Idempotency

Problem

What becomes confusing, fragile, or impossible without understanding idempotency? 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.

Concept explanation

Idempotency belongs to “Caching, identity, and trust”. idempotency is inspected by introducing one realistic failure and proving root cause before repair.

For idempotency, 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.

Example bank

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

Worked example

Start from supplied lab.sh. Focus: Capture healthy idempotency baseline from: 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: Record idempotency behavior at: Compare fresh, stale, and no-store directives in a tiny local response fixture.
4. Failure case: Diagnose this exact idempotency failure before editing: Change representation without changing validator and document stale-result bug.

RESULT
Evidence identifies first divergence, repair changes only proven cause, and rerun restores: 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.

Starter material

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

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

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

Independent exercise

Prove root cause: idempotency

1. Create second case from blank file: Record idempotency behavior at: Compare fresh, stale, and no-store directives in a tiny local response fixture.
2. Then create controlled failure: Diagnose this exact idempotency failure before editing: 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.

Expected result

• Evidence identifies first divergence, repair changes only proven cause, and rerun restores: 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 idempotency failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.