WRWEB ENGINEERING
REFORGING31 · 2/7
ALL LESSONS→WEEK 31→TUESDAY

BUILD · BROWSER PLATFORM

Service workers

Workers and overlooked powers30–60 MINUTESCORE + PRACTICAL
01

GROUND

Problem

What becomes confusing, fragile, or impossible without understanding service workers? This lesson answers that through explanation, a worked example, two runnable exercises, and a reference solution. No teacher-supplied worksheet is required.

Browser APIs grew around the DOM to expose observation, storage, workers, files, history, devices, and offline capabilities under a permissioned security model.
02

LEARN

Concept explanation

Service workers belongs to “Workers and overlooked powers”. Workers, caches, locks, and compression move work or coordination across browser lifecycle boundaries. This lesson turns service workers into a working part of weekly fixture.

For service workers, trace concrete input, state transition, output, and failure through a permissioned browser capability with explicit ownership, lifecycle, event-loop, and storage boundaries.

The browser is a multi-process capability host. APIs cross security, event-loop, storage, and lifecycle boundaries. Apply that model to supplied normal, boundary, and failure cases; each case below names its input and expected evidence.

Observable

Evidence produced by the service workers experiment: output, state, trace, bytes, timing, or diagnostics.

Invariant

Condition that must remain true while inputs or implementation of service workers change.

Boundary

Point where service workers crosses ownership, representation, time, process, network, or trust.

Example bank

Compare normal, boundary, failure, and cross-layer cases. Predict each observation before revealing the explanation.

Baseline · one variable

SETUPBuild service workers around this exact fixture: Send transferable data to worker, cache one response, and coordinate update with Web Lock.

OBSERVEservice workers handles baseline and boundary while preserving this observable result: Main thread stays responsive; ownership transfer/cache hit/lock ordering and recovery are logged.

WHY IT MATTERSThis isolates the normal contract of service workers; preserve its raw evidence as the control for every later comparison.

Boundary · same contract, harder input

SETUPAdd service workers support for weekly boundary case: Reload offline and compare cold/warm path.

OBSERVERecord what remains invariant and the first representation, owner, size, or timing value that changes in Chromium Sources · Application · Performance · Media.

WHY IT MATTERSA boundary example is useful only when one named dimension changes and everything else stays comparable.

Failure · evidence before repair

SETUPMake service workers surface—not hide—this failure: Terminate worker or corrupt cache entry and recover explicitly.

OBSERVECapture the first divergence from the baseline, including exact input, diagnostic, state, and recovery result. Expected recovery: service workers handles baseline and boundary while preserving this observable result: Main thread stays responsive; ownership transfer/cache hit/lock ordering and recovery are logged.

WHY IT MATTERSThe diagnostic is part of the interface. Repair the proven cause, not the most visible symptom.

Cross-layer · follow ownership

SETUPTrace service workers one layer below its usual abstraction through a permissioned browser capability with explicit ownership, lifecycle, event-loop, and storage boundaries.

OBSERVEUse DOM/event breakpoints, Application storage, observer logs, worker inspection, permissions, and lifecycle events.

WHY IT MATTERSThe lower layer is earned when it explains evidence the current layer cannot. Otherwise keep service workers at the simpler boundary.

03

SEE

Worked example

Start from supplied index.html. Focus: Build service workers around this exact fixture: Send transferable data to worker, cache one response, and coordinate update with Web Lock.

  1. Run: Serve the directory locally, open index.html, click Run, then inspect relevant browser panels.
  2. Save baseline evidence. Use DOM/event breakpoints, Application storage, observer logs, worker inspection, permissions, and lifecycle events.
  3. Boundary case: Add service workers support for weekly boundary case: Reload offline and compare cold/warm path.
  4. Failure case: Make service workers surface—not hide—this failure: Terminate worker or corrupt cache entry and recover explicitly.
RESULT
service workers handles baseline and boundary while preserving this observable result: Main thread stays responsive; ownership transfer/cache hit/lock ordering and recovery are logged. Starter-level baseline: Button changes log from idle to a JSON record containing the lesson topic and a numeric timestamp.
04

START HERE

Starter material

PREREQUISITESNode.js 22+ and a modern browser. Local server is required for storage, workers, or permissioned APIs.

ONE-TIME SETUPSave index.html, then run npx --yes serve . and open the printed localhost URL.

Create index.html, paste this exact content, then run the command below.

<!doctype html>
<meta charset="utf-8">
<title>service workers</title>
<button id="run">Run service workers probe</button>
<pre id="log">idle</pre>
<script>
  const log = document.querySelector('#log');
  document.querySelector('#run').addEventListener('click', () => {
    log.textContent = JSON.stringify({ topic: "service workers", time: performance.now() }, null, 2);
  });
</script>
RUNServe the directory locally, open index.html, click Run, then inspect relevant browser panels.

