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ptaoussanis / sente


Clojure(Script) + core.async + WebSockets/Ajax


API docs | CHANGELOG | other Clojure libs | Twitter | contact/contrib | current Break Version:

[com.taoensso/sente "1.6.0"]     ; Stable
[com.taoensso/sente "1.7.0-RC1"] ; Dev, see CHANGELOG for details

Sente, channel sockets for Clojure

Almost sente

Sen-te (先手) is a Japanese Go term used to describe a play with such an overwhelming follow-up that it demands an immediate response, leaving its player with the initiative.

Sente is a small client+server library that makes it easy to build reliable, high-performance realtime web applications with Clojure.

Or: We don't need no Socket.IO
Or: The missing piece in Clojure's web application story
Or: Clojure(Script) + core.async + WebSockets/Ajax = The Shiz

(I'd also recommend checking out James Henderson's Chord and Kevin Lynagh's jetty7-websockets-async as possible alternatives!)

What's in the box™?

  • Bidirectional a/sync comms over both WebSockets and Ajax (auto-fallback).
  • Robust: auto keep-alives, buffering, protocol selection, reconnects. It just works™.
  • Highly efficient design incl. transparent event batching for low-bandwidth use, even over Ajax.
  • Send arbitrary Clojure vals over edn or Transit (JSON, MessagePack, etc.).
  • Tiny, simple API: make-channel-socket! and you're good to go.
  • Automatic, sensible support for users connected with multiple clients and/or devices simultaneously.
  • Realtime info on which users are connected over which protocols (v0.10.0+).
  • Flexible model: use it anywhere you'd use WebSockets/Ajax/Socket.IO, etc.
  • Standard Ring security model: auth as you like, HTTPS when available, CSRF support, etc.
  • Fully documented, with examples.
  • Small codebase: ~1k lines for the entire client+server implementation.
  • Supported servers: http-kit, Immutant v2+, nginx-clojure


Protocol client>server client>server + ack/reply server>user push
WebSockets ✓ (native) ✓ (emulated) ✓ (native)
Ajax ✓ (emulated) ✓ (native) ✓ (emulated)

So you can ignore the underlying protocol and deal directly with Sente's unified API. It's simple, and exposes the best of both WebSockets (bidirectionality + performance) and Ajax (optional evented ack/reply model).

Getting started

Note that there's also a variety of full example projects available

Add the necessary dependency to your Leiningen project.clj. This'll provide your project with both the client (ClojureScript) + server (Clojure) side library code:

[com.taoensso/sente "1.6.0"]

On the server (Clojure) side

First, make sure you're using a supported Ring-compatible async web server. These are currently: http-kit, Immutant v2+, and nginx-clojure.

Welcoming PRs to support additional servers, btw.

Somewhere in your web app's code you'll already have a routing mechanism in place for handling Ring requests by request URL. If you're using Compojure for example, you'll have something that looks like this:

(defroutes my-app
  (GET  "/"            req (my-landing-pg-handler  req))
  (POST "/submit-form" req (my-form-submit-handler req)))

For Sente, we're going to add 2 new URLs and setup their handlers:

(ns my-server-side-routing-ns ; .clj
    ;; <other stuff>
    [taoensso.sente :as sente] ; <--- Add this

    ;; Uncomment a web-server adapter --->
    ;; [taoensso.sente.server-adapters.http-kit      :refer (sente-web-server-adapter)]
    ;; [taoensso.sente.server-adapters.immutant      :refer (sente-web-server-adapter)]
    ;; [taoensso.sente.server-adapters.nginx-clojure :refer (sente-web-server-adapter)]

;;; Add this: --->
(let [{:keys [ch-recv send-fn ajax-post-fn ajax-get-or-ws-handshake-fn
      (sente/make-channel-socket! sente-web-server-adapter {})]
  (def ring-ajax-post                ajax-post-fn)
  (def ring-ajax-get-or-ws-handshake ajax-get-or-ws-handshake-fn)
  (def ch-chsk                       ch-recv) ; ChannelSocket's receive channel
  (def chsk-send!                    send-fn) ; ChannelSocket's send API fn
  (def connected-uids                connected-uids) ; Watchable, read-only atom

(defroutes my-app-routes
  ;; <other stuff>

  ;;; Add these 2 entries: --->
  (GET  "/chsk" req (ring-ajax-get-or-ws-handshake req))
  (POST "/chsk" req (ring-ajax-post                req))

(def my-app
  (-> my-app-routes
      ;; Add necessary Ring middleware:

The ring-ajax-post and ring-ajax-get-or-ws-handshake fns will automatically handle Ring GET and POST requests to our channel socket URL ("/chsk"). Together these take care of the messy details of establishing + maintaining WebSocket or long-polling requests.

