A discovery agent is a bootstrap mechanism that enables a client or message broker to discover other broker instances on a network. A discover agent URI is used on the client side and on the broker side, as follows:

	Note
	A discover agent URI resolves to a list of transport URIs, but the discovery agent URI is not itself a transport URI and cannot be used in place of a transport URI.

Since a discovery agent is not a transport protocol, you cannot use a discovery agent URI directly on the client side. To use a discovery agent on the client side, embed the agent URI,DiscoveryAgentUri, inside a discovery URL, as follows:

discovery://(DiscoveryAgentUri)?TransportOptions

The client recognizes the discovery URL as a transport. It first obtains a list of available endpoint URLs using the specified discovery agent and then connects to one of the discovered URLs. For more details about the discovery protocol, see Dynamic Discovery Protocol .

For certain kinds of discovery agent (for example, multicast or zeroconf), it is necessary to enable the discovery agent in the message broker configuration. For example, to enable the multicast discovery agent on an Openwire endpoint, edit the relevant transportConnector element as follows:

<transportConnectors>
  <transportConnector name="openwire"
    uri="tcp://localhost:61716"
    discoveryUri="multicast://default"/>
</transportConnectors>

Where the discoveryUri attribute on the transportConnector element is initialized to multicast://default. You can associate multiple endpoints with the same discovery agent. For example, to configure both an Openwire endpoint and a Stomp endpoint to use the multicast://default discovery agent:

<transportConnectors>
  <transportConnector name="openwire"
    uri="tcp://localhost:61716"
    discoveryUri="multicast://default"/>
  <transportConnector name="stomp"
    uri="stomp://localhost:61613"
    discoveryUri="multicast://default"/>
</transportConnectors>

Fuse Message Broker currently supports the following discovery agents:

The simple (static) discovery agent provides an explicit list of broker URLs for a client to connect to. For example:

simple://(tcp://localhost:61716,tcp://localhost:61816)

In general, the URI for a simple discovery agent must conform to the following syntax:

simple://(URI1,URI2,URI3,...)

Or equivalently:

static://(URI1,URI2,URI3,...)

The two prefixes, simple: and static:, are exactly equivalent. In order to use the agent URI, it must be embedded inside a discovery URL—for example:

discovery://(static://(tcp://localhost:61716,tcp://localhost:61816))

This discovery agent is only used on the client side. No extra configuration is required in the broker.

The multicast discovery agent uses the IP multicast protocol to find any message brokers currently active on the local network. In order for the protocol to work, a multicast discovery agent must be enabled on each broker you want to advertise and messaging clients must be configured to use a discovery URI.

The URI for a multicast discovery agent must conform to the following syntax:

multicast://GroupID

Where the GroupID is an alphanumeric identifier. All participants in the same discovery network must use the same GroupID. For example, the Fuse Message Broker is usually configured to use the URI, multicast://default.

	Note
	Your local network (LAN) must be configured appropriately for the IP/multicast protocol to work. If your clients fail to discover a broker using the multicast protocol, this could be because IP/multicast is not set up on your network.

The zeroconf discovery agent is derived from Apple’s Bonjour Networking technology, which defines the zeroconf protocol as a mechanism for discovering services on a network. Fuse Message Broker bases its implementation of the zeroconf discovery agent on JmDSN, which is a service discovery protocol that is layered over IP/multicast and is compatible with Apple Bonjour. To enable the protocol, a multicast discovery agent must be configured on each broker you want to advertise and messaging clients must be configured to use a discovery URI.

The URI for a zeroconf discovery agent must conform to the following syntax:

zeroconf://GroupID

Where the GroupID is an alphanumeric identifier. All participants in the same discovery network must use the same GroupID.

For example, to use a zeroconf discovery agent on the client side, where the client needs to connect to the groupA group, you would construct a discovery URL like the following:

discovery://(zeroconf://groupA)

	Note
	Your local network (LAN) must be configured appropriately for the IP/multicast protocol to work. If your clients fail to discover a broker using the zeroconf protocol, this could be because IP/multicast is not set up on your network.