link is the event-driven network library used by slacker. It's a thin wrapper of Netty.
Currently, link only works on the JVM implementation of Clojure. We might support nodejs in future.
In most cases, we use a declarative DSL to define a custom tcp
protocol codec: link.codec.
With the codec, you can read/write Clojure data structure in your handler and don't have to read the message byte by byte, and worry about TCP framing.
user> (require '[link.codec :refer :all])
;; create a custom codec: [version target-id string-message]
user> (def custom-codec
(frame
(byte)
(int32)
(string :encoding :utf8 :prefix (uint16))))
;; create an empty buffer
user> (def buf (unpooled-buffer))
;; encode clojure data structure on to given buffer, by using codec.
;; note that you don't have to call `encode*` and `decode*` by
;; youself, link does it for you.
user> (encode* custom-codec [1 348 "hello world"] buf)
#object[io.netty.buffer.UnpooledByteBufAllocator$InstrumentedUnpooledUnsafeHeapByteBuf 0x4eb69819 "UnpooledByteBufAllocator$InstrumentedUnpooledUnsafeHeapByteBuf(ridx: 0, widx: 18, cap: 256)"]
user> (decode* custom-codec buf)
[1 348 "hello world"]For a more complex codec, check slacker's codec definition.
You need to create a custom handler to process you network message. Link has provided you a dsl that is easier to understand. And also hide complexity of Netty's default handler API.
(require '[link.core :refer :all])
(def echo-handler
(create-handler
(on-message [ch msg]
(send! ch msg))))There are 5 events you can process in a link handler:
(on-active [ch])when channel is open, bound or connected(on-inacitve [ch])when channel is no longer open, bound or connected(on-message [ch msg])when a packet is read in(on-error [ch e])when exception occurs on I/O thread(on-event [ch evt])when netty user defined event triggered
And for the channel ch, you can call following functions as defined
by LinkMessageChannel protocol.
(send! [ch msg])write a msg into channel(channel-addr [ch])get the local socket address of the channel(remote-addr [ch])get the remote socket address of the channel(close! [ch])request to close the channel(valid? [ch])test if channel is still open and active
link only supports non-blocking server and client.
To start a server, you can provide a few argument to customize it:
(require '[link.tcp :refer :all])
(require '[link.threads :refer :all])
;; Just to demo the usage here, there is no need to run a echo-handler
;; in a thread pool.
(def handler-spec {:handler echo-handler :executor (new-executor 10)})
;; you can also provide a few handlers by passing a vector of them
(tcp-server 8081 [handler-spec]
:options {:so-reuseaddr true} ;; netty, ip, tcp and socket options
:host ;; if to bind, default "0.0.0.0"
)From link 0.7, ssl handler and codecs are all normal handlers. You will need to put them at correct position of handlers.
To see a full list of TCP options, you can find it on Netty
doc. Change
the option name to lowercase and replace the underscore with dash, as
in Clojure way. Prefixing a clild- to specify option for child
channels: :child-tcp-nodelay.
You can stop a server by
;; calling stop-server with the value returned by tcp-server
(stop-server *1)To create a TCP client, you need to create a connection factory for it. Note that, clients created from the same factory will share the same selector and event loop. Managing it carefully if you have a large number of connections.
(def client-factory
(tcp-client-factory handlers
:options ...))Create a client
(def client (tcp-client client-factory "localhost" 8081))The value returned by tcp-client is a LinkMessageChannel object so
you can call any functions of the protocol on it.
To send some data:
(send! client [1 345 "hello world"])To close a client, call close! on the channel. To close a client
factory, call stop-clients would work.
link also comes with an HTTP server. Since link is a clojure library, it accepts a ring function, so you can use any HTTP framework on link http server, without pain.
(require '[link.http :refer :all])
(http-server 8080 ring-app-fn
:executor ... ;; the thread pool to run ring functions on)(require '[link.http :as h])
(require '[link.ssl :as ssl])
(import '[io.netty.handler.ssl.util SelfSignedCertificate])
(let [ssc (SelfSignedCertificate.)
ssl-context (ssl/ssl-context-for-http2 (.certificate ssc) (.privateKey ssc)
:jdk)]
(h/h2-server 8443 ring-app ssl-context :threads 8))New in link 0.5. You can start a websocket server with link.
Create a websocket handler:
(require '[link.websocket :refer :all])
(require '[link.tcp :refer :all])
(def ws-echo-handler
(create-ws-handler
(on-open [ch])
(on-close [ch])
(on-text [ch string]
;; you can use (text), (binary), (ping), (pong) to generate
;; different types of response
(send! ch (text string)))
(on-binary [ch ^ByteBuf bytes])
(on-ping [ch ^ByteBuf bytes])
(on-pong [ch ^ByteBuf bytes])))
(tcp-server 8082 (conj (websocket-codecs "/chat") ws-echo-handler))
Copyright (C) 2012-2019 Ning Sun [email protected] and contributors
Distributed under the Eclipse Public License, the same as Clojure.