STOP / CLEANUPReturn to server terminal and press Ctrl+C.

05

DO WITH GUIDANCE

Guided exercise

Complete one useful slice: service workers

  1. Normal case: Build service workers around this exact fixture: Send transferable data to worker, cache one response, and coordinate update with Web Lock.
  2. Write predicted evidence from this named case before running starter.
  3. Implement smallest behavior that satisfies the checks.
  4. Run exact normal case. Save commands, inputs, outputs, and diagnostics in notebook.
  5. Explain changed evidence using lesson mental model in no more than five sentences.
Concrete guided solution
  1. Copy the supplied index.html unchanged and run: Serve the directory locally, open index.html, click Run, then inspect relevant browser panels.
  2. Write this prediction before inspecting output: service workers handles baseline and boundary while preserving this observable result: Main thread stays responsive; ownership transfer/cache hit/lock ordering and recovery are logged.
  3. Perform only the named normal case: Build service workers around this exact fixture: Send transferable data to worker, cache one response, and coordinate update with Web Lock.
  4. Save the raw output, then annotate input → transition → evidence. Use Chromium Sources · Application · Performance · Media to confirm the transition rather than inferring it.
  5. Compare prediction with evidence; if they differ, keep both and write the rule that explains the difference. Reference baseline: Button changes log from idle to a JSON record containing the lesson topic and a numeric timestamp.
06

DO ALONE

Independent exercise

Add one real case: service workers

  1. Create second case from blank file: Add service workers support for weekly boundary case: Reload offline and compare cold/warm path.
  2. Then create controlled failure: Make service workers surface—not hide—this failure: Terminate worker or corrupt cache entry and recover explicitly.
  3. Use Chromium Sources · Application · Performance · Media to prove behavior, then repair controlled failure.
  4. Compare result against supplied acceptance checks and reference approach before marking complete.
Concrete independent solution
  1. Duplicate the starter into a clean comparison case; change only this boundary: Add service workers support for weekly boundary case: Reload offline and compare cold/warm path.
  2. Save its evidence beside the baseline and identify the first changed value. Use DOM/event breakpoints, Application storage, observer logs, worker inspection, permissions, and lifecycle events.
  3. Create the exact controlled failure: Make service workers surface—not hide—this failure: Terminate worker or corrupt cache entry and recover explicitly.
  4. Create smallest file or component named for service workers; accept weekly fixture as input rather than embedding hidden state.
  5. Implement baseline path first: Send transferable data to worker, cache one response, and coordinate update with Web Lock.
  6. Add one explicit branch or state for: Reload offline and compare cold/warm path.
  7. Return, throw, display, or log actionable failure for: Terminate worker or corrupt cache entry and recover explicitly.
  8. Run both successful cases after failure repair; verify: Main thread stays responsive; ownership transfer/cache hit/lock ordering and recovery are logged.
  9. Rerun baseline, boundary, and repaired failure together. Accept only if all reproduce: service workers handles baseline and boundary while preserving this observable result: Main thread stays responsive; ownership transfer/cache hit/lock ordering and recovery are logged.
07

COMPARE

Expected result

  • service workers handles baseline and boundary while preserving this observable result: Main thread stays responsive; ownership transfer/cache hit/lock ordering and recovery are logged.
  • Button changes log from idle to a JSON record containing the lesson topic and a numeric timestamp.
  • Controlled service workers failure produces captured evidence; repair restores stated invariant without hiding error.
08

PROVE

Acceptance checks

Lesson is complete only when every check is true. Each check is stored locally and travels with your JSON backup.

0/5 complete · saved on this device

09

UNSTICK

Hints

Reveal hints
  1. Start with supplied normal case exactly as written: Build service workers around this exact fixture: Send transferable data to worker, cache one response, and coordinate update with Web Lock.
  2. For boundary case, change only named dimension: Add service workers support for weekly boundary case: Reload offline and compare cold/warm path.
  3. If result is confusing, diff raw inputs and evidence before editing implementation.
  4. If tool shows nothing useful, move observation one boundary lower: representation, runtime, OS, or network.
10

VERIFY

Solution

Attempt both exercises before opening reference approach.

Reveal reference solution
  1. Run unmodified starter and preserve baseline evidence: Button changes log from idle to a JSON record containing the lesson topic and a numeric timestamp.
  2. Create smallest file or component named for service workers; accept weekly fixture as input rather than embedding hidden state.
  3. Implement baseline path first: Send transferable data to worker, cache one response, and coordinate update with Web Lock.
  4. Add one explicit branch or state for: Reload offline and compare cold/warm path.
  5. Return, throw, display, or log actionable failure for: Terminate worker or corrupt cache entry and recover explicitly.
  6. Run both successful cases after failure repair; verify: Main thread stays responsive; ownership transfer/cache hit/lock ordering and recovery are logged.
11

PREDICT · INSPECT · BREAK · DEBUG · MEASURE

Interrogate reality

Prediction: write expected output, state transition, ordering, and failure evidence before running either exercise.

