The Backpressure package allows Pony programmers to participate in Pony's runtime backpressure system. The goal of the backpressure system is to prevent an actor's mailbox from growing at an unbounded rate.
At a high level, the runtime backpressure system works by adjusting the scheduling of actors. When an actor becomes overloaded, the Pony runtime will deprioritize scheduling the actors that are sending to it. This change in scheduling allows the overloaded actor to catch up.
The Pony runtime can detect overloading based on message queue size. However,
the overloading of some types of actors is harder to detect. Let's take the
case of actors like
TCPConnection manages a socket for sending data to and receiving data from
another process. TCP connections can experience backpressure from outside
our Pony program that prevents them from sending. There's no way for the Pony
runtime to detect this, so intervention by the programmer is needed.
TCPConnection is a single example. This Backpressure package exists to allow
a programmer to indicate to the runtime that a given actor is experiencing
pressure and sending messages to it should be adjusted accordingly.
Any actor that needs to be able to tell the runtime to "send me messages
slower" due to external conditions can do so via this package. Additionally,
actors that maintain their own internal queues of any sort, say for buffering,
are also prime candidates for using this package. If an actor's internal
queue grows too large, it can call
Backpressure.apply to let the runtime know
it is under pressure.
// Here we have a TCPConnectionNotify that upon construction // is given a BackpressureAuth token. This allows the notifier // to inform the Pony runtime when to apply and release backpressure // as the connection experiences it. // Note the calls to // // Backpressure.apply(_auth) // Backpressure.release(_auth) // // that apply and release backpressure as needed use "backpressure" use "collections" use "net" class SlowDown is TCPConnectionNotify let _auth: BackpressureAuth let _out: StdStream new iso create(auth: BackpressureAuth, out: StdStream) => _auth = auth _out = out fun ref throttled(connection: TCPConnection ref) => _out.print("Experiencing backpressure!") Backpressure.apply(_auth) fun ref unthrottled(connection: TCPConnection ref) => _out.print("Releasing backpressure!") Backpressure.release(_auth) fun ref connect_failed(conn: TCPConnection ref) => None actor Main new create(env: Env) => try let auth = env.root as AmbientAuth let socket = TCPConnection(auth, recover SlowDown(auth, env.out) end, "", "7669") end
The runtime backpressure is a powerful system. By intervening, programmers can
create deadlocks. Any call to
Backpressure.apply should be matched by a
corresponding call to
Backpressure.release. Authorization via the
ApplyReleaseBackpressureAuth capability is required to apply or release
backpressure. By requiring that the caller have a token to apply or release a
backpressure, rouge 3rd party library code can't run wild and unknowingly
interfere with the runtime.