Head of Engineering

James Henderson


How-to: Clojure libraries on Maven Central

July 15, 2021
Crux is now XTDB

September 2021 Update: Crux has recently been renamed XTDB. This post still refers to XTDB as "Crux" but you will find newer versions of the artifacts discussed under the new com.xtdb Maven GroupId ([com.xtdb/xtdb-core "1.19.0"] for example) instead of the pro.juxt GroupId described in the examples below ([pro.juxt.crux/crux-core "1.17.1"] for example).

The official home for XTDB is now https://xtdb.com.

Much as we love Clojure here at JUXT, we’ll shortly be releasing a couple of non-Clojure modules for our database Crux (watch this space!), and we’d love these to be more accessible for people outside of the Clojure community.

Until recently, Crux artifacts were deployed to Clojars - this has been great for us so far, but it doesn’t make life as easy for projects using other JVM build tools.

It’s relatively straightforward to ask our non-Clojure users to add Clojars as a repository, but this has one main drawback: additional repositories aren’t applied transitively in Maven, for security reasons - every downstream dependency of a Clojars library has to have Clojars explicitly added as a repo. That is, from an end-user’s point-of-view, if any library anywhere in your dependency tree happens to be deployed to Clojars, you’ll have to add it.

Artifacts on Central, on the other hand, are trivially accessible to anyone using any Maven build tool.

So, we would like to appeal to Clojure library authors: if your library could be used by a non-Clojure application at some point, directly or indirectly, please do consider deploying your libraries to Central.

(This certainly isn’t to detract from how awesome Clojars and the Clojars folks are, nor the Clojure community funding it, nor is it to say that we’re stopping using it altogether. Indeed, in doing so, we can hopefully all save them some hosting, bandwidth, and administrative hassle!)

This guide aims to help you through the one-time setup required to get started.

What we know as 'Maven Central' is in fact a network of repositories, all sync’d into one entry point at https://repo1.maven.org/maven2. Larger organisations often have their own public repositories that are mirrored here - we’ll be deploying to the 'Open Source Software Repository Hosting' (OSSRH) service.

We’ll focus on Leiningen as it’s what Crux uses - no doubt the process will be similar for your build tool of choice. Leiningen has an excellent sample project.clj which contains examples and documentation of a lot of these configuration values - I highly recommend having this open in a tab any time you’re making non-trivial changes to your project.clj!

One-time setup - verifying your group ID

Maven Central first requires that we pick and verify our group ID. Group IDs can be based on the standard Java 'reverse domain name' pattern (e.g. org.clojure, org.apache, pro.juxt). Or, if you don’t want to verify a domain name, you can use a GitHub-based group ID (e.g. https://github.com/jarohen becomes io.github.jarohen, and similarly for GitLab, Bitbucket etc.). This is a similar, albeit slightly longer-winded, process to the new Clojars group ID verification process.

Buckle up, it’s JIRA time:

  1. Create yourself an account on Sonatype JIRA. You’ll use this same username and password to access the Nexus UI later.

  2. Create a New Project ticket.

  3. Verify your domain/git account:

    • For domains, this involves adding a TXT record containing a ticket number.

    • For git accounts, create a repo with the same name as the ticket number.

They say this could take 2 business days - in practice, for us, it was a couple of hours.

Once you’ve verified your group ID, you can then deploy under any sub-group too - e.g. we’ve only needed to verify pro.juxt but we deploy Crux’s modules under pro.juxt.crux.

There are more details on the requirements and JIRA process on the OSSRH site.

Per-user setup

To add subsequent users (including bot users) to a group, the process is similar to the process for creating the group (although without the need for any re-verification):

  1. Get the user to create a JIRA account. (If it’s a bot user, you’ll probably have to do this for them.)

  2. Open a card requesting access to the group for the new user.

  3. Get someone already on the group to vouch for them in a comment on the card.

Users can then log in to the Nexus UI using those same credentials. While you can also use these same credentials in Leiningen, there are separate deployment credentials available in this UI. Head to your profile - then, in the dropdown, select User Token.

Add these to your Leiningen config (I’ve added them to ~/.lein/credentials.clj.gpg) as outlined here.

 {:username "..."
  :password "..."}}

Finally, you’ll need to publish your public GPG key on a public keyserver. Assuming you have a GPG key set up locally, you can run gpg -K to get your key ID, and then gpg --keyserver keyserver.ubuntu.com --send-key <key-id> to publish it.

Further instructions about setting up GPG for OSSRH are available here.

Per-project setup

Maven Central does have a few more validations than Clojars - thankfully none too onerous! These need to be applied once per project - in Crux, we’ve made use of lein-parent to share as much as possible between our various modules.

