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Chapter 1. Overview
AMQ Interconnect is a lightweight AMQP message router for building scalable, available, and performant messaging networks.
AMQ Interconnect is based on Dispatch Router from the Apache Qpid™ project.
1.1. Key Features
- Connects clients and brokers into an internet-scale messaging network with uniform addressing
- Supports high-performance direct messaging
- Uses redundant network paths to route around failures
- Streamlines the management of large deployments
1.2. Supported Standards and Protocols
AMQ Interconnect supports the following industry-recognized standards and network protocols:
- Version 1.0 of the Advanced Message Queueing Protocol (AMQP)
- Modern TCP with IPv6
The details of distributed transactions (XA) within AMQP are not provided in the 1.0 version of the specification. AMQ Interconnect does not support XA transactions.
1.3. Supported Configurations
AMQ Interconnect is supported on Red Hat Enterprise Linux 6 and 7. See Red Hat AMQ 7 Supported Configurations for more information.
1.4. Theory of Operation
This section introduces some key concepts about AMQ Interconnect
1.4.1. Overview
AMQ Interconnect is an application layer program running as a normal user program or as a daemon.
The router accepts AMQP connections from clients and creates AMQP connections to brokers or AMQP-based services. The router classifies incoming AMQP messages and routes the messages between message producers and message consumers.
The router is meant to be deployed in topologies of multiple routers, preferably with redundant paths. It uses link-state routing protocols and algorithms similar to OSPF or IS-IS from the networking world to calculate the best path from every message source to every message destination and to recover quickly from failures. The router relies on redundant network paths to provide continued connectivity in the face of system or network failure.
A messaging client can make a single AMQP connection into a messaging bus built with routers and, over that connection, exchange messages with one or more message brokers connected to any router in the network. At the same time the client can exchange messages directly with other endpoints without involving a broker at all.
1.4.2. Connections
AMQ Interconnect connects clients, servers, AMQP services, and other routers through network connections.
1.4.2.1. Listener
The router provides listeners that accept client connections. A client connecting to a router listener uses the same methods that it would use to connect to a broker. From the client’s perspective the router connection and link establishment are identical to broker connection and link establishment.
Several types of listeners are defined by their role.
| Role | Description |
|---|---|
|
| The connection is used for AMQP clients using normal message delivery. |
|
| The connection is assumed to be to another router in the network. Inter-router discovery and routing protocols can only be used over inter-router connections. |
|
| The connection is a broker or other resource that holds known addresses. The router will use this connection to create links as necessary. The addresses are available for routing only after the remote resource has created a connection. |
1.4.2.2. Connector
The router can also be configured to create outbound connections to messaging brokers or other AMQP entities using connectors. A connector is defined with the network address of the broker and the name or names of the resources that are available in that broker. When a router connects to a broker through a connector it uses the same methods a normal messaging client would use when connecting to the broker.
Several types of connectors are defined by their role.
| Role | Description |
|---|---|
|
| The connection is used for AMQP clients using normal message delivery. On this connector the router will initiate the connection but it will never create any links. Links are to be created by the peer that accepts the connection. |
|
| The connection is assumed to be to another router in the network. Inter-router discovery and routing protocols can only be used over inter-router connections. |
|
| The connection is to a broker or other resource that holds known addresses. The router will use this connection to create links as necessary. The addresses are available for routing only after the router has created a connection to the remote resource. |
1.4.3. Addresses
AMQP addresses are used to control the flow of messages across a network of routers. Addresses are used in a number of different places in the AMQP 1.0 protocol. They can be used in a specific message in the to and reply-to fields of a message’s properties. They are also used during the creation of links in the address field of a source or a target.
Addresses in this discussion refer to AMQP protocol addresses and not to TCP/IP network addresses. TCP/IP network addresses are used by messaging clients, brokers, and routers to create AMQP connections. AMQP protocol addresses are the names of source and destination endpoints for messages within the messaging network.
Addresses designate various kinds of entities in a messaging network:
- Endpoint processes that consume data or offer a service
- Topics that match multiple consumers to multiple producers
Entities within a messaging broker:
- Queues
- Durable Topics
- Exchanges
The syntax of an AMQP address is opaque as far as the router network is concerned. A syntactical structure may be used by the administrator who creates addresses but the router treats them as opaque strings.