On the client (ClojureScript) side

You'll setup something similar on the client side:

(ns my-client-side-ns ; .cljs
   [cljs.core.async.macros :as asyncm :refer (go go-loop)])
   ;; <other stuff>
   [cljs.core.async :as async :refer (<! >! put! chan)]
   [taoensso.sente  :as sente :refer (cb-success?)] ; <--- Add this

;;; Add this: --->
(let [{:keys [chsk ch-recv send-fn state]}
      (sente/make-channel-socket! "/chsk" ; Note the same path as before
       {:type :auto ; e/o #{:auto :ajax :ws}
  (def chsk       chsk)
  (def ch-chsk    ch-recv) ; ChannelSocket's receive channel
  (def chsk-send! send-fn) ; ChannelSocket's send API fn
  (def chsk-state state)   ; Watchable, read-only atom

Now what?

The client will automatically initiate a WebSocket or repeating long-polling connection to your server. Client<->server events are now ready to transmit over the ch-chsk channel.

Last step: you'll want to hook your own event handlers up to this channel. Please see one of the example projects for details.

Client-side API

  • ch-recv is a core.async channel that'll receive event-msgs.
  • chsk-send! is a (fn [event & [?timeout-ms ?cb-fn]]). This is for standard client>server req>resp calls.

Server-side API

  • ch-recv is a core.async channel that'll receive event-msgs.
  • chsk-send! is a (fn [user-id event]). This is for async server>user PUSH calls.

Term Form
event [<ev-id> <?ev-data>], e.g. [:my-app/some-req {:data "data"}]
server event-msg {:keys [event id ?data send-fn ?reply-fn uid ring-req client-id]}
client event-msg {:keys [event id ?data send-fn]}
<ev-id> A namespaced keyword like :my-app/some-req
<?ev-data> An optional arbitrary edn value like {:data "data"}
:ring-req Ring map for Ajax request or WebSocket's initial handshake request
:?reply-fn Present only when client requested a reply.


  • So clients can use chsk-send! to send events to the server. They can optionally request a reply, with timeout.
  • The server can likewise use chsk-send! to send events to all the clients (browser tabs, devices, etc.) of a particular connected user by his/her user-id.
  • The server can also use an event-msg's ?reply-fn to reply to a particular client event using an arbitrary edn value.

It's worth noting that the server>user push (chsk-send! <user-id> <event>) takes a mandatory user-id argument. See the FAQ later for more info.

Ajax/Sente comparison: client>server

(jayq/ajax ; Using the jayq wrapper around jQuery
 {:type :post :url "/some-url-on-server/"
  :data {:name "Rich Hickey"
         :type "Awesome"}
  :timeout 8000
  :success (fn [content text-status xhr]
             (do-something! content))
  :error   (fn [xhr text-status] (error-handler!))})

(chsk-send! ; Using Sente
  [:some/request-id {:name "Rich Hickey" :type "Awesome"}] ; Event
  8000 ; Timeout
  ;; Optional callback:
  (fn [edn-reply]
    (if (sente/cb-success? edn-reply) ; Checks for :chsk/closed, :chsk/timeout, :chsk/error
      (do-something! edn-reply)

Some important differences to note:

  • The Ajax request is slow to initialize, and bulky (HTTP overhead).
  • The Sente request is pre-initialized (usu. WebSocket), and lean (edn/Transit protocol).

Ajax/Sente comparison: server>user push

  • Ajax would require clumsy long-polling setup, and wouldn't easily support users connected with multiple clients simultaneously.
  • Sente: (chsk-send! "destination-user-id" [:some/alert-id <edn-payload>]).

Example projects

There's a full reference example project in the repo. Call lein start-dev in that dir to get a (headless) development repl that you can connect to with Cider (emacs) or your IDE.

There's also some user-submitted examples which may be handy:

PRs welcome for additional examples!

Author/link Description
@danielsz/sente-boot Same example app, different toolchain (boot)
@danielsz/sente-system Same example app, adapted for danielsz/system
@seancorfield/om-sente ??


What is the user-id provided to the server>user push fn?

There's now also a full user-id, client-id summary up here

For the server to push events, we need a destination. Traditionally we might push to a client (e.g. browser tab). But with modern rich web applications and the increasing use of multiple simultaneous devices (tablets, mobiles, etc.) - the value of a client push is diminishing. You'll often see applications (even by Google) struggling to deal with these cases.

Sente offers an out-the-box solution by pulling the concept of identity one level higher and dealing with unique users rather than clients. What constitutes a user is entirely at the discretion of each application:

  • Each user-id may have zero or more connected clients at any given time.
  • Each user-id may survive across clients (browser tabs, devices), and sessions.

To give a user an identity, either set the user's :uid Ring session key OR supply a :user-id-fn (takes request, returns an identity string) to the make-channel-socket! constructor.

If you want a simple per-session identity, generate a random uuid. If you want an identity that persists across sessions, try use something with semantic meaning that you may already have like a database-generated user-id, a login email address, a secure URL fragment, etc.

Note that user-ids are used only for server>user push. client>server requests don't take a user-id.

