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Drawbridge (HTTP Transport for nREPL)

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HTTP transport support for Clojure’s nREPL implemented as a Ring handler.


The coordinates of the project changed from cemerick/drawbridge to nrepl/drawbridge in version 0.1.

Drawbridge is available in Clojars. Add this :dependency to your Leiningen project.clj:

[nrepl/drawbridge "0.2.1"]

Or, add this to your Maven project’s pom.xml:


Drawbridge is compatible with Clojure 1.7.0+ and nREPL 0.4+.

Upgrade notes

If you’re upgrading from 0.0.7 keep in mind that the namespaces of the project were changed as following:

  • cemerick.drawbridge -> drawbridge.core

  • cemerick.drawbridge.client -> drawbridge.client


While nREPL provides a solid REPL backend for Clojure, typical socket-based channels are often unsuitable. Being able to deploy applications that allow for REPL access via HTTP and HTTPS simplifies configuration and can alleviate security concerns, and works around limitations in various deployment environments where traditional socket-based channels are limited or entirely unavailable.

In a Ring web application

Once you have added Drawbridge to your project’s dependencies, just add its Ring handler to your application. For example, if you’re using Compojure for routing and such:

(require 'drawbridge.core)

(let [nrepl-handler (drawbridge.core/ring-handler)]
  (ANY "/repl" request (nrepl-handler request)))

With this, any HTTP or HTTPS client can send nREPL messages to the /repl URI, and read responses from the same. Conveniently, any security measures applied within your application will work fine in conjunction with Drawbridge; so, if you configure its route to require authentication or authorization to some application-specific role, those prerequisites will apply just as with any other Ring handler in the same context.

Some things to be aware of when using drawbridge.core/ring-handler:

  • It requires GET and POST requests to be routed to whatever URI to which it is mapped; other request methods result in an HTTP error response.

  • It requires these standard Ring middlewares to function properly:

    • keyword-params

    • nested-params

    • wrap-params

Especially if you are going to be connecting to your webapp’s nREPL endpoint with a client that uses Drawbridge’s own HTTP/HTTPS client transport (see below), this is all you need to know.

If you are interested in the implementation details and semantics, perhaps because you’d like to implement support for Drawbridge in non-Clojure nREPL clients, you’ll want to review the documentation for ring-handler, which contains additional important details.

In Clojure tooling

Drawbridge also provides a client-side nREPL transport implementation for the Ring handler in drawbridge.client/ring-client-transport.

Note that the drawbridge.client namespace implicitly adds implementations to the nrepl.core/url-connect multimethod for "http" and "https" schemes. So, once this namespace is loaded, any tool that uses url-connect will use ring-client-transport for connecting to HTTP and HTTPS nREPL endpoints.


The client supports additional HTTP headers, which is useful e.g. for using Bearer authorization to connect to the endpoint. The headers can be set in the nREPL configuration. For example, create .nrepl.edn in the working directory with the contents:

{:drawbridge {:http-headers {:Authorization "Bearer <JWT token>"}}}


The biggest outstanding issues are around the semantics of how HTTP session (might optionally) map onto nREPL sessions. Right now, they don’t at all, though HTTP sessions are significant insofar as they retain the message queue nREPL will dispatch responses to that are emitted by asynchronous or long-running operations.

Secondarily, supporting nontextual REPL interactions over HTTP has not yet been addressed at all.

Need Help?

The primary support channel for Drawbridge is the Clojurians Slack. Feel free to ask any questions on the #nrepl channel there.


Copyright © 2012-2019 Chas Emerick, Bozhidar Batsov and other contributors.

Distributed under the Eclipse Public License, the same as Clojure.


An HTTP/HTTPS nREPL transport, implemented as a Ring handler.







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