Extracted from:Web Development with Clojure,
Third Edition
Build Large, Maintainable Web Applications Interactively

This PDF file contains pages extracted from Web Development with Clojure, Third
Edition, published by the Pragmatic Bookshelf. For more information or to purchasea paperback or PDF copy, please visit http://www.pragprog.com.
Note: This extract contains some colored text (particularly in code listing). Thisis available only in online versions of the books. The printed versions are black
and white. Pagination might vary between the online and printed versions; thecontent is otherwise identical.
Copyright © 2021 The Pragmatic Programmers, LLC.
All rights reserved.
No part of this publication may be reproduced, stored in a retrieval system, or transmitted,
in any form, or by any means, electronic, mechanical, photocopying, recording, or otherwise,
without the prior consent of the publisher.

The Pragmatic Bookshelf
Raleigh, North Carolina

Web Development with Clojure,
Third Edition
Build Large, Maintainable Web Applications Interactively

Dmitri Sotnikov
Scot Brown

The Pragmatic Bookshelf
Raleigh, North Carolina

Many of the designations used by manufacturers and sellers to distinguish their products
are claimed as trademarks. Where those designations appear in this book, and The Pragmatic
Programmers, LLC was aware of a trademark claim, the designations have been printed ininitial capital letters or in all capitals. The Pragmatic Starter Kit, The Pragmatic Programmer,
Pragmatic Programming, Pragmatic Bookshelf, PragProg and the linking g device are trademarks of The Pragmatic Programmers, LLC.
Every precaution was taken in the preparation of this book. However, the publisher assumes
no responsibility for errors or omissions, or for damages that may result from the use of
information (including program listings) contained herein.
For our complete catalog of hands-on, practical, and Pragmatic content for software developers, please visit https://pragprog.com.
The team that produced this book includes:
CEO: Dave Rankin
COO: Janet Furlow
Managing Editor: Tammy Coron
Development Editor: Michael Swaine
Copy Editor: L. Sakhi MacMillan
Indexing: Potomac Indexing, LLC
Layout: Gilson Graphics
Founders: Andy Hunt and Dave Thomas
For sales, volume licensing, and support, please contact support@pragprog.com.
For international rights, please contact rights@pragprog.com.

Copyright © 2021 The Pragmatic Programmers, LLC.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system,or transmitted, in any form, or by any means, electronic, mechanical, photocopying, recording,or otherwise, without the prior consent of the publisher.
ISBN-13: 978-1-68050-682-2
Encoded using the finest acid-free high-entropy binary digits.
Book version: P1.0—July 2021

Multi-User with WebSockets
Instead of just having a refresh messages button, let’s add a push notification
for new messages using WebSockets.
We’ll take a look at using WebSockets for client-server communication. In
the traditional Ajax approach, the client first sends a message to the server
and then handles the reply using an asynchronous callback. WebSockets
allow the web server to initiate the message exchange with the client.
Currently, our guestbook application doesn’t provide a way to display messages
generated by other users without reloading the page. If we wanted to solve
this problem using Ajax, our only option would be to poll the server and checkif any new messages are available since the last poll. This is inefficient since
the clients end up continuously polling the server regardless of whether any
new messages are actually available.
Instead, we’ll have the clients open a WebSocket connection when the pageloads, and then the server will notify all the active clients anytime a newmessage is created. This way the clients are notified in real time and themessages are only sent as needed.

Configuring the Server
WebSockets require support on both the server and the client side. While thebrowser API is standard, each server provides its own way of handling WebSocket connections. In this section we’ll take a look at using the API for the
HTTP Kit web server that Luminus defaults to.
Let’s start by updating the server-side code in the project to provide a WebSocket connection. Once the server is updated, we’ll look at the updates
required for the client.
First we add a new namespace for our WebSocket connection handler. Let’scall it guestbook.routes.websockets and add the following requirements:
guestbook-websockets/src/clj/guestbook/routes/websockets.clj(ns guestbook.routes.websockets(:require [clojure.tools.logging :as log][org.httpkit.server :as http-kit][clojure.edn :as edn][guestbook.messages :as msg]))

We’ll need org.httpkit.server for managing our WebSocket connections.
While we figure out the basics of WebSockets, we’ll just use pr-str and edn/readstring to serialize and de-serialize our data.

