Backpressure For Streaming Pipelines

Last time we showed how to terminate non-cooperative CPU work at the worker boundary. This article stays cooperative but adds the missing piece: backpressure, the runtime contract that lets a producer pause the moment the consumer can't keep up.

A RAG ingest pipeline has a billion candidate documents. You only need the 25 that match a downstream filter. A naive promise collection can materialize far more work than the consumer needs; a hand-rolled async iterator can still fill a prefetch buffer before the first result arrives. With WorkIt:

import { work } from "@workit/core";

async function* billionDocuments() {
  for (let i = 0; i < 1_000_000_000; i++) yield { id: i, text: `doc ${i}` };
}

const results = [];
for await (const processed of work(billionDocuments())
  .inParallel(16)
  .map(async (doc, ctx) => enrich(doc, { signal: ctx.signal }))
  .stream()) {
  results.push(processed);
  if (results.length === 25) break;
}

Two things to notice:

• work() accepts an async iterable directly. No .from(), no Readable.from(...) shim. The signature is Iterable<I> | AsyncIterable<I> -> WorkBuilder<I, I>.
• .map().stream() is the streaming pipeline form. .do(fn) returns a Promise<WorkOutput<R>> (full batch result). .map(fn) returns a new builder; .stream() on a builder returns an AsyncIterable<O> that respects backpressure. Both terminals exist; you pick by what the consumer is doing.

What the producer actually does:

Bench 09-stream-1b-lazy.mjs. 1,000,000,000-row generator. inParallel(16). Consumer takes 25, breaks.

Implementation	Consumed	Items pulled from the generator	maxActive	In-flight after break
Naïve eager prefetch buffer (256-deep)	25	281	1	0 (all let to settle)
work().inParallel(16).map().stream()	25	40	1	0 (cancelled at break)

These are representative captured values. The bench asserts the invariant: produced items stay bounded by TAKE + CONCURRENCY. The naïve baseline pulled 281 items because once the prefetch buffer is full it doesn't pause the producer -- it pauses the worker pool, which is a different question.

That's backpressure: the producer pauses when the consumer slows down or stops, not when the worker pool fills.

work().stream() -- bounded, lazy, cancellable

for await (const summary of work(documents)
  .inParallel(8)
  .withRetry(2)
  .withTimeout("15s")
  .map(async (doc, ctx) => summarize(doc, { signal: ctx.signal }))
  .stream()) {
  ui.append(summary);
}

Properties the runtime guarantees:

• inParallel(N) is a hard cap. maxActive never exceeds N. Property test runs 1..20 wide x 1..100 items, asserts the cap holds across every shape.
• stream() is lazy. The producer iterator pulls only when an inflight slot is free.
• break is cancellation. The remaining inflight tasks abort with CancelReason { kind: "manual", tag: "stream_consumer_closed" }. Their ctx.defer runs. The producer iterator's return() runs.
• A throw inside the body triggers CancelReason { kind: "manual", tag: "stream_failed" } for siblings -- typed, distinguishable from the consumer-break path on a dashboard.
• Slow consumer pauses producer. Tracked under check:stream-memory: 1,000,000 logical items, slow consumer, bounded heap growth, and no unbounded producer advance.

Bench 10-stream-slow-consumer.mjs. 5,000-item source, inParallel(16), consumer ~5 ms per item, take 200.

Metric	Value
Consumed	200
Produced	215
Producer overshoot	15 (bound: CONCURRENCY + 1 = 17)
maxActive	1
In-flight after break	0
Wall time	~3,108 ms

The interesting detail: even with inParallel(16), maxActive stayed at 1 because the consumer was the bottleneck. The runtime didn't speculatively saturate the worker pool -- it paced the producer to consumer demand. That is what "backpressure" actually means. A pool that always runs at capacity isn't backpressure; it's a pool.

Streaming map: stop after 12, produce only what demand requires

The most practical reader-facing form of the same property -- a real summarizer pipeline, the size of a real prompt:

// samples/streaming-summarizer.sample.js
const TAKE = 12;
const CONCURRENCY = 5;

for await (const summary of work(documents())
  .inParallel(CONCURRENCY)
  .withRetry(2)
  .withTimeout("500ms")
  .map(async (doc, ctx) => `summary:${doc.id}`)
  .stream()) {
  summaries.push(summary);
  if (summaries.length === TAKE) break;
}

// Asserted by the sample:
//   summaries.length === TAKE
//   produced     <= TAKE + CONCURRENCY - 1
//   maxActive    === CONCURRENCY
//   active       === 0       // all in-flight cancelled cleanly on break

50-doc generator. Consume 12. Producer never advances past 16. Concurrency cap exact. Active count zero after break. Retry and timeout policy attached without breaking the pull cadence.

npm run sample:stream

Defaults that don't surprise

You opt into resilience. Nothing is implicit.

