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2.2. Discovery Protocol

Abstract

When you want to have the list of available brokers dynamically generated and don't want to hard code the initial connection point, you can use Red Hat JBoss A-MQ's discovery protocol. This protocol uses discovery agents to advertise available brokers and discovery URIs to configure what discovery agent a client will use for discovering brokers.
The failover protocol provides a lot of control over the brokers to which a client can connect. Using dynamic failover adds some ability to make the broker list more transparent. However, it has weaknesses. It requires that you know the address of at least one broker and that an initial broker is active when the client starts up. Using dynamic failover also requires that all of the brokers being used for failover are configured in a network of brokers.
Red Hat JBoss A-MQ's discovery protocol offers an alternative method for dynamically generating a list of brokers that are available for client failover. The protocol allows brokers to advertise their availability and for clients to dynamically discover them. This is accomplished using two pieces:
  • discovery URI—looks up all of the discoverable brokers and presents them as a list of actual URIs for use by the client or network connector
  • discovery agents—components that advertise the list of available brokers

2.2.1. Discovery URI
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Abstract

The discovery URI tells the client, or netowrk connector, which discovery agent to use when resolving its list of real URIs.

Overview
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The discovery URI is a virtual URI that specifies which discovery agent to use for discovering available brokers. The discovery protocol connects to the specified agent and uses that data returned from the agent to build up a list of broker URIs.

URI syntax
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Example 2.4, “Discovery URI” shows the syntax for a discovery URI.

Example 2.4. Discovery URI

discovery://(DiscoveryAgentUri)?Options
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DiscoveryAgentUri is URI for the discovery agent used to build up the list of available brokers. Discovery agents are described in Section 2.2.2, “Discovery Agents”.
The options, ?Options, are specified in the form of a query list. The discovery options are described in Table 2.3, “Dynamic Discovery Protocol Options”. You can also inject transport options as described in the section called “Setting options on the discovered transports”.
Note
If no options are required, you can drop the parentheses from the URI. The resulting URI would take the form discovery://DiscoveryAgentUri

Transport options
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The discovery protocol supports the options described in Table 2.3, “Dynamic Discovery Protocol Options”.
Expand
Table 2.3. Dynamic Discovery Protocol Options
OptionDefaultDescription
initialReconnectDelay 10Specifies, in milliseconds, how long to wait before the first reconnect attempt.
maxReconnectDelay 30000Specifies, in milliseconds, the maximum amount of time to wait between reconnect attempts.
useExponentialBackOff trueSpecifies if an exponential back-off is used between reconnect attempts.
backOffMultiplier 2Specifies the exponent used in the exponential back-off algorithm.
maxReconnectAttempts 0Specifies the maximum number of reconnect attempts before an error is sent back to the client. 0 specifies unlimited attempts.

Sample URI
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Example 2.5, “Discovery Protocol URI” shows a discovery URI that uses a multicast discovery agent.

Example 2.5. Discovery Protocol URI

discovery://(multicast://default)?initialReconnectDelay=100
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Setting options on the discovered transports
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The list of transport options, Options, in the discovery URI can also be used to set options on the discovered transports. If you set an option not listed in Table 2.3, “Dynamic Discovery Protocol Options”, the URI parser attempts to inject the option setting into every one of the discovered endpoints.
Example 2.6, “Injecting Transport Options into a Discovered Transport” shows a discovery URI that sets the TCP connectionTimeout option to 10 seconds.

Example 2.6. Injecting Transport Options into a Discovered Transport

discovery://(multicast://default)?connectionTimeout=10000
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The 10 second timeout setting is injected into every discovered TCP endpoint.

