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In RFC 793, the 1981 TCP specification, a receiver advertises a window stating how many bytes it can currently accept. The sender takes that limit into account even if it has more data ready. Protocols communicated receiver capacity to senders before asynchronous stream libraries appeared. ~~RFC 793~~ ~~§2.6.~~

In RFC 793, the 1981 TCP specification, a receiver advertises a window stating how many bytes it can currently accept. The sender takes that limit into account even if it has more data ready. Protocols communicated receiver capacity to senders before asynchronous stream libraries appeared.[Footnote 1]

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Networking also distinguishes flow control from congestion control. A receiver advertises its own capacity; the sender's congestion control governs what the network path can carry. RFC 9293, the current TCP base specification, distinguishes the send window from the congestion window. ~~RFC 9293~~ ~~§§3.8.~~2 ~~and 3.8.6.~~

Networking also distinguishes flow control from congestion control. A receiver advertises its own capacity; the sender's congestion control governs what the network path can carry. RFC 9293, the current TCP base specification, distinguishes the send window from the congestion window.[Footnote 2]

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In their 2001 SEDA paper, Welsh, Culler, and Brewer divided a server into processing stages and made the queues between them explicit. Each stage could inspect its backlog and adjust its thread count or processing batch. ~~SEDA paper~~ §3~~, publication record.~~

In their 2001 SEDA paper, Welsh, Culler, and Brewer divided a server into processing stages and made the queues between them explicit. Each stage could inspect its backlog and adjust its thread count or processing batch.[Footnote 3]

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The Reactive Streams participants defined interfaces so asynchronous stream libraries could exchange demand and cancellation under common rules. The commit tagged 1.0.0 dates from April 2015, and the official announcement states their goal of standardizing asynchronous stream processing with non-blocking backpressure. ~~1.0.0 record, official~~ ~~announcement.~~

The Reactive Streams participants defined interfaces so asynchronous stream libraries could exchange demand and cancellation under common rules. The commit tagged 1.0.0 dates from April 2015, and the official announcement states their goal of standardizing asynchronous stream processing with non-blocking backpressure.[Footnote 4]

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The opening sentence of the official introduction summarizes the participants' goal, while the specification defines the contract for each signal. ~~Reactive Streams~~ ~~specification.~~ A Publisher sends elements and a Subscriber receives them. The Subscriber calls request(n) on its Subscription to say how many more it can accept. Publisher rule 1.1 prevents cumulative delivery from exceeding cumulative demand, and Subscription rule 3.8 adds each new request to the remaining demand. ~~Reactive Streams 1.0.4 rules 1.1 and~~ ~~3.8.~~

The opening sentence of the official introduction summarizes the participants' goal, while the specification defines the contract for each signal.[Footnote 5] A Publisher sends elements and a Subscriber receives them. The Subscriber calls request(n) on its Subscription to say how many more it can accept. Publisher rule 1.1 prevents cumulative delivery from exceeding cumulative demand, and Subscription rule 3.8 adds each new request to the remaining demand.[Footnote 6]

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While remaining demand is zero, the Publisher sends no additional data elements. New requests use a positive number; request(0) is invalid, not a pause command. Completion and error are separate signals from data elements, so they may be delivered even when no demand remains. ~~Specification rules 1.2 and~~ ~~3.9.~~ Java 9 also provides Publisher, Subscriber, Subscription, and Processor through java.util.concurrent.Flow. Its documentation describes these interfaces as corresponding to Reactive Streams. Library authors must match the contracts for demand, cancellation, and termination as well as the method for sending data if their components are to connect. ~~Java 9 Flow, Subscription~~ ~~contract.~~

While remaining demand is zero, the Publisher sends no additional data elements. New requests use a positive number; request(0) is invalid, not a pause command. Completion and error are separate signals from data elements, so they may be delivered even when no demand remains.[Footnote 7] Java 9 also provides Publisher, Subscriber, Subscription, and Processor through java.util.concurrent.Flow. Its documentation describes these interfaces as corresponding to Reactive Streams. Library authors must match the contracts for demand, cancellation, and termination as well as the method for sending data if their components are to connect.[Footnote 8]

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In a blocking implementation, a thread may wait. In an asynchronous implementation, the runtime can run other work while a coroutine is suspended. Reactive Streams requires non-blocking exchange of control signals when connecting asynchronous components, so a thread need not be held waiting on that exchange. ~~Problem and scope in the official~~ ~~introduction.~~

In a blocking implementation, a thread may wait. In an asynchronous implementation, the runtime can run other work while a coroutine is suspended. Reactive Streams requires non-blocking exchange of control signals when connecting asynchronous components, so a thread need not be held waiting on that exchange.[Footnote 9]

