Prerenderer 0.2.0 released

We are proud to announce the release of version 0.2.0 of our ClojureScript library Prerenderer, a library to do server side pre-rendering of single page applications. In this release, we include:

The two first items in the changelog came hand in hand and they are the biggest changes to keep in mind if you are upgrading. We are very happy that we no longer need a fork of re-frame and we would like to extend our gratitude to Mike Thompson for working with us on having the appropriate API to make this happen.

The change in API means that your Prerenderer module now would look something like this:

(ns projectx.node
  (:require [prerenderer.core :as prerenderer]))

(defn render-and-send [page-path send-to-browser]
  (send-to-browser (render page-path)))

(set! *main-cli-fn* (prerenderer/create render-and-send "ProjectX"))

instead of:

(ns projectx.node
  (:require [cljs.nodejs :as nodejs]
            [prerenderer.core :as prerenderer]))

(defn render [req res]
  (let [page-path (.-path (.parse url (.-url (.-query req))))]
    (.send res (render page-path))))

(set! *main-cli-fn* (prerenderer/create render "ProjectX"))


Isomorphic JavaScript (with ClojureScript) for pre-rendering single-page-applications, part 3

I was not expecting there to be a part 3 to this series and this third part is also going to be quite different to the first two. In parts 1 and 2 I walked you through an exploration of server side pre-rendering with Nashorn. My naive example worked fine with Nashorn but it didn’t survive encountering the real world.

Nashorn is not a headless browser, it’s a plain JavaScript engine. It doesn’t implement document or window for example, which were easy to workaround, but it also doesn’t implement setTimeout, setInterval or XMLHttpRequest which are much harder to workaround.

When I started to look for alternatives I focused on NodeJS because I knew if implemented those things I was missing and V8‘s speed is attractive. Also, the fact the ClojureScript has it as a compilation target made me feel it was well supported, a first class citizen.

At this point someone might interject and exclaim: What about is an attempt to bring all the NodeJS goodness to Nashorn and I think it’s a great idea. Sadly, on GitHub we can find this notice:

This project is no longer being actively maintained. If you have interest in taking over the project, please file an issue.

I’m not sure if the project got far before being abandoned either.

Implementing the missing bits of Nashorn in Clojure was tempting. It looks like fun and it also looks like something that might be popular amongst Java users and thus good for the Clojure echo system. I exercised some restrain and moved on.

In the process of experimenting with NodeJS my code quickly took the form of a library and without further ado, let me introduce you to Prerenderer. The ultimate solution for all your server side pre-rendering needs. I’m not going to show you how to use it here because its page go into a lot of detail already.

My big concern about prerendering is performance and stability. As I showed in part 2, a naive implementation can behave horribly while in production, sometimes taking up to 17 seconds to serve a page. Prerenderer was not developed with a sample project, like I did with Nashorn, but with a project we are working on called Ninja Tools that uses re-frame and AJAX. Before any modifications to it, this was its performance:

Ninja Tools performance with no server side rendering

After enabling Prerenderer, this is how it looks like:

Ninja Tools with Prerenderer

The average response time went up from 51ms to 362ms. This would generally be a very bad thing. The reason for this is explained in Prerenderer’s documentation:

[…] SPAs are never done. Imagine a SPA that has a timer and every second sends a request to the server, and the server replies with the current time, which then the application displays. When is it done rendering? Never. But Prerenderer needs to, at some point, decide that the page is done enough and ship it to the browser. […]

[…] Prerenderer will watch for events and once nothing happened for a period of time (300ms by default) it’ll consider the application done and if a certain amount of time went by (3s by default) even if the application is still active, it’ll stop and send it to the browser.

That’s where the jump in 300ms is coming from and it’s constant. It’s not linear and definitely not exponential. It’s a constant number that can be tuned and tweaked. There are also some potential optimizations to reduce it or remove all together.

The important thing is that all other values remained more or less the same and that the performance characteristics where quite stable. For me, this feels good enough to move on and start producing SPAs and with a bigger codebase we’ll be able to improve this library and make it better.

