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Copyright (C) 2016-2022 The Open Library Foundation

This software is distributed under the terms of the Apache License, Version 2.0. See the file "LICENSE" for more information.


This project contains examples of FOLIO modules (currently a server-side Vert.x-based module, but more, e.g. a UI/front-end module will come later), and some general information about writing, packaging and describing modules.

For background understanding, see the Okapi Guide and Reference.

What is a module

A module is a stand-alone unit of functionality that follows FOLIO's ecosystem guidelines (interfaces and schemas) and conventions; so that Okapi (FOLIO's middleware/API gateway) can forward requests to it and Stripes (FOLIO's UI Toolkit) can produce a user interface for it.

At the moment we support several types of modules:

  • Server-side modules
  • User-interface modules
  • Virtual modules, with only dependencies and other metadata

FOLIO is an open-ended system and we may end up adding more module types later.

Module descriptor

A module must come with a JSON file that contains a descriptor for the module. Typically it is called ModuleDescriptor.json. This will tell what services the module provides, if it depends on some other services (and their versions), what permissions are needed to use the module, and a number of other things.

For exact definitions, see the RAML from the Okapi-core project. The various example modules in this project may be helpful too.

The main parts of a ModuleDescriptor are:

  • id -- Primary key, uniquely identifying this module.
  • name -- A short name to be used in logs and some administrative UIs.
  • tags -- A set of short strings that tell something about the module. See below.
  • provides -- A set of interfaces, and their versions, that the module provides.
  • requires -- A set of interfaces, and their versions, that the module requires.
  • handlers -- Tells which HTTP requests the module is serving, and which permissions are needed to make such a request.
  • uiModuleDescriptor -- Placeholder for module-specific configuration for the UI modules.
  • deploymentDescriptor -- Tells how the module is to be deployed (started).

Module tags

We have not really started to use module tags in the system, but we are likely to end up with at least the following:

  • ServerModule -- Tells that this is a server side module.
  • UiModule -- Tells that this is a UI module.
  • VirtualModule -- Tells that this module is purely virtual, no code involved.

We may later add tags for various other purposes.


The handlers array tells Okapi which requests should be routed to the module (for example, a GET request to /hello), in what order various modules should be invoked for that path (so that we can invoke an authentication check before the module itself, and a logging module after it), and what permission bits will be needed for making this request.

Server-side modules

The server-side modules are typically something Okapi deploys on various nodes on the cluster where it runs. Each of those is a separate process, offering an HTTP endpoint that serves web service requests that conform to these guidelines.

(Side note: Okapi is designed to be quite flexible, so it does not have to be used for deploying the modules, if the cluster management system provides better tools for such. Nor does the thing have to live in a cluster at all, it is quite possible to have a complete system on a single workstation, for example for development work.)

In order to avoid problems with system-level dependencies, we have adopted a policy of running modules in Docker containers. This way, each container can have all the stuff the module needs, nicely isolated from the node itself, and from other modules. But this is not a hard requirement, especially when developing modules it is possible to run them as standard processes on a workstation.

Deployment and discovery

Okapi can deploy modules in several ways:

  • Exec'ing a given program (and killing its PID when undeploying).
  • By use of command lines for starting and stopping a service.
  • Using Docker API calls.

Okapi knows what it has deployed on each node, and will route requests to one of those. It is also possible to tell Okapi's discovery about processes deployed directly, either on the cluster, or even externally.

The deployment options are specified in the DeploymentDescriptor.

Once deployed, Okapi's proxy part must be informed about the module, and then the module can be enabled for tenants to use.

Writing a module

FOLIO is designed so that different modules can be written in different languages with different tools.

Language Choice

Within FOLIO, server-side modules are primarily written in Java (with some in Node.js). These are used a lot within FOLIO, therefore, we have build libraries and utilities to work with them (especially for standard scaffolding and boiler-plate code).

Currently, a lot of these modules are written in Java using Vert.x. FOLIO has two Vert.x based frameworks:

Some newer modules are written with the Spring Way philosophy. This uses Spring Boot, a more popular Java framework, as well as openapi, making it easier to take advantage of more modern Java features. If you are looking to start a new FOLIO module, you will probably want to use this methodology and framework as opposed to something RMB/Vert.x based.

FOLIO is language agnostic because modules and Okapi communicate over APIs only.

However, FOLIO has implemented a Technical Designs and Decisions process to provide consistency in FOLIO and to minimize the use of alternative tech stacks.

