cached telemetry value becomes stale.

Tier 2 · Phone browser · No account

Version tested: Chromium 147.0.7727.15; static site and real DevTools with scoped service worker, 2026-10-09. Date: 2026-10-09.

Open the browser lab, then inspect the real Network, Console, or Accessibility panel in DevTools.

Open the phone lab (new tab)

Steps

Screens captured against Browser (Chromium DevTools) Chromium 147.0.7727.15; static site and real DevTools with scoped service worker, 2026-10-09 on 2026-10-09; the tool may have moved on — the text steps are the contract.

  1. Step 1

    Do
    In the Network panel with Disable cache off, press Fetch cacheable reading. The request is handled by this lab’s service worker.
    You will see
    Input: First cacheable GET and uncached time check. Observed page: Sample 2026-10-09T19:11:33.448Z; current time 2026-10-09T19:11:33.453Z; age 0 s; HTTP 200. Sample age 0 s; HTTP status 200. Observed DevTools: Telemetry HTTP 200 comes from the service worker. Interpretation: The first timestamp is fresh.
    Why it matters
    Evidence: Step 1 screenshot shows the real Chromium page and DevTools panel. Mechanism: The endpoint returns Cache-Control private, max-age=30 and X-Server-Time. Boundary: The time check is an additional no-store request. Next check: Repeat the same URL before expiry.

  2. Step 2

    Do
    In the controls panel, press Fetch cacheable reading again within 30 seconds. Inspect the unchanged sample time and the telemetry row in Network; its Size may say (ServiceWorker), not (disk cache).
    You will see
    Input: Repeated identical URL within 30 seconds. Observed page: Sample 2026-10-09T19:11:33.448Z; current time 2026-10-09T19:11:34.992Z; age 2 s; HTTP 200. Returned sample time is unchanged while current time advances and sample age grows. Observed DevTools: The telemetry row is served by the service worker; its Size may say (ServiceWorker), even though the worker reused its cached response. Interpretation: HTTP 200 does not imply a fresh sample.
    Why it matters
    Evidence: Step 2 screenshot shows the real Chromium page and DevTools panel. Mechanism: The service worker reuses its cacheable response before max-age expires. Boundary: Repeat the same URL within 30 seconds for this observation. Next check: Compare sample and server times.

  3. Step 3

    Do
    In the Current result panel, compare returned sample time with current time.
    You will see
    Input: Compared sample and current timestamps. Observed page: Sample 2026-10-09T19:11:33.448Z; current time 2026-10-09T19:11:34.992Z; age 2 s; HTTP 200. The sample is older than the separate current time. Observed DevTools: The worker still returns the original sample. Interpretation: The reading has aged.
    Why it matters
    Evidence: Step 3 screenshot shows the real Chromium page and DevTools panel. Mechanism: The separate no-store time endpoint avoids using the client clock for age. Boundary: Two seconds is this run only, not a freshness target. Next check: Force a fresh telemetry request.

  4. Step 4

    Do
    In the controls panel, press Fetch with no-store and inspect the new telemetry row in Network.
    You will see
    Input: Fetch cache mode no-store. Observed page: Sample 2026-10-09T19:11:35.624Z; current time 2026-10-09T19:11:35.626Z; age 0 s; HTTP 200. Sample time changes and age returns to 0 s. Observed DevTools: A new service-worker HTTP 200 row appears. Interpretation: no-store bypasses the worker cache.
    Why it matters
    Evidence: Step 4 screenshot shows the real Chromium page and DevTools panel. Mechanism: The request cache mode prevents reuse of the prior response. Boundary: It does not prove all intermediaries honor cache directives. Next check: Try a versioned URL for a second isolation check.

  5. Step 5

    Do
    In the controls panel, press Fetch versioned URL. Its new URL forces a fresh sample even while the original is within its 30-second lifetime.
    You will see
    Input: Versioned telemetry URL. Observed page: Sample 2026-10-09T19:11:35.947Z; current time 2026-10-09T19:11:35.953Z; age 0 s; HTTP 200. Sample time changes and age returns to 0 s. Observed DevTools: A new service-worker HTTP 200 row has a different v query parameter. Interpretation: A distinct URL has its own cache entry.
    Why it matters
    Evidence: Step 5 screenshot shows the real Chromium page and DevTools panel. Mechanism: A versioned URL has a separate worker cache entry. Boundary: DevTools Disable cache does not bypass the Cache API. Next check: Inspect the response headers.

  6. Step 6

    Do
    In the Network panel, select the newest telemetry row in Network and inspect Response headers.
    You will see
    Input: Opened the service-worker response Headers tab. Observed page: Sample 2026-10-09T19:11:35.947Z; current time 2026-10-09T19:11:35.953Z; age 0 s; HTTP 200. The latest sample remains fresh. Observed DevTools: Cache-Control is private, max-age=30; X-Server-Time matches the returned sample time. Interpretation: The policy and timestamp are visible even though Size says (ServiceWorker).
    Why it matters
    Evidence: Step 6 screenshot shows the real Chromium page and DevTools panel. Mechanism: The header is supplied by the scoped service worker. Boundary: One controlled browser run is not a production cache audit. Next check: Record the freshness policy with the dashboard design.

Chapter checks

These questions refer to the chapter’s examples. Use the return links to review their answers.

  1. A latest-value cache makes a dashboard fast, but the durable telemetry store sometimes lags and the cache does not show freshness. What is the strongest review finding?

    Return to the chapter’s knowledge check

  2. A dashboard shows a normal sensor value, but the data record does not include freshness or quality state. What is the main visualization risk?

    Return to the chapter’s knowledge check

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