FRDS.Broker Library

The FRDS.Broker library is a teaching oriented implementation of the Broker architectural pattern for distributed remote method invocation. It defines the central roles of the pattern and provides implementations of those roles that are not domain/use case specific. It provides a JSON based (GSon library) Requestor implementation, and implementations of the ClientRequestHandler and ServerRequestHandler roles in both a Java socket based and a Http/URI tunneling based variants. The latter us based upon the UniRest and Spark-Java libraries.

The Broker pattern and the source code is explained in detail in the book Flexible, Reliable, Distributed Software (FRDS), by Henrik Bærbak Christensen / Aarhus University /

Version history is available here.

License is Apache 2.

I only need the Broker Library, what is the dependency?

The Broker library is available in JCenter.

Get it using Gradle:

dependencies {
  compile group: 'com.baerbak.maven', name: 'broker', version: '1.2'

What is this repository for?

The Broker library used in teaching context based on the book Flexible, Reliable, Distributed Software.

This repository serves multiple purposes.

  1. It has the source code of the library that contains central roles for the Broker pattern, as well as some default implementations for some of these. Folder: broker.

  2. It has the source code of the TeleMed system, which is used in the FRDS book to show the Broker pattern in action, and contains tests of both the Broker and TeleMed implementation. Folder: demo.

  3. It has the source code (and development diary) of the GameLobby system, which is used in FRDS to show how to create remote objects on the server, and handle multi-object method dispatch. Folder: demo2.

  4. It has the source code of an implementation of TeleMed that uses REST instead of Broker for communication. Folder: demo-rest.


The TeleMed system is a small distributed system in which patients may upload blood pressure measurements to a central medical server.

Its primary goal is to show how the Broker pattern is implemented. Note, it is not a full Broker as in Java RMI as it has two limitations (more detail in FRDS):

  • No Name Service is used, so object id must be defined by the code base and/or from interaction with the server.
  • It is pure client/server relation, so server objects can never invoke methods on client side objects (no Observer pattern possible).

Two variants of the Broker is provided

  • Socket: Socket based Client- and ServerRequestHandler implementations.

  • URI Tunnel: HTTP/URI Tunnel based Client- and ServerRequestHandler implementations similar to what most WebService frameworks will produce.


The GameLobby is a more complex distributed system. The domain is players that want to create remote games, that friends can join so distributed play is possible. The game itself is not interesting here, as the learning goal is to demonstrate code that

  • Create objects on the server side, new servants, and allows the clients to bind client proxies to them for remote method calls.

  • Implement the invoker to handle multi-type dispatch in a way that avoids 'blob' invokers, by creating sub-invokers, one for each type of role/servant type in the system.

  • Also my test-driven development diary is included in which I develop the system from scratch in about 14 hours, including the documentation effort.

How do I run TeleMed?

You first start the TeleMed server, next you invoke the client multiple times to upload or fetch blood pressure measurements.

To start the TeleMed server, open a shell and issue

gradle serverSocket

To upload blood pressure (123,99) for patient with id=241248 to the server located at IP localhost, open another shell and issue

gradle -q homeSocket -Pid=241248 -Psys=123 -Pdia=99 -Phost=localhost

To fetch the last week's data for patient with id=87, issue

gradle -q homeSocket -Pop=fetch -Pid=87

If you want to use a HTTP URI Tunnel protocol instead, just replace serverSocket by serverHttp, and homeSocket with homeHttp. The HTTP based version can also be viewed from the web page [http://localhost:4567/bp/pid=87]

Review for default values for the arguments to the server and the client.

How do I run GameLobby?

The GameLobby system is primarily intended to be reviewed through the test cases, but manual tests can be made using the command line.

Start server

gradle lobbyServer

Note the IP of the running server (I use below).

Let user 'Pedersen' create a game

csdev@m31:~/proj/broker$ gradle -q lobbyClient -Pop=create -Phost= -Pplayer=Pedersen
LobbyClient: Asked to do operation create for player Pedersen
 Future created, the join token is game-1

And let 'Findus' join the game, using the provided game token game-1

csdev@m31:~/proj/broker$ gradle -q lobbyClient -Pop=join -Ptoken=game-1 -Pplayer=Findus -Phost=
LobbyClient: Asked to do operation join for player Findus
 Future joined, available is true
 The Game id is 609833b2-bf8d-421c-beb1-9aac1464aac2
 The Game's 1st player is Pedersen
 The Game's 2nd player is Findus

How do I run REST based TeleMed?

Start the REST server

gradle serverRest

The client just executes a CRUD scenario, most of the data is hardwired.

gradle demoRest

You can avoid the delete by

gradle demoRest -Pdelete=false

In addition you can review uploaded observations for a patient, for instance localhost:4666/bp/for/251248-1234/.


Author: Henrik Baerbak Christensen / Aarhus University /

Contribution guidelines

  • Contact Henrik if you want to contribute and he will set things up...