WRWEB ENGINEERING
REFORGING06 · 1/7
ALL LESSONS→WEEK 06→MONDAY

UNDERSTAND · HTTP + HYPERMEDIA

Cache control

Caching, identity, and trust30–60 MINUTESCORE + PRACTICAL
01

GROUND

Problem

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

HTTP and hypertext scale by exchanging representations through standardized methods, status codes, headers, links, and forms.
02

LEARN

Concept explanation

Cache control belongs to “Caching, identity, and trust”. Caches reuse representations only while freshness and validators permit; identity and trust travel in metadata.

For cache control, trace concrete input, state transition, output, and failure through a request/response contract whose method, status, headers, representation, and next actions are observable.

A client sends a request; a server returns a representation plus metadata. Hypermedia controls advertise valid next actions. Apply that model to supplied normal, boundary, and failure cases; each case below names its input and expected evidence.

Observable

Evidence produced by the cache control experiment: output, state, trace, bytes, timing, or diagnostics.

Invariant

Condition that must remain true while inputs or implementation of cache control change.

Boundary

Point where cache control 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

SETUPRecord ETag or Last-Modified, then make a conditional request.

OBSERVEValid conditional request returns 304 or equivalent avoided body; incorrect validator demonstrates stale reuse.

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

Boundary · same contract, harder input

SETUPCompare fresh, stale, and no-store directives in a tiny local response fixture.

OBSERVERecord what remains invariant and the first representation, owner, size, or timing value that changes in curl · Chromium Network · server logs.

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

Failure · evidence before repair

SETUPChange representation without changing validator and document stale-result bug.

OBSERVECapture the first divergence from the baseline, including exact input, diagnostic, state, and recovery result. Expected recovery: Valid conditional request returns 304 or equivalent avoided body; incorrect validator demonstrates stale reuse.

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

Cross-layer · follow ownership

SETUPTrace cache control one layer below its usual abstraction through a request/response contract whose method, status, headers, representation, and next actions are observable.

OBSERVECompare curl -v output, request method, status, cache metadata, redirects, and representation bytes.

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

03

SEE

Worked example

Start from supplied lab.sh. Focus: Record ETag or Last-Modified, then make a conditional request.

  1. Run: chmod +x lab.sh && ./lab.sh
  2. Save baseline evidence. Compare curl -v output, request method, status, cache metadata, redirects, and representation bytes.
  3. Boundary case: Compare fresh, stale, and no-store directives in a tiny local response fixture.
  4. Failure case: Change representation without changing validator and document stale-result bug.
RESULT
Valid conditional request returns 304 or equivalent avoided body; incorrect validator demonstrates stale reuse. Starter-level baseline: headers.txt begins with an HTTP status line; body.txt contains the representation; byte count is greater than zero.
04

START HERE

Starter material

PREREQUISITEScurl plus a POSIX shell. Baseline uses public example.com and needs no account or API key.

ONE-TIME SETUPmkdir reforging-http && cd reforging-http

Create lab.sh, paste this exact content, then run the command below.

#!/usr/bin/env sh
set -eu
url="${1:-https://example.com/}"
printf '%s\n' '# cache control'
curl -sS -D headers.txt -o body.txt "$url"
sed -n '1,12p' headers.txt
printf 'body bytes: ' && wc -c < body.txt
RUNchmod +x lab.sh && ./lab.sh

STOP / CLEANUPNo background process. Remove headers.txt and body.txt only after saving evidence.

05

DO WITH GUIDANCE

Guided exercise

Observe one rule: cache control

  1. Normal case: Record ETag or Last-Modified, then make a conditional request.
  2. Write predicted evidence from this named case before running starter.
  3. Change one input while holding environment constant.
  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 lab.sh unchanged and run: chmod +x lab.sh && ./lab.sh
  2. Write this prediction before inspecting output: Valid conditional request returns 304 or equivalent avoided body; incorrect validator demonstrates stale reuse.
  3. Perform only the named normal case: Record ETag or Last-Modified, then make a conditional request.
  4. Save the raw output, then annotate input → transition → evidence. Use curl · Chromium Network · server logs 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: headers.txt begins with an HTTP status line; body.txt contains the representation; byte count is greater than zero.
06

