An engine for building OpenHIM mediators
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Build Status OpenHIM Core

OpenHIM Mediator Engine

An engine for building OpenHIM mediators based on the Akka framework.

Yeoman Generator

The quickest way to get a new mediator up and running is to use the handy Yeoman generator:

  • npm install -g yo
  • npm install -g generator-mediator-java
  • mkdir my-mediator
  • cd my-mediator
  • yo mediator-java

See the Getting Started guide below otherwise.

Getting Started

Include the jar in your mediator project. If you're using Maven, simply add the following dependency:


The engine provides an HTTP server and Akka handlers for dealing with incoming requests. To fire up the engine, the org.openhim.mediator.engine.MediatorServer class is used. You simply have to pass it the necessary config and run the start() method:

MediatorConfig config = new MediatorConfig("my-mediator", "", 8500);
//add config values...

MediatorServer server = new MediatorServer(config);

Note that start() is non-blocking.

The server constructor also accepts an ActorSystem parameter (e.g. new MediatorServer(myActorSytem, config)), in the event that you want to manage this yourself. If not passed, the server will create a new system. Note though that ActorSystem is a heavy object and there should ideally only be a single instance of this in your application.

On startup server will register your mediator with the HIM core and you must pass the registration json info to the config (see this page and the Configuration section below for details).

You will need at least one actor in your project to receive requests from the engine. If you're starting a new mediator, you can simply create an actor as follows:

import akka.event.Logging;
import akka.event.LoggingAdapter;
import org.apache.http.HttpStatus;
import org.openhim.mediator.engine.messages.FinishRequest;
import org.openhim.mediator.engine.messages.MediatorHTTPRequest;

public class MyActor extends UntypedActor {
    LoggingAdapter log = Logging.getLogger(getContext().system(), this);

    public void onReceive(Object msg) throws Exception {
        if (msg instanceof MediatorHTTPRequest) {
            FinishRequest response = new FinishRequest("A message from my new mediator!", "text/plain", HttpStatus.SC_OK);
            ((MediatorHTTPRequest) msg).getRequestHandler().tell(response, getSelf());
        } else {

An actor is simply a Java Object that can receive messages. The engine will send your actor a MediatorHTTPRequest message whenever a request is received and you can respond by sending back a FinishRequest message. The MediatorHTTPRequest provides a reference to the request handler for receiving this response. The above actor template demonstrates a simple actor that responds immediately with an OK message. Note that the response can be sent asyncronously, not necessarily at the same moment you receive the http request like above.

When running, the server will receive requests as well as handle the response, including the formatting of the standard application/json+openhim response, so you do not need to worry about this. Whatever you return in the FinishRequest will be embedded in the response object.

To link your actor to the engine you have to create a routing table and pass that to the server config. The routing table consists of path/actor pairs, e.g.:

RoutingTable routingTable = new RoutingTable();
routingTable.addRoute("/mymediator", MyActor.class);

When receiving a request on the specified path, the engine will launch a new instance of your actor to handle the request (actor-per-request model). This means that you can safely add request-specific state to your actor.

In summary, the following illustrates an example main method that fires up the engine that'll route to the above MyActor:

import akka.event.Logging;
import akka.event.LoggingAdapter;
import org.openhim.mediator.engine.*;

public class MyMediatorMain {

    public static void main(String... args) throws Exception {
        //setup actor system
        final ActorSystem system = ActorSystem.create("mediator");
        //setup logger for main
        final LoggingAdapter log = Logging.getLogger(system, "main");

        //setup actors"Initializing mediator actors...");

        MediatorConfig config = new MediatorConfig("my-mediator", "", 8500);


        RoutingTable routingTable = new RoutingTable();
        routingTable.addRoute("/mymediator", MyActor.class);

        InputStream regInfo = MyMediatorMain.class.getClassLoader().getResourceAsStream("mediator-registration-info.json");
        RegistrationConfig regConfig = new RegistrationConfig(regInfo);

        final MediatorServer server = new MediatorServer(system, config);

        //setup shutdown hook (will handle ctrl-c)
        Runtime.getRuntime().addShutdownHook(new Thread() {
            public void run() {
      "Shutting down mediator");
        });"Starting mediator server...");
        server.start();"Mediator listening on");

