Building Clients

This page is very incomplete and a work in progress.


This part of the documentation is mostly intended for people who are working on nREPL clients (e.g. command-line tools and editors). We’ll go over the basics of building an nREPL client and some of the best practices that might not be obvious to people who are not familiar with the process.

We’ll try to keep the conversation mostly focused on building clients for the nREPL protocol in general, but we’ll also highlight some specifics of nREPL’s Clojure implementation.


At the very least a client should be able to connect to an nREPL server and support code evaluation (the eval op). Ideally, the client should be able to make use of all the ops that are part of the nREPL protocol.

While, the reference nREPL server for Clojure supports multiple data exchange formats, it’s important for clients to support bencode, as that is the only format embraced the nREPL protocol. If a client is focused only on Clojure(Script) nREPL servers they can safely opt for EDN, or ideally support both bencode and EDN.

A typical message exchange between a client and a server would be:

  • Client sends clone to create a new session.

  • Server responds with the session id.

  • Client sends describe to check the server’s capabilities and version/runtime information.

  • Server responds with a map of the relevant data.

  • Clients starts sending eval messages using the session id that it obtained earlier.

  • Server responds with the appropriate messages (e.g. value and out). Eventually a message with status done signals that the eval message has been fully processed.

If you don’t pass a session explicitly to some op, the server is supposed to create a transient (one-off) session on the fly. That’s fine for most ops, as they don’t interact with the session state, but it’s generally a bad idea for user evaluations.

The reference nREPL implementation bundles a simple command-line client implementation that should give a pretty good overview of how to build a client. There are a few other simple clients that you can explore for inspiration:

It might also be a good idea to see the message exchange between some nREPL and client. CIDER provides a convenient solution for this.

Best Practices

Below is a listing of best practices when it comes to building nREPL clients. A common problem with clients is that they typically assume they are talking to a Clojure server. While this is OK in some cases (e.g. if you’re building a Clojure IDE), it’s always better to build clients that can work with any nREPL server. Note that some of the suggestions here might not apply to all servers.

  • Keep user evaluations in one sessions and tooling-related evaluations (e.g. for code completion) in another. This will ensure that vars like *1 and *e are not messed up.

  • Don’t assume too much about the server. Infer the capabilities of the server using describe. Try to make your client Clojure-agnostic.

  • Leverage streaming of values and output, when supported by the server. This improves a lot the performance when you’re dealing with huge results.

  • Leverage data cut-off limits when supported by the server.

  • Match requests and responses based on their ID, if you need to assign different callbacks to messages of the same type (this is a common situation in programming editors).

  • Use sequential message ids in the scope of a session - this makes it easier to track the flow of messages.

  • Provide some debug mode where you can see a dump of the requests and the responses.

Implementing sideloading

Sideloading is a feature specific to the reference Clojure nREPL server.

Let’s start by looking at an exchange of messages while using sideloading:

;; -> init sideloading
{:op      "sideloader-start"
 :id      "1"
 :session "x"}

;; -> try to require a namespace that's not available locally
{:op      "eval"
 :id      "2"
 :session "x"
 :code    (quote (require '[ :as bar])

;; <- lookup for
{:id      "1"
 :session "x"
 :status  :sideloader-lookup
 :type    "resource"
 :name    "foo/bar.clj"}

;; -> providing resource
{:id      "3"
 :session "x"
 :op      "sideloader-provide"
 :type    "resource"
 :name    "foo/bar.clj"
 :content "<base64 package>"}

;; <- ack of provided resource
{:id      "3"
 :session "x"
 :status  :done}

;; <- result of eval
{:id      "2"
 :session "x"
 :value   "Qaz"
 :status  :done}

Note that between sending the eval op in message 2 and receiving the result, two other messages were dealt with:

  • We received a response from message 1, requesting a resource. This indicates that the the resource was not found on the server JVM, and evaluation is blocked until we provide it, or respond with an empty package (:content should be an empty string).

  • We provide the resource/file using message 3, to which the server responds with a :done.

This unblocks the eval command, which returns the value Qaz in the final message.

It’s thus important, once sideloading has started, to asynchronously handle lookup requests, without these being blocked by waiting on a response from another message.