Picture by Ian Farrel

Isomorphic JavaScript (with ClojureScript) for pre-rendering single-page-applications, part 2

In part 1 I covered the basic problem that SPA (single page applications) face and how pre-rendering can help. I showed how to integrate Nashorn into a Clojure app. In this second part, we’ll get to actually do the rendering as well as improving performance. Without further ado, part 2 of isomorphic ClojureScript.

Rendering the application

Now to the fun stuff! It would be nice if we had a full browser running on the server where we could throw our HTML and JS and tell it go! but unfortunately I’m not aware of such thing. What we’ll do instead is call a JavaScript function that will do the rendering and we’ll inject that into our response HTML.

The function to convert a path into HTML will be called render-page and it’ll be in core.cljs:

(defn ^:export render-page [path]
  (reagent/render-to-string [(parse-path path)]))

We need to mark this function as exportable because JavaScript optimizations can be very aggressive even removing dead code and since this code is called dynamically from Clojure, it’ll look like it’s unused and it’ll be removed.

render-page  is similar to mount-root but instead of causing the result to be displayed to the user, it just returns it. The former takes the path as an argument while the latter reads it from the local state which is in turn set by Pushy by reading the current URL.

To invoke that function, we’ll go back to handler.clj, just after we define js-engine we’ll define a function called render-page:

render-page (fn [path]
                ^Invocable js-engine
                (.eval js-engine "projectx.core")
                (object-array [path])))

and instead of sending a message about the application is loading, we just call it:

[:div#app [:div (render-page path)]]

That extra div is not necessary, it’s there only because projectx.core/current-page adds it and without it you’ll get a funny error in the browser:

React error about server rendering not matching

Aside from that little trip into the internals of React, which is interesting, we now have a snappy, pre-rendered application… that is… if you can wait 3 seconds or so for it to load:

Server side scripting taking too long to load

That is not good, not good at all. We have a serious performance problem here, we need to get serious about fixing it.


The first step to fix any performance problems is making sure you have one, as premature optimization is the root of all evil. I think we are at this point with this little project. The second step is measuring the problem: we need a good repeatable way of measuring the problem that allows us to actually locate it and and verify it was fixed.

To measure the performance behaviour of this app I’m going to use one of Heroku’s bigger instances, the Performance-L, which is a dedicated machine with 14GB of RAM. The reason is that I don’t want out of memory or my virtual CPU affected by other instances to muddy my measurements. That unacceptable 3 seconds load time was measured in that type of server.

To perform the load and the measurement of the response I’m going to use the free version of BlazeMeter, an web application to trigger load testing which I’m falling in love with. The UI is great. I’m going to hit the home and the about page with their default configuration which includes up to 20 virtual users:

BlazeMeter configuration

In all the tests I’m going to make a few requests to the application manually after any restart to make sure the application is not being tested in cold. Ok… go!

Performance with naive script engine

That is terrible! Under load it behaves so much worst! 17.1s response time. Now that we have a way to measure how horrendous our application is behaving, we need to pin-point which bit is causing this. The elephant in the room is of course server-side JavaScript execution.

Disabling the server side JavaScript engine causes load times to go down:

Load time without scripting engine

but what we really care about is the load testing:

Load testing without script engine

40ms vs 17000ms, that’s a big difference! The scripting engine is definitely the problem, so, what now?

Optimizing time

Now it’s time to find optimizations. Poking around Nashorn it seems the issue is that it has a very slow start. We already know that browsers spend a lot of time parsing and compiling JavaScript and the way we are using Nashorn, we are parsing and compiling all our JavaScript in every request. Clearly we should re-use this compiled JavaScript.

Re-using Nashorn is not straightforward because it’s not thread safe while our server is multi-threaded. JavaScript just assumes that there’s one and only thread and when developing Nashorn they decided to respect that and not make any other assumptions, which leads to a non-thread-safe implementation. We need to re-use Nashorn engines, but never at the same time by two or more threads.

Nashorn does provides a way to have binding sets, that is, the state of a program, separate from the Nashorn script engine, so that you could use the same engine with various different states. Unfortunately this is very poorly documented. Fortunately, ClojureScript is immutable, so we don’t have much to worry about breaking state.

