Hi Mitsuo,
I recommend designing the interface like this:
interface EventPublisher{
interface Subscriber {
updateEvent @0 (event: Int32) -> ();
}
interface Subscription {}
subscribe @0 (subscriber: Subscriber) -> (result: Int32, subscription:
Subscription);
# To unsubscribe, drop the returned `subscription`.
}
Here, subscribe() returns a `subscription` object. This object has no
methods. But, when the capability is dropped, then the destructor will run
on the server side. Atn that point, you can remove the subscription.
A big advantage of this approach is that it handles connection failures
gracefully. If the client randomly disconnects, the `subscription`
capability is automatically dropped, thus unsubscribing the client. This
way you don't end up stuck sending messages to a disconnected subscriber.
It also solves your problem because you can remember arbitrary metadata
about the subscriber within the `Subscription` object, so you know what to
remove when it's destroyed.
-Kenton
On Mon, Nov 22, 2021 at 12:40 AM mitsuo <mitsuo.date@woven-planet.global>
wrote:
> Hi,
>
> I'm trying to implement pub/sub like communication model with the
> following scheme.
> *pubsub.capnp*
> *interface EventPublisher{*
> * interface Subscriber {*
> * updateEvent @0 (event: Int32) -> ();*
> * }*
> * subscribe @0 (subscriber: Subscriber) -> (result: Int32);*
> * unsubscribe @1 (subscriber: Subscriber) -> (result: Int32);*
> *}*
>
> I'm using *kj::Vector<EventPublisher::Subscriber::Client> m_subscribers *to
> store the current subscribers.
> When I try to implement unsubscribe and remove the Subscriber from the
> Vector, I couldn't find good method to do that.
> Could you give me some advice?
>
> *server implementation*
> *class EventPublisherImpl final : public EventPublisher::Server {*
> * protected:*
> * ::kj::Promise<void> subscribe(SubscribeContext context) {*
> * cout << "subscribe request received" << endl;*
> * m_subscribers.add(context.getParams().getSubscriber());*
> * return kj::READY_NOW;*
> * }*
>
> * ::kj::Promise<void> unsubscribe(UnsubscribeContext context) {*
> * cout << "unsubscribe request received" << endl;*
> * auto unsub = context.getParams().getSubscriber();*
>
> * // I want to remove usub from subscribers like
> m_subscribers[unsub].erase();*
> * // But I couldn't find a method to compare*
> * // "EventPublisher::Subscriber::Client" such as == operator or public
> method*
> * // to distinguish the client.*
> * //*
> * // One solution is having an additional argument(id) for this purpose
> but*
> * // that requres additional management of ID.*
> * // subscribe @0 (subscriber: Subscriber, id: Int32) -> (result:
> Int32);*
> * // unsubscribe @1 (id: Int32) -> (result: Int32);*
>
> * // what I can do is erase everything but this is not my goal*
> * m_subscribers.clear();*
> * return kj::READY_NOW;*
> * }*
>
> * private:*
> * kj::Vector<EventPublisher::Subscriber::Client> m_subscribers;*
> *};*
> Thank you,
>
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