Development environment

These are some notes to assist developers to prepare for their local development.

To build and run the local instance of Okapi, see the Okapi Guide and Reference and the Contribution guidelines for Okapi.

So that will require:

  • Apache Maven 3.3.1 or later.
  • Java 11 JDK

As shown in the Okapi Guide and these samples, the command-line http client curl is used extensively for demonstration and development.

As explained above, using Docker is not necessary, but certainly is useful, and these samples do use it. So take the plunge. Okapi cleans up its own deployments, but be sure to keep the docker space clean.

Both Docker and Maven can utilise a local repository, to enable faster and more reliable turnaround.

For development machines using "Docker Toolbox" (instead of native) see our FAQ about "localhost" ports.

As explained in the Okapi Guide, Okapi uses HTTP 1.1 with chunked encoding to make the connections to the modules.

(See notes below for additional requirements for developing UI modules.)

Setting things up

There is a little bit that needs to be set up, depending on what kind of system you are working on.

It may be necessary to tell the Docker daemon to listen on a HTTP port, and not just a local socket, since the vertx HTTP client we use for talking to Docker can not talk to local sockets.

We need to specify how the modules may talk back to Okapi. Especially if they run in Docker containers (as we do in most examples) some tricks may be needed, since the default address http://localhost:9130/ does not work from inside a Docker container.

Speaking of docker, the example scripts create new docker images freely. At some point you need to clean them up. A quick command to do that (at least on Linux) is docker images -q |xargs docker rmi. This tries to remove all docker images, but fails on some of the more important ones (since we did not specify -f for the docker rmi command). Docker may need to do some extra work next time you build images, but not too much.

See notes about configuring Docker for all operating systems.


With recent versions of Linux using 'systemd' (e.g. from Debian 8 Jessie, Ubuntu 15.04, etc.) configure the Docker daemon:

  • Edit the file /etc/systemd/system/
  • Locate the line that says #ExecStart=/usr/bin/docker -d -H fd:// $DOCKER_OPTS
  • Edit it to say ExecStart=/usr/bin/dockerd -H tcp:// -H fd://
  • sudo systemctl daemon-reload
  • sudo service docker restart

Debian is a bit behind with the latest versions of Docker. You may want to follow the instruction at to get the latest and finest. Especially if you plan to be pushing docker images to a repository. The one in Debian should be enough to work through these examples.

One good way to start Okapi is with:

   cd .../okapi
   export OKAPIHOST=`hostname`
   java  \
      -Dokapiurl="http://$OKAPIHOST:9130" \
      -Dloglevel=DEBUG \
      -jar okapi-core/target/okapi-core-fat.jar dev

Other ways to get a value for OKAPIHOST are:

  • export OKAPIHOST=$(ifconfig docker0 | awk '/inet addr:/ { print $2 }' | cut -d: -f 2)
  • On many Debian installations it may be or
  • Use the IP address of the public interface of your workstation with something like ip addr show eth0

Sample module: hello-vertx

There is a very minimal "hello, world" module in the hello-vertx directory. Written as an educational example, it may serve as a starting point for a server-side FOLIO module. Its README has some information about its structure and how to run it in a Docker container.

The sample module uses Apache Log4j for its logging, the same way as Okapi itself does, so its logs should be compatible.

Sample module: hello-spring

This is a reimplementation of hello-vertx, but using the Java Spring Boot framework and OpenAPI specification.

Sample module: mod-spring-petstore

This is an example of the FOLIO backend module built using folio-spring-base library. The module has been created using the mod-spring-template. Please find all the details regarding creating new folio Spring based modules using mod-spring-template at

Sample module: simple-vertx

This is a slightly more complex example, again based on Java and vert.x. It has a little bit more structure, and it uses the hello-vertx module to demonstrate how to make calls to other modules.

Sample module: simple-perl

This is another very simple module, written in Perl, just to show that everything does not have to be Java.

Utility libraries

There are several useful classes and utilities for writing modules in Okapi and the core Domain Models project. We have started to extract some of them into a separate module, okapi-common, for easier reuse.

Starting your own module

Assume that you want to write your own module. Here is one way to get started. We take the hello-vertx (or hello-spring) module as a starting point, and produce a new module that we call my-module. These examples are written for Linux, but something similar ought to work on any other platform.