• Click HERE to purchase this book now. discuss•6
If we want to communicate to clients, the first thing we need to do is keep
track of our connections. Let’s create an atom containing our open connections
and write connect! and disconnect! functions to manage it.
guestbook-websockets/src/clj/guestbook/routes/websockets.clj(defonce channels (atom #{}))(defn connect! [channel](log/info "Channel opened")(swap! channels conj channel))(defn disconnect! [channel status](log/info "Channel closed: " status)(swap! channels disj channel))

To keep it simple, let’s assume our WebSocket will only receive save-message!
messages. Let’s copy our logic from guestbook.routes.services but replace HTTP
responses with maps and add serialization and de-serialization where necessary.
guestbook-websockets/src/clj/guestbook/routes/websockets.clj(defn handle-message! [channel ws-message](let [message (edn/read-string ws-message)
response (try(msg/save-message! message)(assoc message :timestamp (java.util.Date.))(catch Exception e(let [{id:guestbook/error-id
errors :errors} (ex-data e)](case id:validation{:errors errors};;else{:errors{:server-error ["Failed to save message!"]}}))))](if (:errors response)(http-kit/send! channel (pr-str response))(doseq [channel @channels](http-kit/send! channel (pr-str response))))))

Finally, we need to write a connection handler:
guestbook-websockets/src/clj/guestbook/routes/websockets.clj(defn handler [request](http-kit/with-channel request channel(connect! channel)(http-kit/on-close channel (partial disconnect! channel))(http-kit/on-receive channel (partial handle-message! channel))))(defn websocket-routes []["/ws"{:get handler}])• Click HERE to purchase this book now. discussMulti-User with WebSockets • 7

We reference it from guestbook.handler, exposing a route in our app for accepting
connections:(ns guestbook.handler(:require;;...
[guestbook.routes.websockets :refer [websocket-routes]];;...
))(mount/defstate app:start(middleware/wrap-base(ring/ring-handler(ring/router[;;...
(websocket-routes)])
;;...
)))

Now, let’s connect from the client.

Connecting from ClojureScript
To connect on the client side, we use js/WebSocket to create a WebSocket connection object, which we use to communicate.
guestbook-websockets/src/cljs/guestbook/websockets.cljs(ns guestbook.websockets(:require [cljs.reader :as edn]))(defonce channel (atom nil))(defn connect! [url receive-handler](if-let [chan (js/WebSocket. url)](do(.log js/console "Connected!")(set! (.-onmessage chan) #(->> %.-dataedn/read-stringreceive-handler))(reset! channel chan))(throw (ex-info "Websocket Connection Failed!"{:url url}))))(defn send-message! [msg](if-let [chan @channel](.send chan (pr-str msg))(throw (ex-info "Couldn't send message, channel isn't open!"{:message msg}))))• Click HERE to purchase this book now. discuss•8In our core namespace, everything stays mostly the same. The only things
we need to do are: update our :message/send event to use our WebSocket, add
a handle-response! function that will deal with responses, and call ws/connect! from
our init! function.
guestbook-websockets/src/cljs/guestbook/core.cljs(rf/reg-event-fx:message/send!
(fn [{:keys [db]} [_ fields]](ws/send-message! fields){:db (dissoc db :form/server-errors)}))(defn handle-response! [response](if-let [errors (:errors response)](rf/dispatch [:form/set-server-errors errors])(do(rf/dispatch [:message/add response])(rf/dispatch [:form/clear-fields response]))))(defn init! [](.log js/console "Initializing App...")(rf/dispatch [:app/initialize])(ws/connect! (str "ws://" (.-host js/location) "/ws")
handle-response!)(mount-components))

Let’s try it out in the browser. Let’s respond to our earlier message via WebSockets.

It automatically loads! Since re-frame is event driven, using WebSockets is
actually a lot simpler! We just dispatch our events from our WebSocket
message handler instead of from our Reagent components. If we look at reframe-10x’s history in the left browser, we can see that [:message/add ...] wasindeed triggered even though we didn’t submit anything.

• Click HERE to purchase this book now. discuss

Upgrading to Sente • 9

From here, we could do a lot to improve our re-frame app, but let’s swap out
our from-scratch WebSocket implementation for a much richer one—Sente.

Upgrading to Sente
Now that we’ve done a scratch implementation of WebSockets, let’s look at a
popular Clojure(Script) WebSockets library: Sente.15
Sente is a little more complicated than our toy implementation, but it brings
a lot of great features in exchange.
Ajax Fallback Support
Sente automatically switches to Ajax polling if WebSockets aren’t available.
Keep-Alives
Sends messages periodically to prevent connections from dropping andto kill stale connections.
Message Buffering
Leverages core.async to buffer messages for us.
Encoding
Serializes and de-serializes data for us.
Security
Supports Ring anti-forgery middleware.
Let’s add the dependency to our project.clj and get started:
guestbook-sente-setup/project.clj[com.taoensso/sente "1.16.0"]

