16 Locating services in a network: Discovery

Andreas Sundström

In order to make it easier to find a service (an Oz server application) one might want to have some sort of yellow pages, or a directory. While a directory is easy to implement in Oz, the Discovery module allows to locate it in a local area network.


The current implementation may be incomplete on certain platforms. Notably, only on Linux and Solaris it is guaranteed that broadcasts are sent to all the available networks. On other platforms this depends on the operating system.

The Discovery service consists of two parts, a server and a client. The server is initialized with a value. The server waits for inquiries from clients and sends the value as an answer. The client sends a broadcast message to all available networks (for example, ethernet and ip over serial link). Then the client waits for answers from servers.

A value the server holds would typically be a ticket to an Oz port that a directory server listens to.

16.1 The Module

The module has three features:


The server class.


The client class.


The number of the default ip port that the server listens to.

The server class Discovery.server has following methods:


init(info:Info port:PortNr <= DefaultServerPort)

Info is the answer to be send on request by clients. It must be a virtual string. The server listens to the ip port PortNr. If the field port is not present, the default port number will be used.



Replaces the answer to be send to clients.



Closes the operation of the server.

The client class Discovery.client has following methods:


init(port:ServerPortNr <= DefaultServerPort)

This method broadcasts a message. Answers to that message can be obtained using the methods getOne and getAll. The port that the server listens can be specified as ServerPortNr. If the field port is not present, the default port number will be used.


getOne(timeOut:TimeOut <= 1000 info:?Info)

There could be several servers listening to broadcasts from a client, therefore there can be several answers. If an answer is received before TimeOut milliseconds the variable Info will hold that answer. Otherwise Info will be timeout. Answers are Oz strings.

Instead of a time in milliseconds TimeOut can be inf. This means that the method will suspend until an answer is received (or forever if no answer is received).

If this method is called again the next answer (if such answer exists) will be returned.


getAll(timeOut:TimeOut <= 1000 info:?Info)

Method getAll can be used instead of method getOne. After calling this method, Info will hold a list of all answers received before TimeOut milliseconds has gone by.

This method is implemented using getOne. So any answers fetched by calling getOne will not reappear in the list Info, and vice versa.

TimeOut can be inf here too. In this case a stream will be returned instead of a list.



Closes the operation of the client class.

Denys Duchier, Leif Kornstaedt, Martin Homik, Tobias Müller, Christian Schulte and Peter Van Roy
Version 1.4.0 (20080702)