DO ALONE

Independent exercise

Find the boundary: cache control

  1. Create second case from blank file: Compare fresh, stale, and no-store directives in a tiny local response fixture.
  2. Then create controlled failure: Change representation without changing validator and document stale-result bug.
  3. Use curl · Chromium Network · server logs 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: Compare fresh, stale, and no-store directives in a tiny local response fixture.
  2. Save its evidence beside the baseline and identify the first changed value. Compare curl -v output, request method, status, cache metadata, redirects, and representation bytes.
  3. Create the exact controlled failure: Change representation without changing validator and document stale-result bug.
  4. Reproduce baseline exactly: Record ETag or Last-Modified, then make a conditional request.
  5. Write observation table with columns input, state transition, output, and failure for cache control.
  6. Run boundary case unchanged: Compare fresh, stale, and no-store directives in a tiny local response fixture.
  7. Trigger controlled failure: Change representation without changing validator and document stale-result bug.
  8. Compare evidence with reference outcome: Valid conditional request returns 304 or equivalent avoided body; incorrect validator demonstrates stale reuse.
  9. Rerun baseline, boundary, and repaired failure together. Accept only if all reproduce: Valid conditional request returns 304 or equivalent avoided body; incorrect validator demonstrates stale reuse.
07

COMPARE

Expected result

  • Valid conditional request returns 304 or equivalent avoided body; incorrect validator demonstrates stale reuse.
  • headers.txt begins with an HTTP status line; body.txt contains the representation; byte count is greater than zero.
  • Controlled cache control 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: Record ETag or Last-Modified, then make a conditional request.
  2. For boundary case, change only named dimension: Compare fresh, stale, and no-store directives in a tiny local response fixture.
  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: headers.txt begins with an HTTP status line; body.txt contains the representation; byte count is greater than zero.
  2. Reproduce baseline exactly: Record ETag or Last-Modified, then make a conditional request.
  3. Write observation table with columns input, state transition, output, and failure for cache control.
  4. Run boundary case unchanged: Compare fresh, stale, and no-store directives in a tiny local response fixture.
  5. Trigger controlled failure: Change representation without changing validator and document stale-result bug.
  6. Compare evidence with reference outcome: Valid conditional request returns 304 or equivalent avoided body; incorrect validator demonstrates stale reuse.
11

PREDICT · INSPECT · BREAK · DEBUG · MEASURE

Interrogate reality

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

Inspection: Use curl verbose mode and Chromium Network to trace methods, redirects, caching, cookies, payloads, initiators, and timing.

Measurement: Count requests, bytes, round trips, cache hits, server time, and client synchronization code.

INSPECT

Capture raw evidence before explaining.

BREAK

Change one assumption and force controlled failure.

DEBUG

Find cause with curl · Chromium Network · server logs before editing fix.

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

MASTERY + FRONTIER + BOUNDARY

Own the knowledge

TEACH

Explain cache control 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 cache control by removing the most convenient abstraction. complete the workflow using links, forms, redirects, and status codes with JavaScript disabled.

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: Record ETag or Last-Modified, then make a conditional request. / Compare fresh, stale, and no-store directives in a tiny local response fixture. / Change representation without changing validator and document stale-result bug.
  2. List every convenience used by the starter; remove the highest-level one while preserving chmod +x lab.sh && ./lab.sh.
  3. Implement the smallest replacement using complete the workflow using links, forms, redirects, and status codes with JavaScript disabled.
  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 cache control: save a complete request/response transcript that another person can replay.

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: Record ETag or Last-Modified, then make a conditional request.
  2. For every row record input, representation, owner, operation, output, and tool evidence from curl · Chromium Network · server logs.
  3. Replay Compare fresh, stale, and no-store directives in a tiny local response fixture.; highlight only changed rows.
  4. Replay Change representation without changing validator and document stale-result bug.; 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 cache control. make every failure response advertise a valid recovery action.

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: Compare fresh, stale, and no-store directives in a tiny local response fixture. AND Change representation without changing validator and document stale-result bug.
  2. Name the invariant that must survive and the user-visible evidence when it cannot: Valid conditional request returns 304 or equivalent avoided body; incorrect validator demonstrates stale reuse.
  3. Implement this original direction: make every failure response advertise a valid recovery action.
  4. Demonstrate baseline, combined failure, recovery, then baseline again; save the sequence as a regression fixture.

Capability frontier

Push cache control 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

Hypermedia becomes strained when interactions require high-frequency local manipulation or long-lived peer state.

If this vanished tomorrow…

Use URLs, links, forms, server-rendered documents, status codes, and explicit refresh.

Why next layer is earned

HTML is earned when the representation must become a navigable, semantic document.