After a lot of experimentation and poking, I came up with an acceptable solution using a pool. My choice was to use Dirigiste through Aleph‘s Flow. To do that, we extract the creation of a JavaScript engine into its own function:

(defn create-js-engine []
  (doto (.getEngineByName (ScriptEngineManager.) "nashorn")
    (.eval "var global = this")
    (.eval (-> "public/js/server-side.js"

Then we define the pool. In Dirigiste, each object in the pool is associated to a key, so that effectively it’s a pool of pools. We don’t need this functionality, so we’ll have a single constant key:

(def js-engine-key "js-engine")

and without further ado, the pool:

(def js-engine-pool
    {:generate   (fn [_] (create-js-engine))
     :controller (Pools/utilizationController 0.9 10000 10000)}))

flow is aleph.flow and Pools is io.aleph.dirigiste.Pools. In this pool you can have different controllers which create new objects in different ways. The utilization controller will attempt to have the pool at 0.9, the first arg, so that if we are using 9 objects, there should be 10 in the pool. The other two args is the maximum per key and the total maximum and they are set two numbers that are essentially infinite.

The reason for such a big pool is that you should never run out of JavaScript engines. If your server is getting too many requests for the amount of RAM, CPU or whatever limit you find, it should be throttled by some other means, not by an arbitrary pool inside it. Normally you’ll throttle it by limiting the amount of worker threads you have or something like that.

The function render-page was promoted to be top level and now takes care of taking a JavaScript engine from the pool and returning it when done:

(defn render-page [path]
  (let [js-engine @(flow/acquire js-engine-pool js-engine-key)]
    (try (.invokeMethod
           ^Invocable js-engine
           (.eval js-engine "projectx.core")
           (object-array [path]))
         (finally (flow/release js-engine-pool js-engine-key js-engine)))))

The function to render the app now doesn’t create any engines, it just uses the previous method:

(defn render-app [path]
      [:meta {:charset "utf-8"}]
      [:meta {:name    "viewport"
              :content "width=device-width, initial-scale=1"}]
      (include-css (if (env :dev) "css/site.css" "css/site.min.css"))]
      [:div#app [:div (render-page path)]]
      (include-js "js/app.js")]]))

Let’s load test this new solution:

Load testing with script engine pool

That is a big difference. It’s almost as fast as no server side scripting! You can find this change in GitHub:… as well as the full final project:


There are a few problems or potential problems with this solution that I haven’t addressed yet. One of those is that at the moment I’m not doing anything to have Nashorn generate the same cookies or session as we would have in the real browser.

This pool works well when it’s under constant use, but for many web apps that do not see than level of usage, the pool will kill all script engines which means every request will have to create a fresh one. Solving this might require creating a brand new controller, a mix between Dirigiste’s Pools.utilizationController  and Pools.fixedController.

A big thanks to DomKM for his Omelette app, that was a source of inspiration.

Another approach worth considering is to implement the rendering system in portable Clojure (cljc), the common language between Clojure and ClojureScript and have it run natively on the server, without the need of a JavaScript engine. I’m very skeptical of this working in the long run as it means none of your rendering function can ever use any JavaScript or if they do, you need to implement Clojure(non-Script) equivalents.

This approach is being explored by David Tanzer and he wrote a blog post about it: Server-Side and Client-Side Rendering Using the Same Code With Re-Frame. David’s approach is to use Hiccup to do the rendering on the server side, where React and Reagent are not available. I personally prefer to steer clear of template engines that are not safe by default, like Hiccup at the time of  this writing, as they make XSS inevitable. The only reason why I’m using it in projectx is because that’s what the template provided and I wanted to do the minimum amount of changes possible.

Another optimization I briefly explored is not doing the server side rendering for browsers that don’t need it, that is, actual browser being used by people, like Chrome, Firefox, Safari, even IE (>10). The problem is that many bots do identify themselves as those types of browsers and Google gets very unhappy when its bots see a different page than the browsers, so it’s dangerous to perform this optimization except, maybe, for pages that you can only see after you log in.

In conclusion I’m happy enough with this solution to start moving forward and using it, although I’m sure it’ll require much tweaking an improvement. Something I’m considering is turning it into a library, but this library would make quite a bit of assumptions about your application, how things are rendered, compiled, etc. What’s your opinion, would you like to see this code expressed as a library or are you happy to just copy and paste?


There’s now a part 3 for this post.

