I use lamina channels in a library that maintains multiple event streams.
The event source is the Asterisk Management Interface, whose raw events are
processed, filtered, collated, and finally pushed into appropriate lamina
channels.
On the client side of my library I want to expose the event streams as a
web service. My challenge is to implement long polling: a request either
finds events already enqueued, returning immediately; or blocks until an
event occurs (with a timeout), then accumulates any further events for a
short period, then returns the accumulated events. This reduces network
overhead for the typical case of events occuring in short bursts (a single
user action triggers several events).
I have implemented this behavior in a piece of code that occupies its
thread for the entire duration of the request:
(require
[lamina.core :as m]
[lamina.core.graph.node :as node]
[lamina.core.channel :as chan])
(let [evs (node/drain (chan/emitter-node ch))
evs (if (seq evs)
evs
(try (let [ev @(m/with-timeout POLL-TIMEOUT (m/read-channel
ch))]
(Thread/sleep EVENT-BURST-PERIOD)
(conj (node/drain (chan/emitter-node ch)) ev))
(catch TimeoutException _ nil)))]
(vec evs))
I would instead like to do this without blocking the thread. Ideally, I'll
use aleph to implement the web service and connect a downstream HTTP
response channel to my library's lamina channel. What I'm missing is, how
do I disconnect the downstream channel without breaking anything in the
upstream channel? I want to cleanly disconnect it, let my channel enqueue
any further events, then later connect another aleph channel, which will
drain those events with no loss.
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