Inspection: Use event breakpoints, DOM breakpoints, Application storage panels, worker inspection, permissions, and performance traces.

Measurement: Measure main-thread blocking, storage cost, observer churn, worker transfer overhead, and lifecycle reliability.

INSPECT

Capture raw evidence before explaining.

BREAK

Change one assumption and force controlled failure.

DEBUG

Find cause with Chromium Sources · Application · Performance · Media before editing fix.

TOOL DRILL · keyboard only · record one retrievable command or shortcut
12

MASTERY + FRONTIER + BOUNDARY

Own the knowledge

TEACH

Explain service workers at beginner, intermediate, and senior depth.

REBUILD

Recreate smallest useful example from blank file without notes or AI.

RETRIEVE

Schedule recall for day 1, 7, 30, and 90.

Creative frontier lab

Try first without opening the solutions. The constraints invite invention; the reference gives one concrete direction, never the only valid answer.

Constraint inversion

Re-solve service workers by removing the most convenient abstraction. provide a reload-safe and offline-capable path before adding a server.

CONSTRAINTKeep the same inputs, observable result, and failure evidence; change the means, not the contract.

ORIGINAL IDEATurn subtraction into a design tool: the missing abstraction should reveal which responsibility it used to hide.

Reveal frontier solution
  1. Freeze the contract as three fixtures: Build service workers around this exact fixture: Send transferable data to worker, cache one response, and coordinate update with Web Lock. / Add service workers support for weekly boundary case: Reload offline and compare cold/warm path. / Make service workers surface—not hide—this failure: Terminate worker or corrupt cache entry and recover explicitly.
  2. List every convenience used by the starter; remove the highest-level one while preserving Serve the directory locally, open index.html, click Run, then inspect relevant browser panels..
  3. Implement the smallest replacement using provide a reload-safe and offline-capable path before adding a server.
  4. Run all fixtures and compare raw evidence. Keep the simpler version unless the removed abstraction has a demonstrated benefit.
Representation x-ray

Build an explanation artifact for service workers: expose lifecycle, permission, event, storage, and cross-tab transitions in an on-page event journal.

CONSTRAINTA peer must be able to locate the first divergence without reading implementation code.

ORIGINAL IDEATreat the explanation itself as a product: make invisible transitions visible, replayable, and diffable.

Reveal frontier solution
  1. Create one row or timestamped event for each transition in: Build service workers around this exact fixture: Send transferable data to worker, cache one response, and coordinate update with Web Lock.
  2. For every row record input, representation, owner, operation, output, and tool evidence from Chromium Sources · Application · Performance · Media.
  3. Replay Add service workers support for weekly boundary case: Reload offline and compare cold/warm path.; highlight only changed rows.
  4. Replay Make service workers surface—not hide—this failure: Terminate worker or corrupt cache entry and recover explicitly.; stop at the first divergent row and attach its recovery action.
Adversarial remix

Combine the boundary and failure into a new user-visible scenario for service workers. make two tabs coordinate without silently overwriting either user's state.

CONSTRAINTDo not merely add more input. Invent a recovery interaction, alternate representation, or self-checking behavior.

ORIGINAL IDEAMake the system teach its own limits: the artifact should expose the invariant and offer a safe next action when it breaks.

Reveal frontier solution
  1. Combine these two pressures without changing them: Add service workers support for weekly boundary case: Reload offline and compare cold/warm path. AND Make service workers surface—not hide—this failure: Terminate worker or corrupt cache entry and recover explicitly.
  2. Name the invariant that must survive and the user-visible evidence when it cannot: service workers handles baseline and boundary while preserving this observable result: Main thread stays responsive; ownership transfer/cache hit/lock ordering and recovery are logged.
  3. Implement this original direction: make two tabs coordinate without silently overwriting either user's state.
  4. Demonstrate baseline, combined failure, recovery, then baseline again; save the sequence as a regression fixture.

Capability frontier

Push service workers until another layer becomes justified. Record one robust technique, one contextual trade-off, and one labeled hack or historical curiosity.

CORE · PRACTICAL · CONTEXTUAL · HACK · FRAGILE · HISTORICAL · GOLF

Boundary

Local browser capabilities do not automatically provide shared truth, durable multi-device state, or trusted authority.

If this vanished tomorrow…

Use server round trips, file import/export, plain media elements, and lower-level platform primitives.

Why next layer is earned

Types are earned when growing browser state and contracts become difficult to keep valid by convention alone.