Photo by Jared Tarbell

Isomorphic JavaScript (with ClojureScript) for pre-rendering single-page-applications, part 1

I don’t think I have found the ultimate solution for this problem yet but I have reached a level in which I’m comfortable sharing what I have because I believe it’ll be useful for other people tackling the same problem.

The reason why I doubt this is the ultimate solution is because it has not been battle tested enough for my taste. I haven’t used it in big applications and I haven’t used in production, maintaining it for months or years.

The problem

We are building SPAs, that is, single page applications. Think Google Maps or GMail. When you request the page, you get a relatively small HTML and a huge JavaScript app. This browser app then renders the page and from now on reacts to your interactions, requesting more data from the server whenever needed but never reloading the whole web page.

The reason to develop an application like this is that the user experience ends up being much better. The app feels faster, snappier, more alive. Reloading the whole page, parsing CSS, JavaScript and HTML is slow, but rendering a snippet of HTML is fast. Furthermore, once you have a full app on the client you can start taking advantage of it, performing, for example, validation, storing data than you won’t request again, etc. which saves talking to the server, making the user experience much better.

The problem, though, is that in the initial request you are not sending any content and many web consumers won’t run JavaScript to render your application. I’m talking about search engine bots, snippet generation bots (like the one Facebook, LinkedIn and Twitter use). Even though it seems Google’s bot is executing some JavaScript, it might not be wise to depend on it.

Snippet and image generated by Facebook
Snippet and image generated by Facebook

The solution is to run the client side of the application on the server up to the point of waiting for user interaction, generating the HTML that matches that page, and shipping that to the browser. This also help with the fresh page experience as the user will quickly get some content instead of having to wait for a lot of JavaScript to be parsed, compiled and executed (take a look at GMail and how long it takes to load and show you content).

GMail loading
GMail loading…

JavaScript, on the server

Running the client JavaScript on the server is often referred to as isomorphic JavaScript, meaning, same form, that is, same code, running on both server and client. There are several server-side (no windows, headless) JavaScript implementations to chose from:

When choosing my approach I was looking for a simple solution, one with the least moving parts to make it easier to deploy and more stable over time. Nashorn was an immediate winner as it ships with Java 8 and it’s well integrated, hiding away secondary processes and inter-process communication (if it’s happening at all, I’m not sure, and this is good).

Nashorn came with two big issues though:

  • It’s slow to create new Nashorn instances (this might be true for all JS implementations).
  • The documentation is not great.

I think I have overcame both of this issues, so, without further ado, let’s jump in. You can create a new script engine like this:

(.getEngineByName (ScriptEngineManager.) "nashorn")

ScriptEngineManager has many methods to get a script engine, some use the mime type, or the extension, and with those, you may or may not get Nashorn. I prefer to explicitly request Nashorn as it should be available on all Java 8 installations and I don’t believe we can transparently switch JavaScript implementations as they might be too different.

Once you have a script engine, evaluating code is very easy:

(.eval js-engine "var hello = 'world'")

The method eval can also take files, streams, etc. Invoking a JavaScript method is a bit more involved:

(.invokeMethod ^Invocable js-engine
               (object-array [arg1 arg2 arg3])

That will invoke the method method_name in the JavaScript object js-object which you can obtain this way:

(.eval js-engine "object_name")

There’s a lot more to Nashorn but that’s all we are going to use for implementing server-side JavaScript/ClojureScript.

The application

We’ll start from a reagent application created by:

lein new reagent projectx

which you can start by running:

lein figwheel

You can find all the code for this little application in GitHub: When you visit the app, you’ll briefly see this:

ClojureScript has not been compiled

That page, which you can find in handler.clj, is the actual HTML sent to the browser, before the ClojureScript/JavaScript kicks in:

(def home-page
     [:meta {:charset "utf-8"}]
     [:meta {:name "viewport"
             :content "width=device-width, initial-scale=1"}]
     (include-css (if (env :dev) "css/site.css" "css/site.min.css"))]
      [:h3 "ClojureScript has not been compiled!"]
      [:p "please run "
       [:b "lein figwheel"]
       " in order to start the compiler"]]
     (include-js "js/app.js")]]))

Or in actual HTML:

    <meta charset="utf-8"/>
    <meta content="width=device-width, initial-scale=1" name="viewport"/>
    <link href="css/site.css" rel="stylesheet" type="text/css"/>

ClojureScript has not been compiled!

please run lein figwheel in order to start the compiler

http://js/app.js </body> </html>

In production, you’ll normally want to show a message about the application being loaded. Here we are going to try to replace it with the actual rendered application.