2.2.2. Discovery Agents
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Abstract

A discovery agent is a mechanism that advertises available brokers to clients and other brokers.
A discovery agent is a mechanism that advertises available brokers to clients and other brokers. When a client, or broker, using a discovery URI starts up it will look for any brokers that are available using the specified discovery agent. The clients will update their lists periodically using the same mechanism.
How a discovery agent learns about the available brokers varies between agents. Some agents use a static list, some use a third party registry, and some rely on the brokers to provide the information. For discovery agents that rely on the brokers for information, 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, you edit the relevant transportConnector element as shown in Example 2.7, “Enabling a Discovery Agent on a Broker”.

Example 2.7. Enabling a Discovery Agent on a Broker

<transportConnectors>
  <transportConnector name="openwire"
    uri="tcp://localhost:61716"
    discoveryUri="multicast://default" />
</transportConnectors>
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Where the discoveryUri attribute on the transportConnector element is initialized to multicast://default.
Important
If a broker uses multiple transport connectors, you need to configure each transport connector to use a discovery agent individually. This means that different connectors can use different discovery mechanisms or that one or more of the connectors can be undiscoverable.
Red Hat JBoss A-MQ currently supports the following discovery agents:

2.2.2.1. Fuse Fabric Discovery Agent
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Abstract

The Fuse Fabric discovery agent uses Fuse Fabric to discovery brokers that are deployed into a fabric.
Overview
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The Fuse Fabric discovery agent uses Fuse Fabric to discover the brokers in a specified group. The discovery agent requires that all of the discoverable brokers be deployed into a single fabric. When the client attempts to connect to a broker the agent looks up all of the available brokers in the fabric's registry and returns the ones in the specified group.
URI
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The Fuse Fabric discovery agent URI conforms to the syntax in Example 2.8, “Fuse Fabric Discovery Agent URI Format”.

Example 2.8. Fuse Fabric Discovery Agent URI Format

fabric://GID
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Where GID is the ID of the broker group from which the client discovers the available brokers.
Configuring a broker
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The Fuse Fabric discovery agent requires that the discoverable brokers are deployed into a single fabric.
The best way to deploy brokers into a fabric is using the management console. For information on using the management console see "Using the Management Console".
You can also use the console to deploy brokers into a fabric. See chapter "Fabric Console Commands" in "Console Reference".
Configuring a client
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To use the agent a client must be configured to connect to a broker using a discovery protocol that uses a Fuse Fabric agent URI as shown in Example 2.9, “Client Connection URL using Fuse Fabric Discovery”.

Example 2.9. Client Connection URL using Fuse Fabric Discovery

discovery://(fabric://nwBrokers)
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A client using the URL in Example 2.9, “Client Connection URL using Fuse Fabric Discovery” will discover all the brokers in the nwBrokers broker group and generate a list of brokers to which it can connect.

2.2.2.2. Static Discovery Agent
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Abstract

The static discovery agent uses an explicit list of broker URLs to specify the available brokers.
Overview
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The static discovery agent does not truly discover the available brokers. It uses an explicit list of broker URLs to specify the available brokers. Brokers are not involved with the static discovery agent. The client only knows about the brokers that are hard coded into the agent's URI.
Using the agent
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The static discovery agent is a client-side only agent. It does not require any configuration on the brokers that will be discovered.
To use the agent, you simply configure the client to connect to a broker using a discovery protocol that uses a static agent URI.
The static discovery agent URI conforms to the syntax in Example 2.10, “Static Discovery Agent URI Format”.

Example 2.10. Static Discovery Agent URI Format

static://(URI1,URI2,URI3,...)
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Example
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Example 2.11, “Discovery URI using the Static Discovery Agent” shows a discovery URI that configures a client to use the static discovery agent to connect to one member of a broker pair.

Example 2.11. Discovery URI using the Static Discovery Agent

discovery://(static://(tcp://localhost:61716,tcp://localhost:61816))
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2.2.2.3. Multicast Discovery Agent
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Abstract

The multicast discovery agent uses the IP multicast protocol to find any message brokers currently active on the local network.
Overview
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The multicast discovery agent uses the IP multicast protocol to find any message brokers currently active on the local network. The agent requires that each broker you want to advertise is configured to use the multicast agent to publish its details to a multicast group. Clients using the multicast agent as part of the discovery URI they use for connecting to a broker will use the agent to receive the list of available brokers advertising in the specified multicast group.
Important
Your local network (LAN) must be configured appropriately for the IP/multicast protocol to work.
URI
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The multicast discovery agent URI conforms to the syntax in Example 2.12, “Multicast Discovery Agent URI Format”.