CSP-style channels -- @workit/core/channel

work().stream() is the right shape when the producer-consumer relationship is one fluent pipeline. When the producer and consumer are independent tasks running side by side -- fan-in, fan-out, work-queue -- you want a channel.

import { createChannel } from "@workit/core/channel";
import { group } from "@workit/core";

const orders = createChannel<Order>({ capacity: 100 });

await group(async (task) => {
  task(async (ctx) => {
    for await (const o of orderSource()) {
      await orders.send(o, { signal: ctx.signal });
    }
    orders.close();
  });

  task(async (ctx) => {
    for await (const o of orders) {
      await processOrder(o, { signal: ctx.signal });
    }
  });
});

Channel contract, all five rows verified by 11-channel-contract.mjs:

Cancellation composes with the parent scope. If the consumer task throws inside group, sibling cancellation aborts the producer's pending send. The producer's for await exits cleanly through the rejection. No orphaned sends, no leaked consumers, no half-drained buffer.

This is Go's chan with structured-concurrency parents. Kotlin's Channel without coroutines. It fills the gap between "raw async iterator" and "RxJS observable" for owned producer-consumer work.

Bad-batch bisection -- one rotten document doesn't poison the embedding

A real RAG pipeline failure mode: the provider returns 400 for a mixed batch because one of the documents is malformed. With Promise.all, the whole batch fails, the budget is spent on nothing, and the next 99 documents get re-embedded on retry.

WorkIt ships embedAllBisection that splits the failed batch and recovers the good vectors:

// samples/embed-bisection.sample.js
const result = await group(
  async () => embedAllBisection(["alpha", "bad-doc", "gamma"], {
    async embedBatch(inputs) {
      if (inputs.includes("bad-doc")) throw new BadBatchError("provider rejected mixed batch");
      return inputs.map((input) => [input.length]);
    },
  }, {
    batchSize:   3,
    onError:     "continue",
    countTokens: (input) => input.length,
  }),
  { context }
);

// Asserted by the sample:
//   result.results contains the vectors for "alpha" and "gamma"
//   result.errors  contains exactly one entry pointing at "bad-doc"
//   tokensSpent reflects only the successful work

BadBatchError is the contract. Throw it from embedBatch and the helper bisects: split the batch in halves, retry each half, isolate the rotten document, keep the good vectors. Token budget accounting follows the actual successful work -- you don't pay for the failed mixed batch twice.

npm run sample:bisection

This is the difference between "batch job dies at 2 a.m. and the on-call resyncs the warehouse" and "batch job logs the bad ID and keeps going."

Streaming STT with disconnect cleanup (revisited)

Article 1 showed this. Now you can read the backpressure underneath it:

import { transcribeStream } from "@workit/core/ai";

for await (const text of transcribeStream(microphone, {
  async transcribe(chunk, ctx) {
    return provider.transcribe(chunk, { signal: ctx.signal });
  },
}, { signal: socket.signal })) {
  socket.send(text);
}

When the user closes their laptop:

1. socket.signal aborts.
2. transcribeStream propagates the abort to the inflight transcribe() body.
3. The provider's HTTP request aborts at the AbortSignal boundary.
4. The async generator's finally runs, closing the microphone source.
5. The for await loop exits.

Tracked sample: sample:stt-disconnect -- disconnects mid-second-chunk, asserts the provider was cancelled, the source was closed, and the cancel reason kind is manual.

How WorkIt's streaming primitives compare

WorkIt's streaming and channel primitives are the only ones in the table that tie backpressure to ownership -- cancel the scope, the channel closes, the in-flight work aborts, and cleanup runs.

Receipts

node benchmarks/articles/09-stream-1b-lazy.mjs        # naive 281 vs WorkIt 40
node benchmarks/articles/10-stream-slow-consumer.mjs  # producer overshoot 15 vs bound 17
node benchmarks/articles/11-channel-contract.mjs      # 5 channel scenarios
node benchmarks/articles/run-all.mjs                  # full article suite

Production-side gates that back the same primitives:

Run them:

npm run sample:1b
npm run sample:stream
npm run sample:embed100k
npm run sample:bisection
npm run sample:stt-disconnect

What's coming

Now you have a producer that paces itself to the consumer, a channel that closes when its scope cancels, and a stream that exits cleanly when the user closes the tab.

Tomorrow we add the next ownership primitive on top: the budget.

A $0.50 CostBudget. A 100,000-token OpenAITokens. A 5-tool-call AgentToolCalls. Atomic across all parallel children. Inheritable through scope context. Shadowed by inner scopes for sub-budgets. Overrun cancels with CancelReason { kind: "budget" } and partial results stay.

The runtime change underneath this is context overlay lookup: 100 .with() calls over a 5,000-key context bag moved from tens of milliseconds in the inline clone baseline to well under the 10 ms gate, without changing a line of public API. The bench in the next article shows the representative timing.

The point is not simply "we have budgets." Many frameworks expose budgets. The stronger claim is budgets that compose with cancellation, race, retry, hedge, fallback, channels, and streams under one ownership tree.

Source, Benchmarks, And Evidence

• Source: https://github.com/WorkRuntime/workit
• Article source: https://github.com/WorkRuntime/workit/blob/main/articles/04-backpressure-for-streaming-pipelines.md
• Reproduce: npm run bench:articles and npm run test:evidence