Request Handler Reference

When receiving a request, the engine will send your actor a MediatorHTTPRequest message. This message contains a reference to the request handler actor, and it is this actor that you send messages to in order to manage the final mediator response. The messages it supports are as follows:

  • FinishRequest - Finalize the request and send the response. Note that the request actor instances will be stopped at this point (the engine will send a PoisonPill message).
  • ExceptError - An exception has occurred and the request should end with a 500 Internal Server Error response.
  • AddOrchestrationToCoreResponse - Add orchestration details to the request response. This message can be sent as many times as required.
  • PutPropertyInCoreResponse - Put a name/value pair property in the request response. This message can be sent as many times as required.
  • AcceptedAsyncRequest - Indicates to the request handler that the request will be processed asyncronously. When this message is sent, the request handler will immediately respond to the client with a 202 (Accepted) status, and processing can continue. The request can still be finalized by sending a FinishRequest message. When this is done, the engine will send an update with the response details to the HIM core.


The mediator engine provides several connectors that you can use in order to connect to other services. These are running as actors on the root context, so can be looked up as an ActorSelection:

ActorSelection connector = getContext().actorSelection(config.userPathFor("http-connector"));
MediatorHTTPRequest httpRequest = new MediatorHTTPRequest(...);
connecter.tell(httpRequest, getSelf());

The connectors are loaded on the path: "/user/{mediator-name}/{connector-name}" during mediator initialization. All the connectors will automatically add an orchestration item to the final mediator response (except for the udp-fire-forget-connector).


Provides connection to HTTP services. Accepts MediatorHTTPRequest messages and will respond with MediatorHTTPResponse.


An adaptor to the http-connector that adds the authentication headers as required by the OpenHIM Core API. Accepts MediatorHTTPRequest messages and will respond with MediatorHTTPResponse. It will use the auth details provided in the mediator config supplied to the mediator server. It also supports the RegisterMediatorWithCore message, although this is automatically called by the engine.


Provides connection to TCP services using the MLLP protocol. Accepts MediatorSocketRequest messages and will respond with MediatorSocketResponse.


Provides connection to UDP services for fire-and-forget messages. Accepts MediatorSocketRequest messages with no response.

Error Handling

It's important for a mediator to have robust error handling. The engine provides mechanisms for handling exceptions, see the above Request Handler Reference section. When you encounter an exception, simply send a message to the reguest handler, and it'll log and respond to the client with a 500 status. If however you encounter a logic error, such as a validation error of a received message body, rather use the FinishRequest message to respond appropriately:

try {
} catch (ParseException ex) {
  FinishRequest response = new FinishRequest("Invalid message received", "text/plain", HttpStatus.SC_BAD_REQUEST);
  msg.getRequestHandler().tell(response, getSelf());
} catch (VerySeriousProblem ex) {
  msg.getRequestHandler().tell(new ExceptError(ex), getSelf());

If an exception occurs that is not handled by an actor, then the request handler will automatically respond with a 500 status.


The mediator engine provides you with several configuration mechanisms, both for setting up the mediator within the engine and core in order to get it up and running, as well as options that you can use to setup your own custom settings:

  • Mediator config: As seen in the Getting Started section, the MediatorConfig class is used to setup the mediator within the context of the engine.
  • Registration config: On startup, the mediator must register itself with core.
  • Properties: The engine provides a mechanism for loading custom settings from a standard java properties file. This is useful for settings that you require at mediator startup, such as the OpenHIM Core configuration
  • Dynamic config: Also provided is a dynamic config map that can be used for runtime settings.

Each mechanism is discussed below.

Mediator Config

An instance of MediatorConfig is used to setup your mediator within the context of the engine. For convenience, any actors that are setup in the routing table can get a handle to the mediator config if it has a constructor that takes MediatorConfig as a parameter. If available, the engine will pass through a reference when routing to the actor:

public class MyActor extends UntypedActor {
    LoggingAdapter log = Logging.getLogger(getContext().system(), this);

    private final MediatorConfig config;

    public MyActor(MediatorConfig config) {
        this.config = config;

    public void onReceive(Object msg) throws Exception {
        if (msg instanceof MediatorHTTPRequest) {
        } else {

SSL Context

SSL context configuration can be specified via the MediatorConfig (in particular .setSSLContext(...)) before starting up the mediator server. This allows you to include specific certificates in the trust store and/or include a client certificate for mutual authentication. This context will also affect HTTPS connections to the OpenHIM Core, e.g. for heartbeats and registration, so this will provide you with a mechanism to trust Core's certificate if it's self-signed.