Example 2.12. Multicast Discovery Agent URI Format

multicast://GroupID
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Where GroupID is an alphanumeric identifier. All participants in the same discovery group must use the same GroupID.
Configuring a broker
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For a broker to be discoverable using the multicast discovery agent, you must enable the discovery agent in the broker's configuration. To enable the multicast discovery agent you set the transportConnector element's discoveryUri attribute to a mulitcast discovery agent URI as shown in Example 2.13, “Enabling a Multicast Discovery Agent on a Broker”.

Example 2.13. Enabling a Multicast Discovery Agent on a Broker

<transportConnectors>
  <transportConnector name="openwire"
    uri="tcp://localhost:61716"
    discoveryUri="multicast://default" />
</transportConnectors>
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The broker configured in Example 2.13, “Enabling a Multicast Discovery Agent on a Broker” is discoverable as part of the multicast group default.
Configuring a client
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To use the multicast agent a client must be configured to connect to a broker using a discovery URI that uses a multicast agent URI as shown in Example 2.14, “Client Connection URL using Multicast Discovery”.

Example 2.14. Client Connection URL using Multicast Discovery

discovery://(multicast://default)
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A client using the URI in Example 2.14, “Client Connection URL using Multicast Discovery” will discover all the brokers advertised in the default multicast group and generate a list of brokers to which it can connect.

2.2.2.4. Zeroconf Discovery Agent
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Abstract

The zeroconf discovery agent uses an open source implementation of Apple's Bonjour networking technology to find any brokers currently active on the local network.
Overview
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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. Red Hat JBoss A-MQ 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.
The agent requires that each broker you want to advertise is configured to use a multicast discovery agent to publish its details to a multicast group. Clients using the zeroconf agent as part of the discovery URI they use for connecting to a broker will use the agent to receive the list of available brokers advertising in the specified multicast group.
Important
Your local network (LAN) must be configured to use IP/multicast for the zeroconf agent to work.
URI
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The zeroconf discovery agent URI conforms to the syntax in Example 2.15, “Zeroconf Discovery Agent URI Format”.

Example 2.15. Zeroconf Discovery Agent URI Format

zeroconf://GroupID
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Where the GroupID is an alphanumeric identifier. All participants in the same discovery group must use the same GroupID.
Configuring a broker
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For a broker to be discoverable using the zeroconf discovery agent, you must enable a multicast discovery agent in the broker's configuration. To enable the multicast discovery agent you set the transportConnector element's discoveryUri attribute to a mulitcast discovery agent URI as shown in Example 2.16, “Enabling a Multicast Discovery Agent on a Broker”.

Example 2.16. Enabling a Multicast Discovery Agent on a Broker

<transportConnectors>
  <transportConnector name="openwire"
    uri="tcp://localhost:61716"
    discoveryUri="multicast://NEGroup" />
</transportConnectors>
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The broker configured in Example 2.16, “Enabling a Multicast Discovery Agent on a Broker” is discoverable as part of the multicast group NEGroup.
Configuring a client
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To use the agent a client must be configured to connect to a broker using a discovery protocol that uses a zeroconf agent URI as shown in Example 2.17, “Client Connection URL using Zeroconf Discovery”.

Example 2.17. Client Connection URL using Zeroconf Discovery

discovery://(zeroconf://NEGroup)
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A client using the URL in Example 2.17, “Client Connection URL using Zeroconf Discovery” will discover all the brokers advertised in the NEGroup multicast group and generate a list of brokers to which it can connect.
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