Backpressure

Problem

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

Event loops let one thread coordinate many delayed operations. Promises standardized continuation and error composition over callback-heavy code.

Concept explanation

Backpressure belongs to “Cancellation, streams, backpressure”. backpressure is inspected by introducing one realistic failure and proving root cause before repair.

For backpressure, trace concrete input, state transition, output, and failure through a state-and-time rule spanning synchronous frames, host tasks, microtasks, rendering, cancellation, or backpressure.

Synchronous frames run to completion. Hosts schedule tasks; promise reactions enter microtask queues; rendering occurs only at host-selected opportunities. 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.mjs. Focus: Capture healthy backpressure baseline from: Stream numbered chunks through transform to slow writable with AbortSignal.

1. Run: node lab.mjs
2. Save baseline evidence. Record ordering, async stacks, queue delay, cancellation signals, stream state, and cleanup paths.
3. Boundary case: Record backpressure behavior at: Compare desiredSize and memory with fast versus slow consumer.
4. Failure case: Diagnose this exact backpressure failure before editing: Abort mid-stream and prove every resource closes once.

RESULT
Evidence identifies first divergence, repair changes only proven cause, and rerun restores: Bounded flow honors pressure; abort reason propagates; cleanup trace has no duplicate or leaked ownership. Starter-level baseline: Order ends as sync:start > sync:end > microtask > task. Each line is a prefix of that final sequence.

Starter material

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

const events = [];
const record = (value) => { events.push(value); console.log(events.join(" > ")); };

record("sync:start");
queueMicrotask(() => record("microtask:backpressure"));
setTimeout(() => record("task:backpressure"), 0);
record("sync:end");

STOP / CLEANUPScripts exit after queued work completes. Press Ctrl+C only if your experiment creates an intentional infinite loop.

Independent exercise

Prove root cause: backpressure

1. Create second case from blank file: Record backpressure behavior at: Compare desiredSize and memory with fast versus slow consumer.
2. Then create controlled failure: Diagnose this exact backpressure failure before editing: Abort mid-stream and prove every resource closes once.
3. Use Chromium Performance · Network · async debugger 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: Bounded flow honors pressure; abort reason propagates; cleanup trace has no duplicate or leaked ownership.
• Order ends as sync:start > sync:end > microtask > task. Each line is a prefix of that final sequence.
• Controlled backpressure failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.