Registration config

Upon startup your mediator must register itself with core. The engine will handle this automatically, but you must supply the engine with a json string containing the configuration for your mediator. See this page for details. This string can be passed to your instance of MediatorConfig using an instance of RegistrationConfig:

InputStream json = ... //or String
RegistrationConfig regConfig = new RegistrationConfig(json);

The mediator will reattempt the registration request if there is an error, e.g. if the mediator starts up before core and core is unavailable. It will continue attempting to send requests until successful.


You can load a standard properties file into MediatorConfig using the overloaded setProperties method. These are then availabe via getProperties() and getProperty(String) for looking up a key/value pair. If you are using dynamic configuration (described below) then these properties can be useful for any static configuration that you may have.

Dynamic config

In addition to properties, MediatorConfig also provides a String -> Object map (on getDynamicConfig()) that can be used for runtime settings. When MediatorConfig is initialized, this map will be empty and you can then use it to load any settings.

[Only supported with OpenHIM Core version 1.4 or greater] If supported by the version of core you are using, and if heartbeats are enabled, then the engine will also automatically update the dynamic config map with any updates that are made core-side. This option can be used in order to dynamically change the mediator's config using the OpenHIM Console. To make use of this feature, ensure that heartbeats are enabled on the mediator (see Heartbeats section) and that you supply the configDefs and (optional) default config fields in your registration config json. In addition, if you have specified config values in your registration config, then those will be loaded into the map on startup (and overwritten with any changes from core if there are any).


[Only supported with OpenHIM Core version 1.4 or greater]

Mediator heartbeats can be enabled with the setHeartbeatsEnabled option in MediatorConfig. If enabled, the engine will periodically send an "aliveness" request to core. If there are any config changes (e.g. a user changed a setting for the mediator in the console), the dynamic config (MediatorConfig#getDynamicConfig()) will be updated with the changes.

Actor Model

The engine is based on Akka and is designed to be an easy way to bootstrap an actor system for your mediator. However you are under no obligation to follow the actor model in your project! In this case, simply bootstrap your project as explained in the Getting Started section with a single actor for receiving requests and link to your own non-actor classes from there.

However, once you've got the base mediator running, it's very easy to integrate more actors into your application. This provides you with benefits such fault tolerance, thread and lock management and asyncronous processing. See the Akka documentation for details on using actors. Here we will discuss the conventions followed by the mediator engine.

Adding actors

To add new actors, create an actor class, such as the discussed in the Getting Started section. To instantiate the actor in the current context, use actorOf:

ActorRef actor = getContext().actorOf(Props.create(SomeActor.class));
actor.tell(aMsg, getSelf());

The actor will be instantiated as a child of the current actor. Note that any actors that form part of the request handler hierarchy will be stopped once the request finishes. For example MyActor in Getting Started and any child actors that it, and they in turn, instantiate would form part of the hierarchy. The stop happens with a PoisonPill message, so the actors will have a chance to finish processing any current messages first.

Another option, useful for any single instance actors, is to launch them on startup and then reference them using actorSelection:

MediatorConfig config = new MediatorConfig("my-mediator", "", 8500);
StartupActorsConfig startupActors = new StartupActorsConfig();
startupActors.addActor("some-actor", SomeActor.class);
MediatorServer server = new MediatorServer(config);

ActorSelection actor = getContext().actorSelection("/user/my-mediator/some-actor");
actor.tell(aMsg, getSelf());

Although deciding on when to instantiate actors ultimately depends on your own implementation, we follow the convention of using per-request (actorOf used within the request context) for any actors that require any state that's specific to the request and single instance actors (startup actors and actorSelection) otherwise.

Error Handling Strategies

By default, if an unhandled exception occurs in an actor then the request handler will automatically respond to the client with a 500 status. However this does not apply to single instance actors (startup actors); for these the default Akka supervisor strategy will be used.

You can also override the error handling strategies in any of your own actors (both per-request and single instance); see the Akka documentation.


This software is licensed under the Mozilla Public License Version 2.0.