As of Sente v0.13.0+ it's also possible to send events to :sente/all-users-without-uid.

How do I integrate Sente with my usual login/auth procedure?

This is trivially easy as of Sente v0.13.0+. Please see one of the example projects for details.

Will Sente work with Reactjs/Reagent/Om/Pedestel/etc.?

Sure! Sente's just a client<->server comms mechanism so it'll work with any view/rendering approach you'd like.

I have a strong preference for Reagent myself, so would recommend checking that out first if you're still evaluating options.

What if I need to use JSON, XML, raw strings, etc.?

As of v1, Sente uses an extensible client<->server serialization mechanism. It uses edn by default since this usu. gives good performance and doesn't require any external dependencies. The reference example project shows how you can plug in an alternative de/serializer. In particular, note that Sente ships with a Transit de/serializer that allows manual or smart (automatic) per-payload format selection.

How do I add custom Transit read and write handlers?

To add custom handlers to the TransitPacker, pass them in as writer-opts and reader-opts when creating a TransitPacker. These arguments are the same as the opts map you would pass directly to transit/writer. The code sample below shows how you would do this to add a write handler to convert Joda-Time DateTime objects to Transit time objects.

  (:require [cognitect.transit :as transit]
            [taoensso.sente.packers.transit :as sente-transit])
  (:import [org.joda.time DateTime ReadableInstant]))

;; From
(def joda-time-writer
    (constantly "m")
    (fn [v] (-> ^ReadableInstant v .getMillis))
    (fn [v] (-> ^ReadableInstant v .getMillis .toString))))

(def packer (sente-transit/->TransitPacker :json {:handlers {DateTime joda-time-writer}} {}))

How do I route client/server events?

However you like! If you don't have many events, a simple cond will probably do. Otherwise a multimethod dispatching against event ids works well (this is the approach taken in the reference example project).

Security: is there HTTPS support?

Yup, it's automatic for both Ajax and WebSockets. If the page serving your JavaScript (ClojureScript) is running HTTPS, your Sente channel sockets will run over HTTPS and/or the WebSocket equivalent (WSS).

Security: CSRF protection?

This is important. Sente has support, but you'll need to use middleware like ring-anti-forgery to generate and check CSRF codes. The ring-ajax-post handler should be covered (i.e. protected).

Please see one of the example projects for a fully-baked example.

Pageload: How do I know when Sente is ready client-side?

You'll want to listen on the receive channel for a [:chsk/state {:first-open? true}] event. That's the signal that the socket's been established.

How can server-side channel socket events modify a user's session?

Update: @danielsz has kindly provided a detailed example here.

Recall that server-side event-msgs are of the form {:ring-req _ :event _ :?reply-fn _}, so each server-side event is accompanied by the relevant[*] Ring request.

For WebSocket events this is the initial Ring HTTP handshake request, for Ajax events it's just the Ring HTTP Ajax request.

The Ring request's :session key is an immutable value, so how do you modify a session in response to an event? You won't be doing this often, but it can be handy (e.g. for login/logout forms).

You've got two choices:

  1. Write any changes directly to your Ring SessionStore (i.e. the mutable state that's actually backing your sessions). You'll need the relevant user's session key, which you can find under your Ring request's :cookies key. This is flexible, but requires that you know how+where your session data is being stored.

  2. Just use regular HTTP Ajax requests for stuff that needs to modify sessions (like login/logout), since these will automatically go through the usual Ring session middleware and let you modify a session with a simple {:status 200 :session <new-session>} response. This is the strategy the reference example takes.

Lifecycle management (component management/shutdown, etc.)

Using something like stuartsierra/component or palletops/leaven?

Most of Sente's state is held internally to each channel socket (the map returned from client/server calls to make-channel-socket!). The absence of global state makes things like testing, and running multiple concurrent connections easy. It also makes integration with your component management easy.

The only thing you may[1] want to do on component shutdown is stop any router loops that you've created to dispatch events to handlers. The client/server side start-chsk-router! fns both return a (fn stop []) that you can call to do this.

[1] The cost of not doing this is actually negligible (a single parked go thread).

There's also a couple lifecycle libraries that include Sente components:

  1. danielsz/system for use with stuartsierra/component.
  2. palletops/bakery for use with palletops/leaven.

If you do want a lifecycle management lib, I'm personally fond of Leaven since it's simpler (no auto dependencies) and adds ClojureScript support (which is handy for Sente).

Any other questions?

If I've missed something here, feel free to open a GitHub issue or pop me an email!

Contact & contributing

lein start-dev to get a (headless) development repl that you can connect to with Cider (emacs) or your IDE.

Please use the project's GitHub issues page for project questions/comments/suggestions/whatever (pull requests welcome!). Am very open to ideas if you have any!

Otherwise reach me (Peter Taoussanis) at or on Twitter. Cheers!


Copyright © 2012-2014 Peter Taoussanis. Distributed under the Eclipse Public License, the same as Clojure.