[com.taoensso/sente "1.2.0"] ; Please see CHANGELOG for details

Sente, channel sockets for Clojure

Sen-te (先手) is a Japanese Go term used to describe a play with such an overwhelming follow-up that it elicits an immediate response, thus 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: currently only http-kit but a pluggable interface is planned.

Capabilities

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 full reference example project in this repo.

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 "1.2.0"]

On the server (Clojure) side

First, make sure you're using http-kit as your Clojure web server. If you're using the standard Ring server (Jetty), http-kit is almost a drop-in replacement.

Why http-kit? Besides being a great web server, it currently offers by far the best high-concurrency support which is something Sente needs to lean on for WebSocket and long-polling connections.

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
  (:require
    ;; <other stuff>
    [taoensso.sente :as sente] ; <--- Add this
   ))

;;; Add this: --->
(let [{:keys [ch-recv send-fn ajax-post-fn ajax-get-or-ws-handshake-fn
              connected-uids]}
      (sente/make-channel-socket! {})]
  (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
  ;; <other stuff>

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

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
  (:require-macros
   [cljs.core.async.macros :as asyncm :refer (go go-loop)])
  (:require
   ;; <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?

You're good to go! The client will automatically initiate a WebSocket or repeating long-polling connection to your server.

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"}]`
event-msg	`{:ring-req _ :event _ :?reply-fn _ :push-fn _ <...>}` (server-side)
`<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 (otherwise logs a warning)

Summary

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)
      (error-handler!))))

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>]).

FAQ

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

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 clients without a user-id (they simply have a nil user-id, and you can send to that as you would any other id).

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

This is trivially easy as of Sente v0.13.0+. Please see the reference example project which now includes a basic login form.

Will Sente work with React/Reagent/Om/etc.?

Sure! I use it with Reagent myself. Sente's just a client<->server comms mechanism.

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 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).

The reference example project has 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.

Examples: wherefore art thou?

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.

Further instructions are provided in the relevant namespace.

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

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:

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.
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.

Any other examples?

Here's some more unofficial/user-submitted examples for those interested! (PRs welcome!):

Example	Author	Comments
om-mouse	@tf0054	Basic example: tracks mouse pos using Om.

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 taoensso.com or on Twitter. Cheers!

valichek/sente

Sente, channel sockets for Clojure

What's in the box™?

Capabilities

Getting started

On the server (Clojure) side

On the client (ClojureScript) side

Now what?

Client-side API

Server-side API

Summary

Ajax/Sente comparison: client>server

Ajax/Sente comparison: server>user push

FAQ

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

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

Will Sente work with React/Reagent/Om/etc.?

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

How do I route client/server events?

Security: is there HTTPS support?

Security: CSRF protection?

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

Examples: wherefore art thou?

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

Any other examples?

Any other questions?

Contact & contributing

License

valichek/sente

Sente, channel sockets for Clojure

What's in the box™?

Capabilities

Getting started

On the server (Clojure) side

On the client (ClojureScript) side

Now what?

Client-side API

Server-side API

Summary

Ajax/Sente comparison: client>server

Ajax/Sente comparison: server>user push

FAQ

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

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

Will Sente work with React/Reagent/Om/etc.?

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

How do I route client/server events?

Security: is there HTTPS support?

Security: CSRF protection?

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

Examples: wherefore art thou?

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

Any other examples?

Any other questions?

Contact & contributing

License

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