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Flow control

HTTP/2 defines two levels of flow control (RFC 9113 §5.2): the connection-level window and a per-stream window. PureHTTP2.jl models both as FlowControlWindow instances. The FlowController ties them together — it owns one connection window and a dictionary of stream windows keyed by stream ID, and its operations correctly decrement both windows when a stream sends or receives DATA.

Above that, DataSender and DataReceiver layer frame-size limits on top of the flow controller, splitting outgoing DATA into frames no larger than the peer's MAX_FRAME_SIZE.

Role signalling

Flow control is role-neutral. A FlowControlWindow is a sliding window regardless of who created it, and the FlowController distinguishes only connection-level from stream-level — never server from client. The apply_settings_initial_window_size! function responds to a peer's SETTINGS frame the same way whether that frame came from a server or a client.

Client-role code that sends DATA constructs the same DataSender shape as server-role code; the roles diverge only in which side originally advertises the initial window.

Window

PureHTTP2.FlowControlWindow — Type
FlowControlWindow(initial_size::Int = DEFAULT_INITIAL_WINDOW_SIZE)

A single HTTP/2 flow-control window (RFC 9113 §5.2). Can represent either a connection-level window or a stream-level window. Thread- safe via an internal ReentrantLock.

Fields

  • available::Int: Available window size
  • initial_size::Int: Initial window size
  • pending_updates::Int: Pending WINDOW_UPDATE bytes to send
  • lock::ReentrantLock: Thread-safe access

Example

julia> using PureHTTP2

julia> window = FlowControlWindow(65535);

julia> consume!(window, 1000)
true

julia> available(window)
64535

julia> release!(window, 1000);

julia> available(window)
65535
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Window operations

PureHTTP2.consume! — Function
consume!(window::FlowControlWindow, size::Int) -> Bool

Consume bytes from the flow control window. Returns true if consumption was successful, false if insufficient window.

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PureHTTP2.try_consume! — Function
try_consume!(window::FlowControlWindow, size::Int) -> Int

Try to consume up to size bytes from the window. Returns the actual number of bytes consumed.

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PureHTTP2.release! — Function
release!(window::FlowControlWindow, size::Int)

Release bytes back to the flow control window (for WINDOW_UPDATE).

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PureHTTP2.should_send_update — Function
should_send_update(window::FlowControlWindow; threshold_ratio::Float64=0.5) -> Bool

Check if a WINDOW_UPDATE should be sent based on pending updates.

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PureHTTP2.update_initial_size! — Function
update_initial_size!(window::FlowControlWindow, new_initial_size::Int)

Update the initial window size (from SETTINGS frame). Adjusts the current available window proportionally.

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Multi-stream controller

PureHTTP2.FlowController — Type
FlowController

Manages flow control for an HTTP/2 connection.

Fields

  • connection_window::FlowControlWindow: Connection-level window
  • stream_windows::Dict{UInt32, FlowControlWindow}: Per-stream windows
  • initial_stream_window::Int: Initial window size for new streams
  • lock::ReentrantLock: Thread-safe access to stream windows dict
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Controller operations

PureHTTP2.create_stream_window! — Function
create_stream_window!(controller::FlowController, stream_id::UInt32) -> FlowControlWindow

Create a flow control window for a new stream.

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PureHTTP2.get_stream_window — Function
get_stream_window(controller::FlowController, stream_id::UInt32) -> Union{FlowControlWindow, Nothing}

Get the flow control window for a stream.

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PureHTTP2.consume_send! — Function
consume_send!(controller::FlowController, stream_id::UInt32, size::Int) -> Bool

Consume bytes from both connection and stream windows for sending.

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PureHTTP2.max_sendable — Function
max_sendable(controller::FlowController, stream_id::UInt32) -> Int

Get the maximum sendable bytes considering both connection and stream windows.

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PureHTTP2.apply_window_update! — Function
apply_window_update!(controller::FlowController, stream_id::UInt32, increment::Int)

Apply a received WINDOW_UPDATE to the appropriate window. Stream ID 0 updates the connection window.

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PureHTTP2.generate_window_updates — Function
generate_window_updates(controller::FlowController;
                        threshold_ratio::Float64=0.5) -> Vector{Frame}

Generate WINDOW_UPDATE frames for windows that need updating.

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High-level senders and receivers

PureHTTP2.DataSender — Type
DataSender

Manages sending data with flow control and frame size limits.

Fields

  • controller::FlowController: Flow controller
  • max_frame_size::Int: Maximum frame payload size
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PureHTTP2.send_data_frames — Function
send_data_frames(sender::DataSender, stream_id::UInt32, data::Vector{UInt8};
                 end_stream::Bool=false) -> Vector{Frame}

Split data into frames respecting flow control and frame size limits. Returns empty vector if no data can be sent due to flow control.

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PureHTTP2.DataReceiver — Type
DataReceiver

Manages receiving data with flow control.

Fields

  • controller::FlowController: Flow controller
  • max_frame_size::Int: Maximum allowed frame payload size
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