/ 
Linux-Event-0.104
/ docs/STREAM-DESIGN.md

Stream design

Linux::Event::Stream is an owned buffered byte-stream object above the Linux::Event::Loop reactor. It is intentionally Linux-only and uses native code for repetitive I/O, queue, buffer, and built-in framing work.

Type model

Linux::Event::Stream is a base class. An application defines each stream type as an ordinary Perl package:

package ChatStream;
use parent 'Linux::Event::Stream';
use Linux::Event::Framer 'Delimiter', "\n";

sub on_message ($stream, $message) { $stream->send($message) }
sub on_drain ($stream) {
    $stream->data->{blocked} = 0 if $stream->data;
}
sub on_eof ($stream) { $stream->end }
sub on_error ($stream, $error) { warn "$error\n" }
sub on_close ($stream) {
    $stream->data->{closed} = 1 if $stream->data;
}

A package does not imply one global connection. It defines shared behavior. Each object remains a distinct connection and carries its application value in data:

my $stream = $loop->add(ChatStream->new(
    fh   => $socket,
    data => {
        user_id    => $user_id,
        permissions => $permissions,
        rooms       => {},
    },
));

The established constructor also accepts loop => $loop. Outbound ChatStream->connect(host => 'chat.example', port => 443) follows the same rule: pass loop for immediate attachment or omit it and use Loop->add(). Inbound acquisition belongs to Linux::Event::Listener, which is constructed with stream_class => 'ChatStream'.

connect() keeps one Stream identity across unattached, connecting, active, and closed states. See STREAM-CONNECTIONS.md, LISTENER-DESIGN.md, and OBJECT-LIFECYCLE.md.

Class descriptor

The first construction of each concrete subclass performs all class-level work once:

  1. resolve on_data, on_message, and optional lifecycle methods through the class MRO;
  2. resolve the nearest inherited native framer declaration;
  3. call and validate stream_options;
  4. validate that the class is either raw or framed, never both;
  5. create one immutable XS descriptor holding parser config, Stream policy, and callback references.

Subsequent construction retrieves that descriptor from the class cache. The Perl object and native connection state each retain a reference to it.

Immutable descriptor state includes:

Per-connection native state includes:

Per-connection Perl state includes the loop, handle, native registration, optional data, constructor timeout overrides, one optional operation deadline, effective acquisition socket policy, local/peer Addresses, and semantic lifecycle flags. It does not contain callback option hashes or a framer object.

Socket configuration

Stream applies constructor overrides over cached class socket policy before connect, accepted readiness, or adopted transport setup. TCP_NODELAY, keepalive tuning, TCP_USER_TIMEOUT, buffers, optional local source binding, and SO_BINDTODEVICE are supported. A cached configure_socket hook covers advanced Linux options without adding an open-ended constructor hash.

Established getter/setters read back effective kernel values. Acquisition policy remains attached to the connection and is not reapplied during protocol transition_to. See SOCKET-CONFIGURATION.md for the complete matrix and ordering relative to TLS and framing.

Established deadlines

Idle, read, and write inactivity policy is cached with each subclass and may be overridden per connection. One explicit overall-operation deadline may be set at construction or runtime. Established timing starts only after plain or TLS transport readiness; acquisition and TLS setup retain separate deadline owners.

At most one private Timer per Stream represents the earliest current condition. All such Timers share the Loop timerfd and native heap. Native I/O records successful progress only when inactivity policy is enabled, and does not enter Perl merely to move a deadline. The Timer callback validates the latest activity snapshot before expiring or rescheduling itself. See STREAM-DEADLINES.md for the public contract and reset semantics.

Input paths

All mechanical byte movement enters through the native transport boundary. The current plain provider specializes directly to fd syscalls; the parser and queue do not depend on that provider identity.

Raw subclass

EPOLLIN -> XS read drain -> on_data($stream, $bytes)

The reusable native read buffer is copied into a Perl byte scalar only after a successful read. XS continues until EAGAIN unless the callback pauses or closes the Stream.

Framed subclass

EPOLLIN -> XS read drain -> native input storage -> native parser
        -> on_message($stream, $message)

Complete built-in frames are detected without delivering intermediate read chunks to Perl. Multiple messages can be emitted from one readiness event. Partial prefixes, delimiters, and payloads remain in connection-local native state. The loop rechecks pause and close state after every callback.

Output path

write($bytes) first attempts an immediate native write. Partial output or EAGAIN creates independent native queue segments. EPOLLOUT drains those segments with writev() without first concatenating them in Perl.

