=encoding UTF-8

=head1 NAME

PAGI::Spec::Lifespan - The PAGI Lifespan protocol for startup and shutdown events

=head1 Lifespan Protocol

B<Version>: 0.3 (Draft)

The Lifespan B<PAGI> sub-specification outlines how to communicate
lifespan events such as startup and shutdown within PAGI.

The lifespan messages allow for an application to initialise and
shutdown in the context of a running event loop. An example of this
would be creating a connection pool and subsequently closing the
connection pool to release the connections.

=head2 Execution Model

Lifespan events B<must be sent by every PAGI server instance or worker>
that manages an application event loop. This ensures that application-level
resources are initialized and cleaned up in the context in which they run.

=over

=item -

In B<multi-process deployments>, each worker process must independently
send C<lifespan.startup> on init and C<lifespan.shutdown> on exit.

=item -

In B<in-process async mode>, the main event loop will send the events.

=back

This matches the ASGI model and ensures that applications have the opportunity
to manage state correctly in async-safe scopes.

Lifespans and requests B<must occur on the same event loop>. This guarantees
that async-local state (such as DB pools or background tasks) is not shared
unsafely across threads or processes.

A possible implementation of this protocol in B<modern Perl> using C<Future::AsyncAwait>
could look like this:

    use strict;
    use warnings;
    use Future::AsyncAwait;

    async sub app {
        my ($scope, $receive, $send) = @_;
        # Only handle 'lifespan' protocol
        if ($scope->{type} eq 'lifespan') {

            while (1) {
                my $message = await $receive->();

                if ($message->{type} eq 'lifespan.startup') {
                    # ... Do some startup actions here ...
                    await $send->({
                        type => 'lifespan.startup.complete',
                    });
                }
                elsif ($message->{type} eq 'lifespan.shutdown') {
                    # ... Do some shutdown/cleanup actions here ...
                    await $send->({
                        type => 'lifespan.shutdown.complete',
                    });
                    return; # End the loop
                }
                else {
                    # Unknown or additional message type; you can handle it or ignore it
                }
            }

        }
        else {
            die "Unsupported protocol type: $scope->{type}";
        }
    }

=head2 Scope

The lifespan scope exists for the duration of the event loop.

The scope information passed in C<scope> contains basic metadata:

=over

=item -

C<type> (String) - C<"lifespan">.

=item -

The C<version> key of the C<pagi> hashref (accessed as C<< $scope->{pagi}{version} >>) (String) - The version of the PAGI core spec that governs the worker.

=item -

The C<spec_version> key of the C<pagi> hashref (accessed as
C<< $scope->{pagi}{spec_version} >>) (String) - The version of this lifespan
sub-specification. Optional; if missing, applications MUST assume C<'0.1'>.
Omitting it declares support for version 0.1 only, so the server MUST NOT use
behaviour introduced by a later lifespan version. The core PAGI C<version>
does not affect this default.

=item -

The C<is_worker> key of the C<pagi> hashref (accessed as C<< $scope->{pagi}{is_worker} >>) (Int, optional) - C<1> when running as a worker in a multi-process deployment; C<0> in single-process mode. May be absent on servers that do not distinguish workers. Applications can use this to avoid duplicate initialization (e.g., only print startup messages from worker 1).

=item -

The C<worker_num> key of the C<pagi> hashref (accessed as C<< $scope->{pagi}{worker_num} >>) (Int, optional) - Worker identifier (1, 2, 3, ...) in multi-process mode. Absent or C<undef> for single-process mode. Combined with C<is_worker>, applications can identify which worker they are running in.

=item -

C<state> (HashRef, optional) - A namespace where the application can persist values to be copied (as a shallow copy; see L</Lifespan State>) into subsequent request scopes. Servers omit this key if the feature is unsupported.

=back

If the application B<declines> the lifespan protocol -- either by raising an
exception when called with the C<lifespan> scope, or by returning without ever
sending C<lifespan.startup.complete> or C<lifespan.startup.failed> -- the server
B<must> treat lifespan as unsupported: it B<must> continue startup, B<must not>
send further lifespan events for that worker, and B<should> log this at an
informational level. A raised exception and a clean return are resolved
identically, and the server B<must not> block startup indefinitely waiting for a
lifespan signal.

