A guide to hiring a Clojure developer
Clojure is a functional programming language in the Lisp family. It runs on the Java Virtual Machine, so it works with Java libraries and the same servers and tools as Java code. Teams use it for back end services, data processing and web apps. A Clojure developer builds and looks after those systems. If your product is built around data that changes shape, or you want less code that is easier to follow, this is the role to look at.
What the job covers day to day
Writing functions is only part of it. A Clojure developer shapes the data that moves through the system, splits the app into small namespaces and decides where state lives, if it needs to live anywhere. They write tests, follow logs from a running service and keep the build easy to repeat. They also look after what users never notice: how quickly the app starts, how a newcomer reads the code and how safely a change can go out.
Clojure on the Java platform
Clojure compiles to Java bytecode and runs on the JVM. That means it can call any Java library and be packaged and hosted much like other Java apps. A good developer knows where the two languages meet. They wrap Java calls in plain Clojure functions, keep that layer thin and avoid spreading Java types through the rest of the code. If your team already runs Java, this makes Clojure easier to adopt. If you are still choosing a language for the server, our back end developers can advise on options.
Data first, functions second
Clojure programs are built from plain data: maps, vectors, sets and lists. The data is immutable by default, so a function returns a new value instead of changing an old one. That makes behaviour easier to predict and easier to test. Code in Clojure is also data, which is why the language can build tools such as macros. Strong developers use these features with restraint and prefer a simple function to a clever one.
REPL-driven development
The REPL is a live session connected to a running program. A developer writes a small piece of code, sends it to the REPL and sees the result at once. Over a day this replaces many slow cycles of build, restart and click through. It also helps when taking over old code, since you can try a function with real input before changing it. Ask candidates how they set up their editor and REPL, and what they do to keep the files and the session in sync.
Back ends, APIs and data
Most Clojure work on the server is web services and data jobs. Common building blocks include Ring for handling requests, Reitit for routing, next.jdbc for SQL databases and Datomic, a database designed around immutable facts, on projects that use it. APIs often speak JSON, and sometimes EDN, which is Clojure’s own data format. A good developer keeps routes thin, checks input at the edge and writes queries that are easy to read.
ClojureScript
ClojureScript compiles Clojure to JavaScript, so the same language can be used in the browser. Teams often pair it with libraries such as Reagent and re-frame for the interface and shadow-cljs for builds. It can share code with the back end, such as data checks. Not every Clojure developer has built a browser app, so ask for an example if you need one.
State and concurrency
Because data is immutable, many tasks need no shared state at all. When state is needed, Clojure gives a few clear tools: atoms for a single value that changes, and other reference types and core.async channels for more complex flows. A strong developer can explain when each is the right choice, and why most code should stay as plain functions. This is also a good area to probe in an interview.
Testing and data checks
Clojure ships with clojure.test, and small pure functions are simple to test. Tools such as clojure.spec and Malli describe the shape of data and catch bad input early. Useful tests are quick, check what the code does and stand in fakes for outside services. When a team trusts its tests, it can change code without fear.
Where this role sits next to others
A back end developer can work in any language, and a Clojure developer is chosen for depth in one. Common Lisp is a relative with older roots, and our Lisp developers cover it. An Elixir developer also writes functional code, on a platform made for many live connections. A Java developer uses a more object-based style on the same platform as Clojure. Tell us which code you hold, and we will match the right person.
Typical reasons to hire
Some clients need a new data-driven service, such as an API that pulls from many sources. Others inherit a Clojure app with thin notes and few tests. Some want a ClojureScript front end tidied up, and some are moving one part of a Java system into Clojure, or back. The skills differ for each, so tell us which one is yours.
When a Clojure developer makes sense
- Your system works with data that changes shape often.
- You already run on the Java platform and want less code to maintain.
- You have a Clojure codebase that needs someone who knows it.
- You want a small team to build and test quickly in a live REPL.
- You need a browser app that shares code with the back end.
For a plain brochure site with a few forms, a common stack will be cheaper to run and simpler to staff. An engineer who says so at the start is working for you.
How to read a candidate’s history
Ask about one system they built or maintained, who relied on it and what broke. Have them trace how data travels from a request to the database and back. A list of tools says little, since anyone can write Ring on a resume. A stronger sign is a person who can describe a production fault in detail and the change that stopped it returning.
Questions that open up a good talk
- How do you decide whether something needs an atom or can stay a plain value?
- How would you call a Java library without letting Java types leak through the code?
- How do you describe and check the shape of data coming into an API?
- What does your REPL setup look like, and how do you keep it matching the files?
- How do you test code that reaches a database or an outside service?
Look for reasons and real examples. Be careful with anyone who praises the elegance of Clojure but cannot say how they ship and run it.
A short paid task
Set finalists the same small job taken from your own project, such as fixing a broken test, adding one endpoint or reviewing a short namespace for risks. Pay for the hours. Then compare the code, the tests, the questions each person asked and how plainly they explain their choices.
Keeping the work in good shape
Settle how code gets reviewed, how releases go out and who watches alerts. Request a short note on how the app is laid out and how to start a REPL and run the tests. Hold the repository, servers and database accounts under your own name, with one login per person, so nothing is lost when people change.
The first weeks
Give your new developer the repository, the build tool, the tests and a staging copy of the app. A weekly call and one shared board make progress visible. Bring up the rough spots early, such as a namespace nobody wants to touch or a data shape that keeps shifting, since that is where a Clojure specialist helps first.
Common slips
- Choosing on the language name alone, with no sign of running a live system.
- Letting clever macros replace plain functions anyone can read.
- Skipping tests because the functions look small.
- Picking Clojure for a site that has no need for it.
Related help: our software engineers suit wider product teams, and our database developers suit work where the data layer comes first.