http2/writesched_common.go

// Copyright 2026 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package http2

// WriteScheduler is the interface implemented by HTTP/2 write schedulers.
// Methods are never called concurrently.
//
// Deprecated: User-provided write schedulers are deprecated.
type WriteScheduler interface {
	// OpenStream opens a new stream in the write scheduler.
	// It is illegal to call this with streamID=0 or with a streamID that is
	// already open -- the call may panic.
	OpenStream(streamID uint32, options OpenStreamOptions)

	// CloseStream closes a stream in the write scheduler. Any frames queued on
	// this stream should be discarded. It is illegal to call this on a stream
	// that is not open -- the call may panic.
	CloseStream(streamID uint32)

	// AdjustStream adjusts the priority of the given stream. This may be called
	// on a stream that has not yet been opened or has been closed. Note that
	// RFC 7540 allows PRIORITY frames to be sent on streams in any state. See:
	// https://tools.ietf.org/html/rfc7540#section-5.1
	AdjustStream(streamID uint32, priority PriorityParam)

	// Push queues a frame in the scheduler. In most cases, this will not be
	// called with wr.StreamID()!=0 unless that stream is currently open. The one
	// exception is RST_STREAM frames, which may be sent on idle or closed streams.
	Push(wr FrameWriteRequest)

	// Pop dequeues the next frame to write. Returns false if no frames can
	// be written. Frames with a given wr.StreamID() are Pop'd in the same
	// order they are Push'd, except RST_STREAM frames. No frames should be
	// discarded except by CloseStream.
	Pop() (wr FrameWriteRequest, ok bool)
}

// OpenStreamOptions specifies extra options for WriteScheduler.OpenStream.
//
// Deprecated: User-provided write schedulers are deprecated.
type OpenStreamOptions struct {
	// PusherID is zero if the stream was initiated by the client. Otherwise,
	// PusherID names the stream that pushed the newly opened stream.
	PusherID uint32
	// priority is used to set the priority of the newly opened stream.
	priority PriorityParam
}

// PriorityWriteSchedulerConfig configures a priorityWriteScheduler.
//
// Deprecated: User-provided write schedulers are deprecated.
type PriorityWriteSchedulerConfig struct {
	// MaxClosedNodesInTree controls the maximum number of closed streams to
	// retain in the priority tree. Setting this to zero saves a small amount
	// of memory at the cost of performance.
	//
	// See RFC 7540, Section 5.3.4:
	//   "It is possible for a stream to become closed while prioritization
	//   information ... is in transit. ... This potentially creates suboptimal
	//   prioritization, since the stream could be given a priority that is
	//   different from what is intended. To avoid these problems, an endpoint
	//   SHOULD retain stream prioritization state for a period after streams
	//   become closed. The longer state is retained, the lower the chance that
	//   streams are assigned incorrect or default priority values."
	MaxClosedNodesInTree int

	// MaxIdleNodesInTree controls the maximum number of idle streams to
	// retain in the priority tree. Setting this to zero saves a small amount
	// of memory at the cost of performance.
	//
	// See RFC 7540, Section 5.3.4:
	//   Similarly, streams that are in the "idle" state can be assigned
	//   priority or become a parent of other streams. This allows for the
	//   creation of a grouping node in the dependency tree, which enables
	//   more flexible expressions of priority. Idle streams begin with a
	//   default priority (Section 5.3.5).
	MaxIdleNodesInTree int

	// ThrottleOutOfOrderWrites enables write throttling to help ensure that
	// data is delivered in priority order. This works around a race where
	// stream B depends on stream A and both streams are about to call Write
	// to queue DATA frames. If B wins the race, a naive scheduler would eagerly
	// write as much data from B as possible, but this is suboptimal because A
	// is a higher-priority stream. With throttling enabled, we write a small
	// amount of data from B to minimize the amount of bandwidth that B can
	// steal from A.
	ThrottleOutOfOrderWrites bool
}