Before attachment or connection readiness, accepted output remains in the Stream's Perl acquisition queue. pending_bytes includes those bytes and write() applies the same high-watermark rule as established native output. When acquisition transfers the queue into the transport, a blocked interval ends with exactly one on_drain after the Stream is ready and pending output has reached the low watermark. Producers can therefore use one backpressure contract before and after connection establishment.

Crossing high_watermark makes write() return false even though the bytes are accepted. When pending bytes reach low_watermark or less, XS clears the blocked state before calling on_drain, allowing reentrant writes to begin a new backpressure interval safely.

max_pending_bytes is a separate optional safety policy. Zero means unlimited. For a positive limit, XS checks the unsent remainder before allocating its queue segment. If the resulting pending count would exceed the limit, Stream does not queue the remainder, creates an output_limit error, calls on_error, and closes. A direct kernel write may have sent a prefix before a partial result reveals that the remainder is too large; the native queue itself never exceeds the configured limit. The error records both the attempted pending_bytes and the limit.

This does not change cooperative flow control. A false write() return still means all bytes were accepted and on_drain will end that blocked interval. Limit overflow is terminal and is distinguishable by its typed error.

send($payload) is framed-only. It applies the class's outbound built-in encoding and then uses write(). Writable half-close also belongs to the transport contract. Consequently end() expresses one lifecycle operation whether a provider maps it to socket shutdown(SHUT_WR) or a multi-step TLS close-notify exchange.

Protocol transitions

transition_to($class, input => $bytes) changes the protocol type of a live connection without reconstructing it. The Perl object is reblessed and its native state swaps one retained class descriptor reference for another. These connection-local resources remain unchanged:

The target descriptor supplies subsequent callbacks, parser configuration, read size, watermarks, hard output limit, input limit, and outbound send() framing. Existing queued output is already encoded and remains unchanged.

Native input storage is deliberately retained. A framed-to-framed transition reinterprets the unread suffix with the new native parser. A framed-to-raw transition delivers that suffix as one raw chunk. Raw callbacks receive their current kernel-read chunk directly, so they pass their unconsumed suffix with input => $bytes; it is appended after any native suffix.

Every native parser snapshots its descriptor before invoking on_message. If the callback transitions, that parser returns immediately. The input driver then resumes under the new descriptor without issuing another read(). This prevents old parser constants from being used after a reentrant descriptor swap and supports clients that pipeline new-protocol bytes with an upgrade request.

Transition validation and replacement are atomic. The new raw scratch buffer and optional preserved-input storage are allocated before live state changes. The target max_buffer is checked against existing plus explicit input. A nonzero target max_pending_bytes is also checked against existing queued output. On failure, the old Perl type, descriptor, and buffers remain active.

When called during on_data or on_message, target input callbacks are delayed until the old callback returns; callers should return immediately after the transition. Outside input dispatch, already-complete target input may be delivered before transition_to() returns. Pause state always gates delivery.

Lifecycle

Application callback exceptions propagate. They are not treated as transport or framing errors.

Class Stream policy

stream_options may return a key/value list or hash reference:

sub stream_options ($class) {
    return {
        read_size         => 65_536,
        high_watermark    => 1_048_576,
        low_watermark     => 262_144,
        max_pending_bytes => 0,
        max_buffer        => 8_388_608,
    };
}

It runs once per concrete subclass. Unknown options, invalid integers, and a low watermark above the high watermark fail before connection registration. The hard output limit is a non-negative byte count; zero disables it.

Framing policy

Native framing is declared by exact package name. No keyword registry, factory object, arbitrary next_frame object, or Buffer facade exists. This keeps the hot model singular: built-ins are native; application-specific rules use raw on_data.

A generally useful new family should add a named module and corresponding XS parser mode. The naming convention lets the loader find that package without a second list.

Performance consequences

The redesign improves construction and retained memory by moving immutable work from every object to one class descriptor. It removes per-connection callback hash entries, framer allocation, option parsing, repeated validation, and copies of native transport/framing configuration. Named callbacks also avoid fresh closure allocation and are invoked through cached CVs rather than method lookup.

It does not remove the intentional Perl crossing for semantic on_data or on_message work. It also does not make application state global: data, input storage, parser progress, output, and lifecycle remain connection-local.

Use bench/run-stream-lifecycle-bench.pl for constructor and retained-memory effects, bench/run-stream-microbench.pl for raw transport overhead and the cost of enabling a hard output limit, and the framing benchmarks for parser work.

The provider contract and the bundled TLS implementation are specified in TRANSPORT-BOUNDARY.md.