After seeing that page briefly, ClojureScript gets compiled to JavaScript, served to the browser, executed and it renders the homepage, which looks like this:

Rendered homepage

This template conveniently ships with two pre-built pages, the home page and the about page. Click in the link to go to the about page and you’ll see its content but no request was sent to the server. All content was shipped before and the rendering happens client side:

About page with Network traffic

If we request that URL, we’ll se the same loading message and then the about page is going to be shown, but there’s a problem. The server doesn’t know that the about page was being requested because the fragment, the bit after the # in the URL, is not sent to the server.

Proper URLs

The reason why a fragment is used that way is because we don’t want to send a request to the server when we click a link and that’s what browsers do when you go from /blah#bleh to /blah#blih. Thankfully HTML 5 comes to the rescue with its history API. You can learn more about it in Dive into HTML5: Manipulating History for Fun & Profit. If you are wondering whether it’s safe to use this feature already, all current browsers support it (except Opera Mini) and IE since version 10:History Browsers support 2015-09

To move forward with server side rendering of SPAs you need to switch to HTML5 History, which is implemented in ClojureScript by a library called Pushy. While you are at it, I also recommend to switch to an bidirectional routing library like bidi or silk. To make the long story short, you can look at the diff to implement bidi and Pushy in projectx.

Now that the we are using sane URLs, we need to process them on the server side. In the file handler.clj we’ll find the main HTML template, the routes and the app:

(def home-page
     [:meta {:charset "utf-8"}]
     [:meta {:name "viewport"
             :content "width=device-width, initial-scale=1"}]
     (include-css (if (env :dev) "css/site.css" "css/site.min.css"))]
      [:h3 "ClojureScript has not been compiled!"]
      [:p "please run "
       [:b "lein figwheel"]
       " in order to start the compiler"]]
     (include-js "js/app.js")]]))

(defroutes routes
  (GET "/" [] home-page)
  (resources "/")
  (not-found "Not Found"))

(def app
  (let [handler (wrap-defaults #'routes site-defaults)]
    (if (env :dev) (-> handler wrap-exceptions wrap-reload) handler)))

home-page will stop being a constant as it’ll be a function on the path and while we are at it, let’s rename it to something more appropriate, like render-app:

(defn render-app [path]
      [:meta {:charset "utf-8"}]
      [:meta {:name    "viewport"
              :content "width=device-width, initial-scale=1"}]
      (include-css (if (env :dev) "css/site.css" "css/site.min.css"))]
       [:h3 "ClojureScript has not been compiled!"]
       [:p "please run "
        [:b "lein figwheel"]
        " in order to start the compiler"]]
      (include-js "js/app.js")]]))

The reason why it’s taking the path and not the full URL is that the ClojureScript part of this app works with paths instead of URLs and we’ll need them to be consistent. This is due to how Pushy and likely HTML5 History behave.

The routes will now pass the path to render-app:

(defroutes routes
  (GET "*" request (render-app (path request)))
  (resources "/")
  (not-found "Not Found"))

The function that turns the request into a path is similar to ring.util.request/request-url:

(defn- path [request]
  (str (:uri request)
       (if-let [query (:query-string request)]
         (str "?" query))))

When this change is done, you should see no effect in the running application at all. If you want to confirm things are working properly, you could add this to the render-app  function:

[:p path]

and you’ll see the path the server sees before the ClojureScript kicks in. You can see the diff for this step in GitHub:….

The JavaScript engine

Now things get interesting. The render-app method needs to run some JavaScript, so it’ll create the script engine. First, we need to import it (and also require , which we’ll be using soon):

(ns projectx.handler
  (:require ; ...
           [ :as io])
  (:import [javax.script ScriptEngineManager]))

After creating the engine, we need to define the variable global because Nashorn doesn’t specify it and reagent needs it. Once that’s done, we are ready to load the JavaScript code:

(defn render-app [path]
  (let [js-engine (doto (.getEngineByName (ScriptEngineManager.) "nashorn")
                    (.eval "var global = this")
                    (.eval (-> "public/js/app.js"
    ; ...