Upgrading the Server
We can now update the guestbook.routes.websockets namespace to use Sente to
manage the server-side WebSocket connection. Let’s update the dependencies
to add taoensso.sente and taoensso.sente.server-adapters.http-kit references. Also, since

15. https://github.com/ptaoussanis/sente• Click HERE to purchase this book now. discuss• 10Sente manages the serialization of data and the management of our connections, let’s remove clojure.edn and org.httpkit.server.
guestbook-sente-setup/src/clj/guestbook/routes/websockets.clj(ns guestbook.routes.websockets(:require[clojure.tools.logging :as log]
[guestbook.messages :as msg]
[guestbook.middleware :as middleware][mount.core :refer [defstate]][taoensso.sente :as sente][taoensso.sente.server-adapters.http-kit :refer [get-sch-adapter]]))We initialize Sente by calling the sente/make-channel-socket! function. This function
accepts the server adapter and a map of initialization options. We pass in the
HTTP Kit server adapter, since that’s the server we’re using, and we set the:user-id-fn option to use the :client-id key in the request parameters. The reason
we have to specify our :user-id-fn is that Sente defaults to using the :uid key fromthe session. Since we aren’t creating Ring sessions for our clients, we’ll need
to use something else. The :client-id is a UUID that’s automatically generated
for each Sente client, so it’s a perfect fit for us.
guestbook-sente-setup/src/clj/guestbook/routes/websockets.clj(defstate socket:start (sente/make-channel-socket!
(get-sch-adapter){:user-id-fn (fn [ring-req](get-in ring-req [:params :client-id]))}))(defn send! [uid message](println "Sending message: " message)((:send-fn socket) uid message))

The sente/make-channel-socket! function returns a map that contains a numberof variables that were initialized.
:ajax-post-fn

The function that handles Ajax POST requests.
:ajax-get-or-ws-handshake-fn

The function that negotiates the initial connection.
:ch-recv

The receive channel for the socket.
:send-fn

The function that’s used to send push notifications to the client.

• Click HERE to purchase this book now. discuss

Upgrading to Sente • 11:connected-uids

An atom containing the IDs of the connected clients.
We’ll usually access the keys on our socket map using helper functions suchas send!.
In our first implementation, we sent our guestbook message as a map of fields
with no metadata. This restricted us to only one type of message. We shouldchange this so that we can accept multiple message types. We also mustchange this because Sente will call our handler function whenever an event
occurs, passing it a map with a bunch of metadata describing the event.
Let’s do this with a multimethod:
guestbook-sente-setup/src/clj/guestbook/routes/websockets.clj(defmulti handle-message (fn [{:keys [id]}]
id))(defmethod handle-message :default[{:keys [id]}](log/debug "Received unrecognized websocket event type: " id))(defmethod handle-message :message/create!
[{:keys [?data uid] :as message}](let [response (try(msg/save-message! ?data)(assoc ?data :timestamp (java.util.Date.))(catch Exception e(let [{id:guestbook/error-id
errors :errors} (ex-data e)](case id:validation{:errors errors};;else{:errors{:server-error ["Failed to save message!"]}}))))](if (:errors response)(send! uid [:message/creation-errors response])(doseq [uid (:any @(:connected-uids socket))](send! uid [:message/add response])))))(defn receive-message! [{:keys [id] :as message}](log/debug "Got message with id: " id)(handle-message message))

We’ve replaced our old handle-message! function with a receive-message! function
and a handle-message multimethod. We place any logic that applies to all eventsin our receive-message! wrapper function. It will call handle-message, which will
dispatch to different methods based on the :id of the message.
Sente’s event-message maps have some useful keys on them besides just :id.

• Click HERE to purchase this book now. discuss

• 12
event
The full event vector.
id
The ID keyword (first event).
?data
The data sent in the event (second event).
send-fn
A function to send a message via the socket this message was received
from.
?reply-fn (Server Only)
Sends an arbitrary response body to the callback function specified clientside (only exists if the client specified a callback function).
uid (Server Only)
A user-id (that is, may correspond to one or many connections. Is managed
based on :user-id-fn; compare :client-id).
ring-req (Server Only)
The Ring request received in an Ajax post or the initial WebSocket handshake.
client-id (Server Only)
A client-id that is specific to a single connection.
Since send! only communicates with a single user, we must use a doseq if wewant to broadcast messages. Now that we’ve got the meat of it set up, let’s
update how it connects to the rest of our application.
guestbook-sente-setup/src/clj/guestbook/routes/websockets.clj(defstate channel-router:start (sente/start-chsk-router!
(:ch-recv socket)#'receive-message!):stop (when-let [stop-fn channel-router](stop-fn)))(defn websocket-routes []["/ws"{:middleware [middleware/wrap-csrf
middleware/wrap-formats]:get (:ajax-get-or-ws-handshake-fn socket):post (:ajax-post-fn socket)}])In addition to managing our socket, Sente provides a helper for creating amessage router that passes incoming messages to our handler function. Since• Click HERE to purchase this book now. discuss