Firstly, your project needs to contain a :license, :url and :description - it arguably should do anyway, but Maven Central insists on it. [1] These are passed down from Leiningen into the Maven pom.xml file, so adding them is straightforward:

  :description "General purpose bitemporal database for SQL, Datalog & graph queries."
  :url "https://github.com/juxt/crux"
  :license {:name "The MIT License"
            :url "http://opensource.org/licenses/MIT"}

Next, <scm> tags. Leiningen does a good job of adding these by default for GitHub repos, but I ran into a couple of issues:

  • Crux modules aren’t in the root of the git repo, so I needed to add :scm {:dir ".."} to the project.clj.

  • When I originally cloned the Crux repo, I cloned it as git@github.com:juxt/crux (i.e. without a .git suffix). Leiningen doesn’t pick this up, so I had to git remote set-url origin git@github.com:juxt/crux.git.

Heh :) Hopefully that saves someone else some time!

Another easy one: we’re required to add a <developers> tag. Leiningen doesn’t have any way of passing these through (as far as I know), so we fall back to :pom-addition to manually add XML to the pom.xml:

  :pom-addition ([:developers
                   [:id "juxt"]
                   [:name "JUXT"]]])

If your project contains any Java source, you’ll need to upload 'javadoc' and 'sources' JARs too. For this, we bring in the lein-javadoc plugin, and make use of :classifiers:

  :plugins [[lein-javadoc "0.3.0"]]

  :javadoc-opts {:package-names ["crux.api"]
                 :output-dir "target/javadoc/out"
                 :additional-args ["-windowtitle" "Crux Javadoc"
                                   "-link" "https://docs.oracle.com/javase/8/docs/api/"
                                   "-link" "https://www.javadoc.io/static/org.clojure/clojure/1.10.3"]}

  :classifiers {:sources {:prep-tasks ^:replace []}
                :javadoc {:prep-tasks ^:replace ["javadoc"]
                          :omit-source true
                          :filespecs ^:replace [{:type :path, :path "target/javadoc/out"}]}}

Finally, we’ll need to add :repositories/:deploy-repositories for OSSRH:

  {"snapshots" {:url "https://oss.sonatype.org/content/repositories/snapshots"}}

  {"releases" {:url "https://oss.sonatype.org/service/local/staging/deploy/maven2"
               :username [:gpg :env/sonatype_username]
               :password [:gpg :env/sonatype_password]}
   "snapshots" {:url "https://oss.sonatype.org/content/repositories/snapshots"
                :username [:gpg :env/sonatype_username]
                :password [:gpg :env/sonatype_password]}}

We use [:gpg :env/sonatype_username] so that Leiningen first checks ~/.lein/credentials.clj.gpg, then the SONATYPE_USERNAME environment variable.


If you’ve named your repositories releases and snapshots, lein deploy (without any further parameters) will select the right repository depending on whether your project version ends with -SNAPSHOT. For snapshots, we’re all done - you should find your artifacts in the snapshot repo (Crux’s are at https://oss.sonatype.org/content/repositories/snapshots/pro/juxt/crux/).

For releases, artifacts are first uploaded to a 'staging' area. Once your lein deploy finishes, head back to the Nexus UI, and click 'Staging Repositories'. You should see a staging repository, and can browse the uploaded files.

There are two more steps to release:

  • 'Closing' the staging area - this performs a suite of validations on the files you’ve uploaded.

  • 'Releasing' the staging area - this copies the staging area to the OSSRH releases repository, which is then sync’d to the main Central repo.

There are apparently tools out there to automate these steps too - personally, for Crux releases, I prefer to keep these as a manual step!

We’ve now re-released Crux 1.17.1 onto Maven Central under the pro.juxt.crux group:

;; before
[juxt/crux-core "21.06-1.17.1-beta"]

;; after
[pro.juxt.crux/crux-core "1.17.1"]

More details, as always, in the Crux release notes.

Clojars dependencies

Naturally, while we do minimise dependencies in Crux, we do depend on a few libraries that are released on Clojars (Clojure itself, and other org.clojure libraries, are already on Central). Because the addition of extra repositories doesn’t automatically propagate from a library to its dependents, these would not be accessible to anyone depending on Crux through Maven Central.

We’ve solved this, in the short-term, by releasing mirrored versions of these libraries on Central, under our pro.juxt.clojars-mirrors group. The wiring for these mirrored libraries is available at our clojars-mirrors repo - please do feel free to either copy these, or submit PRs to add other libraries.

Get in touch!

We’d love to hear your thoughts - if you’ve any feedback or questions, we’ve opened up a thread over on Reddit.

Similarly, if you’d like any help or advice with migrating your libraries, please do get in touch with us on the Clojurians' Slack #juxt channel.

1. If you’re having trouble choosing a license, this blog from Steven Deobald may be of interest!

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