It doesn’t yet render anything, but let’s give it a try, let’s see it load the code or… well… fail:

javax.script.ScriptException: ReferenceError: "document" is not defined in <eval> at line number 2

What’s happening here is that app.js is referring document and Nashorn implements JavaScript, but it’s not a browser, it doesn’t have the global, window or document global objects. Let’s look at the offending file:

if(typeof goog == "undefined") document.write('http://js/out/goog/base.js');
document.write('if (typeof goog != "undefined") { goog.require(""); } else { console.warn("ClojureScript could not load :main, did you forget to specify :asset-path?"); };');

This is a generated JavaScript file that is loaded by our small HTML file. It in turns causes the rest of the JavaScript files to be loaded but the mechanism it uses works in a browser, not in Nashorn. This is where things get hard.

From the project definition, this is how app.js  is built:

:cljsbuild {:builds {:app {:source-paths ["src/cljs" "src/cljc"]
                           :compiler {:output-to     "resources/public/js/app.js"
                                      :output-dir    "resources/public/js/out"
                                      :asset-path   "js/out"
                                      :optimizations :none
                                      :pretty-print  true}}}}

It’s built with no optimizations. One of the optimizations, called whitespace, puts all the JavaScript in a single file, so there’s no document trick to load them, but sadly, it will not work in Figwheel.

The solution I came up with, a hack, is to have two builds. One called app which is what I consider the JavaScript app itself and the other one called server-side, which is the one prepared to run on the server:

:cljsbuild {:builds {:app {:source-paths ["src/cljs" "src/cljc"]
                           :compiler     {:output-to     "resources/public/js/app.js"
                                          :output-dir    "resources/public/js/app"
                                          :asset-path    "js/app"
                                          :optimizations :none
                                          :pretty-print  true}}
                     :server-side {:source-paths ["src/cljs" "src/cljc"]
                                   :compiler     {:output-to     "resources/public/js/server-side.js"
                                                  :output-dir    "resources/public/js/server-side"
                                                  :optimizations :whitespace}}}}

For sanity’s sake, I changed the output of app to go to the directory called app, instead of out. Running Figwheel will auto-compile app, but not server-side; for that, you also need to run lein cljsbuild auto. Now the application loads with no errors.

We also need to properly configure server-side for the dev and uberjar profiles:

:cljsbuild {:builds {:app         {:source-paths ["src/cljs" "src/cljc"]
                                   :compiler     {:output-to  "resources/public/js/app.js"
                                                  :output-dir "resources/public/js/app"
                                                  :asset-path "js/app"}}
                     :server-side {:source-paths ["src/cljs" "src/cljc"]
                                   :compiler     {:output-to     "resources/public/js/server-side.js"
                                                  :output-dir    "resources/public/js/server-side"
                                                  :optimizations :whitespace}}}}

:profiles {:dev     {;...
                     :cljsbuild    {:builds {:app         {:source-paths ["env/dev/cljs"]
                                                           :compiler     {:optimizations :none
                                                                          :source-map    true
                                                                          :pretty-print  true
                                                                          :main          ""}}
                                             :server-side {:compiler {:optimizations :whitespace
                                                                      :source-map    "resources/public/js/"
                                                                      :pretty-print  true}}}}}

           :uberjar {;...
                     :cljsbuild   {:jar    true
                                   :builds {:app         {:source-paths ["env/prod/cljs"]
                                                          :compiler     {:optimizations :advanced
                                                                         :pretty-print  false}}
                                            :server-side {:compiler     {:optimizations :advanced
                                                                         :pretty-print  false}}}}}}

You might have notice that we are not including env/dev/cljs  and env/dev/cljs  for server-side. That is because those files call projectx.core/init!, which triggers the whole application to start working, which depends on global objects, like window, which are not present in Nashorn.

With this, even the uberjar loads properly and creates JavaScript engines, but so far, we are not doing any server side rendering. That’s the next step. You can see the full diff for this change in GitHub:….

To be continued…

Part 2 has now been published.

Photo by Jared Tarbell