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The following runs in Python 3.11 or later. The producer puts values into a queue with maxsize=2, awaiting each put. When the queue fills, await queue.put() suspends the producer coroutine, and the event loop can run other work such as the consumer. ~~asyncio.Queue.~~

The following runs in Python 3.11 or later. The producer puts values into a queue with maxsize=2, awaiting each put. When the queue fills, await queue.put() suspends the producer coroutine, and the event loop can run other work such as the consumer.[Footnote 10]

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If the consumer fails with an ordinary exception, TaskGroup also cancels the producer. To stop both tasks together, use a path that ends their shared lifetime, such as cancelling the parent task. Cancellation of an individual consumer with CancelledError is handled differently from an ordinary child failure. ~~TaskGroup failure and cancellation~~ ~~rules.~~

If the consumer fails with an ordinary exception, TaskGroup also cancels the producer. To stop both tasks together, use a path that ends their shared lifetime, such as cancelling the parent task. Cancellation of an individual consumer with CancelledError is handled differently from an ordinary child failure.[Footnote 11]

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Compare immediate rejection when the queue is full. This Python 3 example uses put_nowait: instead of waiting to admit the second item, it receives QueueFull and reports that the item was not accepted. The final read returns first, the item already in the queue. ~~put_nowait~~ ~~contract.~~

Compare immediate rejection when the queue is full. This Python 3 example uses put_nowait: instead of waiting to admit the second item, it receives QueueFull and reports that the item was not accepted. The final read returns first, the item already in the queue.[Footnote 12]

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Some sources, such as sensor measurements or mouse movements, cannot slow the rate at which input originates. At the stream boundary, an application can briefly buffer, combine values, or drop some of them. The Reactive Streams specification likewise says buffering or dropping may be needed to respect demand bounds when the original source cannot be slowed. ~~Subscriber-controlled queue~~ ~~bounds.~~

Some sources, such as sensor measurements or mouse movements, cannot slow the rate at which input originates. At the stream boundary, an application can briefly buffer, combine values, or drop some of them. The Reactive Streams specification likewise says buffering or dropping may be needed to respect demand bounds when the original source cannot be slowed.[Footnote 13]

Footnote 1

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RFC 793 (1981). [RFC 793 §2.6](https://www.rfc-editor.org/rfc/rfc793.html#section-2.6).

Footnote 2

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RFC 9293 (2022). [RFC 9293 §§3.8.2 and 3.8.6](https://www.rfc-editor.org/rfc/rfc9293.html#section-3.8.6).

Footnote 3

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Welsh, Culler, Brewer. SEDA: An Architecture for Well-Conditioned, Scalable Internet Services (2001). [SEDA paper §3](https://www.sosp.org/2001/papers/welsh.pdf#page=4); [publication record](https://doi.org/10.1145/502059.502057).

Footnote 4

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Reactive Streams 1.0.0. [1.0.0 record](https://github.com/reactive-streams/reactive-streams-jvm/releases/tag/v1.0.0); [official announcement](https://www.reactive-streams.org/announce-1.0.0).

Footnote 5

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Reactive Streams 1.0.4. [Reactive Streams specification](https://github.com/reactive-streams/reactive-streams-jvm/tree/v1.0.4).

Footnote 6

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Reactive Streams 1.0.4. [Reactive Streams 1.0.4 rules 1.1 and 3.8](https://github.com/reactive-streams/reactive-streams-jvm/blob/v1.0.4/README.md#specification).

Footnote 7

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Reactive Streams 1.0.4. [Specification rules 1.2 and 3.9](https://github.com/reactive-streams/reactive-streams-jvm/blob/v1.0.4/README.md#specification).

Footnote 8

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Oracle. Java SE 9 API. [Java 9 Flow](https://docs.oracle.com/javase/9/docs/api/java/util/concurrent/Flow.html); [Subscription contract](https://docs.oracle.com/javase/9/docs/api/java/util/concurrent/Flow.Subscription.html).

Footnote 9

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Reactive Streams. [Problem and scope in the official introduction](https://www.reactive-streams.org/).

Footnote 10

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Python 3.12. [asyncio.Queue](https://docs.python.org/3.12/library/asyncio-queue.html#asyncio.Queue).

Footnote 11

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Python 3.12. [TaskGroup failure and cancellation rules](https://docs.python.org/3.12/library/asyncio-task.html#task-groups).

Footnote 12

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Python 3.12. [put_nowait contract](https://docs.python.org/3.12/library/asyncio-queue.html#asyncio.Queue.put_nowait).

Footnote 13

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Reactive Streams 1.0.4. [Subscriber-controlled queue bounds](https://github.com/reactive-streams/reactive-streams-jvm/blob/v1.0.4/README.md#subscriber-controlled-queue-bounds).

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Backpressure

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