Upgrading to Sente • 13

our router depends on the initialization of our socket, we have to define it as a
defstate so that Mount knows to start our socket first before starting our router.
Once we have our router initialized, the last step is to connect the socket toour web server. Unlike our from-scratch implementation Sente works wellwith Ring middlewares, so we use wrap-csrf and wrap-formats. It also has both:get and :post handler functions so that the client can use Ajax if it doesn’t
support WebSockets.

Upgrading the Client
Now that we’ve changed our server to use Sente, we need to update the client
as well. Our client WebSocket connection will look similar to our server, witha few key differences. We’ll use Mount on the client, as well, so we won’t haveto manually connect in our init! function. We’ll have two defstate definitions:
our socket and our router. We’ll have a receive-message! function wrapping ahandle-message multimethod. And we’ll have a send! function for sending messages
over our socket.
Let’s start by updating our namespace declaration and creating our socket
and send! function:
guestbook-sente-setup/src/cljs/guestbook/websockets.cljs(ns guestbook.websockets(:require-macros [mount.core :refer [defstate]])(:require [re-frame.core :as rf][taoensso.sente :as sente]
mount.core))(defstate socket:start (sente/make-channel-socket!
"/ws"(.-value (.getElementById js/document "token")){:type :auto:wrap-recv-evs? false}))(defn send! [message](if-let [send-fn (:send-fn @socket)](send-fn message)(throw (ex-info "Couldn't send message, channel isn't open!"{:message message}))))

Our call to make-channel-socket! looks a bit different. As the first argument, theweb server adapter is replaced by a URL. The second argument, our CSRF
token, is new. Since we’re in the browser now, we need to send our CSRF
token to ensure that our connection is secure. The last argument is an options
map like we had on the server, but it has a different set of options available.
The only ones we’re passing for now are :type and wrap-recv-evs?. The :type option• Click HERE to purchase this book now. discuss• 14determines whether we use WebSockets or Ajax as the underlying connection
method. Choose :auto to let Sente use whichever method it prefers. The wraprecv-evs? option specifies whether we want to receive all application messages
wrapped in an outer :chsk/recv event. Turn this off by passing false so that ourclient events are structured like our server events.
Our send! function looks similar to the server, but not quite the same. We’redereferencing our socket before using its :send-fn. This is a minor detail of how
Mount works when targeting JavaScript rather than Java. This isn’t for any
interesting reason, so we’ll just have to remember that we need to dereference
any Mount states before using them in cljs.
Now that we have our socket set up, let’s write our handle-message and receivemessage! functions and connect them to a channel-router.

• Click HERE to purchase this book now. discuss

Upgrading to Sente • 15
guestbook-sente-setup/src/cljs/guestbook/websockets.cljs(defmulti handle-message(fn [{:keys [id]} _]id))(defmethod handle-message :message/add[_ msg-add-event](rf/dispatch msg-add-event))(defmethod handle-message :message/creation-errors[_ [_ response]](rf/dispatch[:form/set-server-errors (:errors response)]));; --------------------------------------------------------------------------;; Default Handlers(defmethod handle-message :chsk/handshake[{:keys [event]} _](.log js/console "Connection Established: " (pr-str event)))(defmethod handle-message :chsk/state[{:keys [event]} _](.log js/console "State Changed: " (pr-str event)))(defmethod handle-message :default[{:keys [event]} _](.warn js/console "Unknown websocket message: " (pr-str event)));; --------------------------------------------------------------------------;; Router(defn receive-message!
[{:keys [id event] :as ws-message}](do(.log js/console "Event Received: " (pr-str event))(handle-message ws-message event)))(defstate channel-router:start (sente/start-chsk-router!
(:ch-recv @socket)#'receive-message!):stop (when-let [stop-fn @channel-router](stop-fn)))

Our handle-message function is structured similarly to the one on our server,
but instead of interacting with the database, it dispatches re-frame events
based on the message received.
We also have to handle a couple Sente-specific events. The :chsk/handshake and:chsk/state events are related to the status of our connection. While they’reimportant for notifying users about lost or spotty connections, we’ll just log
them in the console for now.