To prevent startup entirely, an application B<must> send
C<lifespan.startup.failed> with a message; on receiving it the server B<must>
abort startup (it B<must not> begin accepting connections) and B<should> log the
message. This is the B<only> application signal that prevents the server from
starting -- a clean return B<must not>.

If the application raises an exception B<after> it has sent
C<lifespan.startup.complete> -- for example, a long-lived background task started
during lifespan crashes -- the server B<should> log the error at error level and
B<may> continue running in a degraded state. It is not required to shut down.

=head2 Lifespan State

Applications often want to persist data from the lifespan cycle to request/response handling.
For example, a database connection can be established in the lifespan cycle and persisted to
the request/response cycle.

The C<scope["state"]> namespace provides a place to store these sorts of things.
The server will ensure that a I<shallow copy> of the namespace is passed into each subsequent
request/response call into the application.
Since the server manages the application lifespan and often the event loop as well this
ensures that the application is always accessing the database connection (or other stored object)
that corresponds to the right event loop and lifecycle, without using context variables,
global mutable state or having to worry about references to stale/closed connections.

PAGI servers that implement this feature will provide
C<state> as part of the C<lifespan> scope:

    my $scope = {
        ...
        state => {},
    };

The namespace is controlled completely by the PAGI application; the server will not
interact with it other than to copy it.
Nonetheless, applications should be cooperative by properly naming their keys such that they
will not collide with other frameworks or middleware.

Because the copy is I<shallow>, take care how you share B<mutable> data:

=over

=item *

The B<values> in C<state> (object references -- a database pool, a queue, a
pub/sub hub) are shared. Every request scope sees the same object, so mutating
it through that reference (calling a method, C<push>-ing onto an arrayref) is
visible everywhere.

=item *

The B<top-level keys> are private to each copy. Assigning a top-level key in one
scope -- C<< $scope->{state}{count}++ >> in a request, or
C<< $state->{queue} = [] >> in the lifespan once requests are running -- changes
only that scope's copy. Other scopes keep their own and silently desync.

=back

So to share mutable data, store a container or object B<once> at startup and
mutate it B<through its reference>; never B<replace> a top-level C<state> key and
expect other scopes to see it. This mirrors the database-connection example
above: you store the pool once and use it everywhere -- you never reassign it
per request.

=head2 Startup - C<receive> event

Sent to the application when the server is ready to startup and receive connections,
but before it has started to do so.

B<Keys>:

=over

=item -

C<type> (String) - C<"lifespan.startup">.

=back

=head2 Startup Complete - C<send> event

Sent by the application when it has completed its startup. A server
must wait for this message before it starts processing connections.

B<Keys>:

=over

=item -

C<type> (String) - C<"lifespan.startup.complete">.

=back

=head2 Startup Failed - C<send> event

Sent by the application when it has failed to complete its startup. If a server
sees this it should log/print the message provided and then exit.

B<Keys>:

=over

=item -

C<type> (String) - C<"lifespan.startup.failed">.

=item -

C<message> (String) - Optional; if missing defaults to C<"">.

=back

=head2 Shutdown - C<receive> event

Sent to the application when the server has stopped accepting connections and closed
all active connections.

B<Keys>:

=over

=item -

C<type> (String) - C<"lifespan.shutdown">.

=back

=head2 Shutdown Complete - C<send> event

Sent by the application when it has completed its cleanup. A server
must wait for this message before terminating.

B<Keys>:

=over

=item -

C<type> (String) - C<"lifespan.shutdown.complete">.

=back

=head2 Shutdown Failed - C<send> event

Sent by the application when it has failed to complete its cleanup. If a server
sees this it should log/print the message provided and then terminate.

B<Keys>:

=over

=item -

C<type> (String) - C<"lifespan.shutdown.failed">.

=item -

C<message> (String) - Optional; if missing defaults to C<"">.

=back

=head2 Version History

=over

=item -

0.3 (Draft): No changes from 0.2

=item -

0.2 (Draft): Added is_worker and worker_num fields for multi-worker support

=item -

0.1 (Draft): Initial lifespan specification

=back

=head2 Copyright

This document has been placed in the public domain.
