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more.async

More core.async idioms and abstractions.

It is important to have the proper set of abstractions and idioms for problem solving and building systems. It allows the programmer to focus more on the system and its language and less on boilerplate, ceremony and logistics.

This library tries to enrich the language of core.async by providing the useful idioms which often repeat across code bases in some variation, among which are:

  • producing/consuming to channels at the edges of a system
  • dispatching/routing channels
  • batching
  • waiting for a group of tasks

Extra Batteries

Naming conventions

  • !/!!: names ending in ! are for async operations which happen in go-blocks. Names ending in !! are blocking and run on the calling thread
  • -call: Names ending in -call are for functions taking other functions as arguments. An equivalent name without the -call suffix is a wrapper macro which wraps a body in a function. see future and future-call as an example.

Producing and Consuming

The abstraction of a process sitting at the edge of a pipeline and producing to or consuming from a channel is quite common. It is how we connect our pipelines to the rest of the world.

more.async provides the following abstractions while adhering to Clojure's core and core.async naming conventions:

Blocking? Producing Consuming
no produce-call! consume-call!
no produce! consume!
yes produce-call!! consume-call!!
yes produce!! consume!!
no consume-checked-call!
no consume-checked!
yes consume-checked-call!!
yes consume-checked!!
  • produce-call!: takes a channel and a function, repeatedly produces to the channel the results of applying the function as long as it returns a non nil value and the channel is open.
  • produce!: like produce-call! but takes a body which is evaluated every iteration and its result is put into the channel.
  • consume-call!: takes a channel and a function, repeatedly consumes from the channel and applies the function to the consumed value until the channel closes.
  • consume!: takes a channel, binding name and a body, repeatedly evaluates the body with the variable name bound to values consumed from the channel.
  • consume-checked!: same as consume! but recurs only when the provided function returns a truth-y value.

Waiting

Another common pattern is waiting for a group of tasks, represented as returned channels (from threads or go blocks) to finish.

wait! and wait!! take a collection of channels and waits for them all to deliver. wait! returns a channel, wait!! blocks the calling thread.

wait-group-call takes a number n, function and cleanup function, will call the function n times in a thread and wait for it to finish. Once all calls are finished the cleanup function will be called once. Returns a channel which closes after cleanup is done.

wait-group is a macro which wraps wait-group-call and allows writing an execution body directly instead of passing functions around, for example:

(wait-group
 8
 (let [n (+ 1000 (rand-int 1000))]
   (Thread/sleep n)
   (println n))
 :finally
 (println "goodbye!"))

:finally is the delimiter used to separate the cleanup portion which will be executed only once.

Batching

A proper batching process with timeout semantics is essential.

batch! and batch!! provide transducer-like semantics in non-blocking and blocking contexts respectively, and pipeline-like semantics with regards to connecting to other channels:

batch! takes:

  • input channel
  • output channel
  • batch size
  • timeout ms
  • reducing function: rf :: batch -> input -> batch
  • init function which provides the initial value (NOTE: this is a deviation from transducers semantics, don't get it wrong)
  • an indicator should the output channel be closed if the input channel closes.

batch! is written to be very flexible and can even work with stateful accumulators:

(defn ticker
  []
  (let [a (atom 0)]
    (fn []
      (swap! a inc))))

(def in (a/chan))
(def out (a/chan))
(def n 3)
(def f (ticker))
(produce-call! in f)
(a/thread
  (batch!! in out n 1000 (fn [^StringBuilder sb x] (.append sb x)) #(StringBuilder.) true))

(str (a/<!! out))
;; => "123"
(str (a/<!! out))
;; => "456"

Routing

split! and split?! are similar to core.async/split but take an input channel, a map of values to output channels, and a routing function. Every value from the input channel is routed to the output channel corresponding to the result of the routing function.

The difference between the versions is that split?! drops values which can't be routed while split! throws.

Errata

periodically! - invoke a function periodically (every timeout) and put its results into returned channel. Essentially a timer channel. reductions! - like core/reductions from input channel to output channel. Produces an intermediate state only when consuming from output channel. Useful for maintaining internal state while processing a stream.

Usage

Dependency

Add the following dependency in your leiningen project:

[bsless/more.async "0.0.5"]

NOTE: breaking change as of 0.0.6: more.async namespaces have been renamed and removed the clojure prefix

Require

(require '[clojure.core.async :as a]
         '[more.async :as ma])

Use

For example, with kinsky's kafka client:

Channel Producer

(require '[kinsky.client :as client])

(defn make-consumer
  []
  (let [c (client/consumer {:bootstrap.servers "localhost:9092"
                            :group.id          "mygroup"}
                           (client/keyword-deserializer)
                           (client/edn-deserializer))]
    (client/subscribe! c "account")
    c))

(def msg-ch (a/chan))

(a/thread
  (let [consumer (make-consumer)]
    (ma/produce-blocking
     msg-ch
     (client/poll! consumer 100))
    (client/close!)))

Channel Consumer

(def out-ch (a/chan))

(let [p (client/producer {:bootstrap.servers "localhost:9092"}
                         (client/keyword-serializer)
                         (client/edn-serializer))
      topic "account"]
  (ma/consume out-ch v (client/send! p topic v)))

Status

In active development. Might contain bugs. PRs welcome.

License

Copyright © 2019-2020 Ben Sless

This program and the accompanying materials are made available under the terms of the Eclipse Public License 2.0 which is available at http://www.eclipse.org/legal/epl-2.0.

This Source Code may also be made available under the following Secondary Licenses when the conditions for such availability set forth in the Eclipse Public License, v. 2.0 are satisfied: GNU General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version, with the GNU Classpath Exception which is available at https://www.gnu.org/software/classpath/license.html.

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