First, make sure you have all the development tools you need. Check out Okapi itself, and these folio-sample-modules. We assume all your projects live under a projects directory, here denoted by .../proj. For convenience we keep the root directory in an environment variable $ROOTDIR.

  cd .../proj
  export ROOTDIR=`pwd`
  git clone --recursive

Next build Okapi itself:

  cd $ROOTDIR/okapi
  mvn install

Check that you see the BUILD SUCCESS line near the end of the output. Next check out the folio-sample-modules to get the module we want to start from, and make a new copy of it:

  git clone
  cp -a folio-sample-modules/hello-spring/ mymodule # spring way
  cp -a folio-sample-modules/hello-vertx/ mymodule # vert.x

Open the project in your favourite IDE, in this example NetBeans. Use its rename function to rename the display name and ArtifactId to "mymodule". You should probably rename the source package to something else like "", unless you are starting up a new FOLIO sample module.

Now you can compile the module in your IDE or with mvn install. Again, check for the "BUILD SUCCESS" message.

Next, edit the ModuleDescriptor.json. Find all occurrences of "hello" and change them to "mymodule".

Edit also the Dockerfile. If you are using hello-vertx, change the ENV VERTICLE_FILE line to refer to mymodule-fat.jar and edit the comments in the beginning. If you are using hello-spring, modify the APP_FILE ENV.

Now you can walk through the examples in, substituting "mymodule" for "hello" where proper. You should be able to create the docker image, see that it can run in isolation, start Okapi, launch the module and access it.

Congratulations, you have your own module! Now you just need to make it do what ever you want it to do, and for that we can not give detailed instructions. Some useful hints:

  • You probably want to make the module respond to some other path(s) than /hello. Change the RoutingEntries in the ModuleDescriptor, and, for vert.x, the vertx router in the file. For spring, modify the api.yaml per openapi specifications.
  • You probably should move the actual processing methods away from the current main class, likely into a class of its own, and make the routes/controller point to it. Most likely you will create other classes to support your operations.
  • Rewrite most of the README to reflect your module.

Running your module

There are three different ways to run your module. They differ in the way that the module is started up, everything else is the same. In all cases you need to declare the module to Okapi and enable it for your test tenant, who has to exist. The methods differ in the LaunchDescriptor you give to Okapi. This can be done in two ways, either inside the ModuleDescriptor, as we did in the hello-vertx module, or in the DeploymentDescriptor, as we did in simple-perl module. Where ever the LaunchDescriptor comes from, it needs to specify how the module gets started.

Run it yourself

You are responsible for starting and stopping the module, possibly from your IDE or a separate console window. You can choose the port yourself, usually 8080 is a good default. You should not provide a LaunchDescriptor at all, since Okapi is not launching the service for you. Instead you need to provide the URL where your module can be reached, often something like http://localhost:8080

This is a good way while you are working with your module. You set Okapi up once and leave it running. You can start and stop your module as many times as you like, and see all its debug output.

Let Okapi start the module

If you put a LaunchDescriptor in your ModuleDescriptor, Okapi will start the module for you, every time you request it to be deployed. This way, you can write a small curl script to get everything up and running. If you do this kind of thing in production environment, you probably have the modules running at some fixed location, and can put an absolute path in the LaunchDescriptor. But while developing, it is nicer to be flexible, and use relative paths, so everything can be run under your home directory. That is no problem, if Okapi and your module are running under the same project directory. Then you can specify something like "../folio-sample-modules/simple-perl/" as the exec line, and Okapi will find the module all right.

Run in a Docker

In a cloud based production environment, we recommend running all modules in their own Docker containers. That way, there is no need to be precise with the paths. You can distribute modules as Docker images, maybe using a public (or your own private) Docker repository. The drawback is that you need to create the Docker image. See the hello-vertx module for a simple example of how this works.

UI modules

The UI modules are quite different from the server-side modules, and rely on the browser technology stack (React/Redux). The system's API is still under development. Refer to the FOLIO user-interface toolkit, called Stripes.

Virtual modules

Virtual modules are pure metadata, with no code to write. All that is needed is to create a good ModuleDescriptor, e.g. one that lists dependencies of other, concrete modules.

Further reading

For more about Okapi, refer to its documentation and even the source code at

Also consult the README for the hello-vertx example which covers some useful ground.

Aside from the sample modules in this repository, there are various other server-side modules and client-side UI modules, which would also be instructive to browse.

Consult the individual repositories' documentation for the status of the code in these modules.

See project OKAPI at the FOLIO issue tracker.

Other FOLIO Developer documentation is at