Holding up your web app

One to watch: Pedestal, a new Clojure web toolset

Elliot Bentley

We speak to the lead developer of Pedestal, a new open source project taking a fresh approach to Clojure web app development.

Last week saw the
high-profile launch of Pedestal, a new open-source Clojure
toolset. Developed by software house Relevance, and revealed at
Clojure/West, it immediately gathered plenty of attention (and
criticism). We got in touch with Tim Ewald,
who led the development of Pedestal, to find out more.

Why did you develop Pedestal?

Tim: We built Pedestal because the
existing infrastructure for Web development in clojure didn’t
provide features we needed. Specifically, for developing Web
services, we needed the ability to decouple HTTP request processing
from a specific thread os that we could handle long polling and
server-sent events efficiently. Ring is the de facto standard
library for HTTP request processing on a server, but it can’t
provide this functionality because its processing model couples a
request to a single Web server thread. That said, Ring does provide
a ton of useful functionality for HTTP request processing (e.g.,
cookie and session management, file and resource handling, etc). So
the Pedestal service library builds on Ring. We worked with the
Ring maintainers to refactor the HTTP processing code so that it
would work across threads in Pedestal, but also still work with
Ring. In addition, we needed a request routing layer that can
generate URLs, so we built one of those too.

On the application side of things, in the browser, we
needed a way to organize and structure clojurescript code “in the
large”. The Pedestal application library is focused on creating
interactive dynamic browser-based apps using a data flow
programming model. The biggest feature in the application model is
the separation of application logic from rendering, and support for
representing changes to the rendering as data. This allows us to
step forward and backward through rendering changes to see how the
UI reacts to changes in the underlying application state. This is
very useful for development and debugging. The split also allows us
to build and test the bulk of the application outside the

Why is it important to work with the same
language on both the client and server?

Using the same language on both the client and server
simplifies development and makes it possible to share code. We do
most of our app development outside the browser. That code is
written as clojure, but when we use it in the browser it is
compiled as clojurescript. We can write logic that can be moved
from the browser to the server, if desired, or can run in both
places if necessary. It also helps unify the testing story – the
bulk of the client application code is tested outside the browser
using the same testing tools we use for server-side code.

Why Clojure?

Clojure is a simple, expressive, powerful dynamic
language that makes it easier for us to build solutions for
customers. It takes advantage of an industrial strength runtime,
the JVM, and fits directly into the Java ecosystem widely used by

How does Pedestal help with

Pedestal applications move work into the browser,
which aids service scaling – the services aren’t doing any UI
rendering work. When we use Pedestal services with Datomic, we get
a great horizontal scale-out story. We can scale read access
without having to add replicas, as you would with a traditional SQL

Do you use Pedestal in-house?

Yes. We built Pedestal to make it easier to for us to
build the kinds of systems our customers are interested in.

What is the learning curve like?

Pedestal is brand new, and it’s an early release –
there is definitely a learning curve. The service side of things
isn’t too difficult to pick up – on the surface it looks like a lot
of other HTTP processing stacks. The application model is more
complex – partly because it takes a different approach to
structuring an app based on data flow programming that many
developers aren’t familiar with. There is definitely work to do to
make the application story simpler, and we’re working on it.

Where do you hope to take

We built Pedestal because we needed a way to build systems for
ourselves and our customers. We released it because we think it’s
generally useful. Hopefully others will think so too and will use
it to build their systems.

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