• Click HERE to purchase this book now. discuss• 16
Now that we’ve sorted out our guestbook.websockets namespace, let’s update
guestbook.core to use it correctly so that we can get back to coding interactively.
guestbook-sente-setup/src/cljs/guestbook/core.cljs(ns guestbook.core(:require;;...
[mount.core :as mount]));;...
(rf/reg-event-fx:message/send!
(fn [{:keys [db]} [_ fields]](ws/send! [:message/create! fields]){:db (dissoc db :form/server-errors)}));;...
(defn init! [](.log js/console "Initializing App...")(mount/start)(rf/dispatch [:app/initialize])(mount-components))

We required mount.core, called mount/start from our init! function and changed
the value we sent from ws/send! in our :message/send! event. We finally have ourapp functionally migrated from our from-scratch WebSockets implementation
over to Sente. Since we required a new library, we need to restart our app toload the new dependency. Once that’s done, let’s try it out. You might’ve
noticed that our fields don’t clear when we submit our form. That was intentionally left out because it’s well suited to the callback feature of Sente.

Leveraging Sente Callbacks
Sente is primarily a WebSockets library, but it allows you to get the best ofboth the Ajax and WebSocket workflows. Generally speaking, having theserver push the results of an action to all concerned parties is incredibly
powerful, as it separates the concerns of keeping state synchronized from thelogistics of client-server communication. But this starts to break down when
we want information about our actions. In this case, the request/responsemodel of Ajax is a better fit. Sente allows you to mix the two by specifying acallback function when you send a message from a client.
We have two behaviors that are related to the state of our actions: clearing
fields after a message is successfully sent and displaying server errors after
a message is rejected.
Let’s update our code to use a reply function to accomplish these tasks.

• Click HERE to purchase this book now. discuss

Upgrading to Sente • 17

First, we need to allow our client-side send! function to take multiple arguments:
guestbook-sente-cb/src/cljs/guestbook/websockets.cljs(defn send! [& args](if-let [send-fn (:send-fn @socket)](apply send-fn args)(throw (ex-info "Couldn't send message, channel isn't open!"{:message (first args)}))))

Next, we need to require [guestbook.websockets :as ws] and pass our timeout andour callback function from :message/send!:
guestbook-sente-cb/src/cljs/guestbook/core.cljs(rf/reg-event-fx:message/send!
(fn [{:keys [db]} [_ fields]](ws/send!
[:message/create! fields]
10000(fn [{:keys [success errors] :as response}](.log js/console "Called Back: " (pr-str response))(if success(rf/dispatch [:form/clear-fields])(rf/dispatch [:form/set-server-errors errors])))){:db (dissoc db :form/server-errors)}))Finally, we need to update our server to invoke the :?reply-fn on the message:
guestbook-sente-cb/src/clj/guestbook/routes/websockets.clj(defmulti handle-message (fn [{:keys [id]}]
id))(defmethod handle-message :default[{:keys [id]}](log/debug "Received unrecognized websocket event type: " id){:error (str "Unrecognized websocket event type: " (pr-str id))
:id
id})
(defmethod handle-message :message/create!
[{:keys [?data uid] :as message}](let [response (try(msg/save-message! ?data)(assoc ?data :timestamp (java.util.Date.))(catch Exception e(let [{id:guestbook/error-id
errors :errors} (ex-data e)](case id:validation{:errors errors};;else{:errors{:server-error ["Failed to save message!"]}}))))]• Click HERE to purchase this book now. discuss• 18(if (:errors response)(do(log/debug "Failed to save message: " ?data)
response)(do(doseq [uid (:any @(:connected-uids socket))](send! uid [:message/add response])){:success true}))))(defn receive-message! [{:keys [id ?reply-fn]:asmessage}](log/debug "Got message with id: " id)(let [reply-fn (or ?reply-fn (fn [_]))](when-some [response (handle-message message)](reply-fn response))))There we go, that’s much better. Our handle-message multimethod now returns
a map that gets passed to the connection’s reply-fn by receive-message! if itexists. Not only does this change handle the clearing of our fields, but it
handles a bug we might’ve encountered later. We were sending the message
errors to the :uid of the sender, which is correct as long as a :uid is exactly one
connected client. This is currently the case, but when we add user accounts
it won’t be. A connection’s :uid corresponds to all connections belonging tothat user. This means that if a user were to submit a message from theirdesktop, errors would display in every one of their active connections (forexample, their phone, their five other browser tabs,…). It would be even worse
if we’d implemented clearing fields on success in the same way—users could
lose long drafted messages in other windows on other devices! Luckily, using:?reply-fn is an easy and elegant way to handle client-specific concerns.

• Click HERE to purchase this book now. discuss