Modifying middleware

To add a middleware that’s already available on the server’s classpath, it’s as simple as sending the message

{:op "add-middleware"
 :middleware ["cider.nrepl.middleware/wrap-version"]}

However, if the middleware is loaded, it’s very likely that it hasn’t been included as a dependency on the server, and thus unavailable on its classpath. Furthermore. In this case, we can use the dynamic-loader in conjunction with the sideloader:

{:op "sideloader-start"}
;; handle sideloading separately...
;; now we add the middleware
{:op "add-middleware"
 :middleware ["cider.nrepl.middleware/wrap-version"]}
;; confirm it's being loaded..
{:op "ls-middleware"}
;; and we should get something like...
{:status #{:done}
 :middleware [... "#'cider.nrepl.middleware/wrap-version" ...]}

However, if we tried to use the middleware with an cider-version op, we’d get an error, because the middleware is implemented in a different namespace, which is only loaded on the first use of the cider-version op. This is a practice in many middleware to improve startup performance. One method of getting around this is to request the extra namespace to be loaded at add-middleware time too:

;; after starting the sideloader...
{:op "add-middleware"
 :middleware ["cider.nrepl.middleware/wrap-version"]
 :extra-namespaces ["cider.nrepl.middleware.version"]}
 ;; now, the following should work
 {:op "cider-version"}

There is no operation to remove a single middleware, but it’s possible to reset the stack to a baseline with the swap-middleware operation. If the goal is to simply reset the middleware stack, use this in conjunction with nrepl.server/default-middleware.

Also note that updating the middleware stack may also destroy/re-create middleware state. As an example, sideloading would need to be re-started. The impact on each middleware differs, however, as some of them, e.g. session holds their state globally.

Debugging the Communication with nREPL

Occasionally it’s be useful to inspect the communication between your client and nREPL. nREPL clients are generally encouraged to provide some nREPL message logging functionality, as that’d be beneficial for their end users as well. (e.g. CIDER has a *nrepl-messages* where you can monitor all requests and responses)

There’s also nrepl-proxy, that allows you to intercept the communication between an nREPL server and a client. Its output looks something like this:

nil ---> 1 clone {}
J <=== 1 #{:done} {:new-session "002914a8-db79-408d-807a-c5b3955ab6f9"}
nil ---> 2 clone {}
X <=== 2 #{:done} {:new-session "6a7e7b99-1b8e-4008-bbe5-ddddf46672a9"}
Y ---> 3 describe {}
Y <=== 3 #{:done} {:aux {:current-ns "user"}, :ops {:stdin {}, :add-middleware {}, :lookup {}, :swap-middleware {}, :sideloader-start {}, :ls-middleware {}, :close {}, :sideloader-provide {}, :load-file {}, :ls-sessions {}, :clone {}, :describe {}, :interrupt {}, :completions {}, :eval {}}, :versions {:clojure {:incremental 3, :major 1, :minor 10, :version-string "1.10.3"}, :java {:version-string "17"}, :nrepl {:incremental 0, :major 0, :minor 9, :version-string "0.9.0"}}}
Y ---> 4 eval {:nrepl.middleware.print/buffer-size 4096, :file "*cider-repl lambdaisland/nrepl-proxy:localhost:5424(clj)*", :nrepl.middleware.print/quota 1048576, :nrepl.middleware.print/print "cider.nrepl.pprint/pprint", :column 1, :line 10, :code "(clojure.core/apply clojure.core/require clojure.main/repl-requires)", :inhibit-cider-middleware "true", :nrepl.middleware.print/stream? "1", :nrepl.middleware.print/options {:right-margin 80}}
G ---> 5 eval {:code "(seq (.split (System/getProperty \"java.class.path\") \":\"))"}
Y <--- 4 #{:nrepl.middleware.print/error} #:nrepl.middleware.print{:error "Couldn't resolve print-var cider.nrepl.pprint/pprint"}
Y <--- 4 #{} {:value "nil"}
Y <--- 4 #{} {:ns "user"}
Y <=== 4 #{:done} {}

You can read more on the topic here.