About Interface Specification - 14.02

Warning: This is not the latest interface definition.

Go here for latest About Interface Definition



This document describes the specification of the AllJoyn® About interface. This interface is required by an application to provide the discovery mechanism for the service framework interfaces that it supports, as well as providing the basic identification information.


This document is targeted to the developers for AllJoyn applications.


Except for RFCs, the following are reference documents found on the AllSeen Alliance web site's Docs/Downloads section.

Specificationion Overview

The About interface is to be implemented by an application on a target device. This interface allows the app to advertise itself so other apps can discover it. Figure 1 illustrates the relationship between a client app and a service app.


Figure: About feature architecture within the AllJoyn framework


A client can discover the app via an announcement which is a sessionless signal containing the basic app information like app name, device name, manufacturer, and model number. The announcement also contains the list of object paths and service framework interfaces to allow the client to determine whether the app provides functionality of interest.

In addition to the sessionless announcement, the About interface also provides the on-demand method calls to retrieve all the available metadata about the app that are not published in the announcement.

Discovery Call Flows

Typical discovery flow

Figure 2 illustrates a typical call flow for a client to discover a service app. The client merely relies on the sessionless announcement to decide whether to connect to the service app to use its service framework offering.


Figure: Typical discovery flow (client discovers a service app)

Nontypical discovery flow

Figure 3 illustrates a call flow for a client to discover a service app and make a request for more detailed information.


Figure: Nontypical discovery call flow

Error Handling

The method calls in the About interface will use the AllJoyn error message handling feature (ER_BUS_REPLY_IS_ERROR_MESSAGE) to set the error name and error message.

Table 1 lists the possible errors raised by the About interface.

Table 1: About interface error handling

Error name Error message
org.alljoyn.Error.LanguageNotSupported The language specified is not supported

About Interface

Interface name Version Secured
org.alljoyn.About 1 no


Property name Signature List of values Writable Description
Version q Positive integers no Interface version number


The following methods are exposed by a BusObject that implements the org.alljoyn.About interface.



Parameter name Mandatory Signature List of values Description
languageTag yes s IETF language tags specified by RFC 5646. The desired language.


Return signature Mandatory Description
a{sv} yes A dictionary of the available metadata fields. If language tag is not specified (i.e., ""), metadata fields based on default language are returned.
If a language tag is not supported, the error org.alljoyn.Error.LanguageNotSupported is returned.


Retrieve the list of available metadata fields based on the language tag.

Field information

Table 2 lists the names of the metadata fields. The fields with a yes value in the Announced column will also be published via the Announce signal. See Signals for more information.

Table 2: About interface data fields

Field name Required Announced Localized Signature Description
appId yes yes no ay The globally unique identifier for the application.
DefaultLanguage yes yes no s The default language supported by the device. Specified as an IETF language tag listed in RFC 5646.
DeviceName yes yes yes s Name of the device set by platform-specific means (such as Linux and Android).
DeviceId yes yes no s Device identifier set by platform-specific means.
AppName yes yes yes s Application name assigned by the app manufacturer (developer or the OEM).
Manufacturer yes yes yes s The manufacturer's name of the app.
ModelNumber yes yes no s The app model number.
SupportedLanguages yes no no as List of supported languages.
Description yes no yes s Detailed description expressed in language tags as in RFC 5646.
DateOfManufacture no no no s Date of manufacture using format YYYY-MM-DD (known as XML DateTime format).
SoftwareVersion yes no no s Software version of the app.
AJSoftwareVersion yes no no s Current version of the AllJoyn SDK used by the application.
HardwareVersion no no no s Hardware version of the device on which the app is running.
SupportUrl no no no s Support URL (populated by the manufacturer).





Return signature Description
a(oas) Return the array of object paths and the list of supported interfaces provided by each object.


Retrieve the object paths and the list of all interfaces implemented by each of objects.


Signal name Parameters Sessionless Description
Name / Signature
Announce (listed below) yes This signal is used to announce the application information and the service framework interfaces that it supports. The following information is provided in the signal:
version / q * version - Version number of the About interface.
port / q * port - Session port. The app will listen on this port for incoming sessions.
objectDescription / a(oas) * objectDescription --Array of object paths and the list of supported interfaces provided by each object.
metaData / a{sv} * metaData - Metadata. All the fields listed in Field information with a yes value in the Announced column are provided in this signal.

AllJoyn Introspection XML

<node name="/About" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="http://www.allseenalliance.org/schemas/ introspect.xsd">
   <interface name="org.alljoyn.About">
      <property name="Version" type="q" access="read"/>
      <method name="GetAboutData">
         <arg name="languageTag" type="s" direction="in"/>
         <arg name="aboutData" type="a{sv}" direction="out"/>
      <method name="GetObjectDescription">
         <arg name="objectDescription" type="a(sas)" direction="out"/>
      <signal name="Announce">
         <arg name="version" type="q"/>
         <arg name="port" type="q"/>
         <arg name="objectDescription" type="a(sas)"/>
         <arg name="metaData" type="a{sv}"/>