The router maintains several classes of address based on how the address is configured or discovered.
| Address Type | Description |
|---|---|
| Mobile | The address is a rendezvous point between senders and receivers. The router aggregates and serializes messages from senders and distributes messages to receivers. |
| Link route | The address defines a private messaging path between a sender and a receiver. The router simply passes messages between the end points. |
1.4.3.1. Mobile Addresses
Routers consider addresses to be mobile such that any users of an address may be directly connected to any router in a network and may move around the topology. In cases where messages are broadcast to or balanced across multiple consumers, the address users may be connected to multiple routers in the network.
Mobile addresses are rendezvous points for senders and receivers. Messages arrive at the mobile address and are dispatched to their destinations according to the routing defined for the mobile address. The details of these routing patterns are discussed later.
Mobile addresses may be discovered during normal router operation or configured through management settings.
1.4.3.1.1. Discovered Mobile Addresses
Mobile addresses are created when a client creates a link to a source or destination address that is unknown to the router network.
Suppose a service provider wants to offer my-service that clients may use. The service provider must open a receiver link with source address my-service. The router creates a mobile address my-service and propagates the address so that it is known to every router in the network.
Later a client wants to use the service and creates a sending link with target address my-service. The router matches the service provider’s receiver having source address my-service to the client’s sender having target address my-service and routes messages between the two.
Any number of other clients can create links to the service as well. The clients do not have to know where in the router network the service provider is physically located nor are the clients required to connect to a specific router to use the service. Regardless of how many clients are using the service the service provider needs only a single connection and link into the router network.
Another view of this same scenario is when a client tries to use the service before service provider has connected to the network. In this case the router network creates the mobile address my-service as before. However, since the mobile address has only client sender links and no receiver links the router stalls the clients and prevents them from sending any messages. Later, after the service provider connects and creates the receiver link, the router will issue credits to the clients and the messages will begin to flow between the clients and the service.
The service provider can connect, disconnect, and reconnect from a different location without having to change any of the clients or their connections. Imagine having the service running on a laptop. One day the connection is from corporate headquarters and the next day the connection is from some remote location. In this case the service provider’s computer will typically have different host IP addresses for each connection. Using the router network the service provider connects to the router network and offers the named service and the clients connect to the router network and consume from the named service. The router network routes messages between the mobile addresses effectively masking host IP addresses of the service provider and the client systems.
1.4.3.1.2. Configured Mobile Addresses
Mobile addresses may be configured using the router autoLink object. An address created via an autoLink represents a queue, topic, or other service in an external broker. Logically the autoLink addresses are treated by the router network as if the broker had connected to the router and offered the services itself.
For each configured mobile address the router will create a single link to the external resource. Messages flow between sender links and receiver links the same regardless if the mobile address was discovered or configured.
Multiple autoLink objects may define the same address on multiple brokers. In this case the router network creates a sharded resource split between the brokers. Any client can seamlessly send and receive messages from either broker.
Note that the brokers do not need to be clustered or federated to receive this treatment. The brokers may even be from different vendors or be different versions of the same broker yet still work together to provide a larger service platform.
1.4.3.2. Link Route Addresses
Link route addresses may be configured using the router linkRoute object. An link route address represents a queue, topic, or other service in an external broker similar to addresses configured by autoLink objects. For link route addresses the router propagates a separate link attachment to the broker resource for each incoming client link. The router does not automatically create any links to the broker resource.
Using link route addresses the router network does not participate in aggregated message distribution. The router simply passes message delivery and settlement between the two end points.
1.4.4. Message Routing
Addresses have semantics associated with them that are assigned when the address is provisioned or discovered. The semantics of an address control how routers behave when they see the address being used. Address semantics include the following considerations:
- Routing pattern - balanced, closest, multicast
- Routing mechanism - message routed, link routed
1.4.4.1. Routing Patterns
Routing patterns define the paths that a message with a mobile address can take across a network. These routing patterns can be used for both direct routing, in which the router distributes messages between clients without a broker, and indirect routing, in which the router enables clients to exchange messages through a broker.
| Pattern | Description |
|---|---|
| Balanced | An anycast method which allows multiple receivers to use the same address. In this case, messages (or links) are routed to exactly one of the receivers and the network attempts to balance the traffic load across the set of receivers using the same address. This routing delivers messages to receivers based on how quickly they settle the deliveries. Faster receivers get more messages. |
| Closest | An anycast method in which even if there are more receivers for the same address, every message is sent along the shortest path to reach the destination. This means that only one receiver will get the message. Each message is delivered to the closest receivers in terms of topology cost. If there are multiple receivers with the same lowest cost, deliveries will be spread evenly among those receivers. |
| Multicast | Having multiple consumers on the same address at the same time, messages are routed such that each consumer receives one copy of the message. |
1.4.4.2. Routing Mechanisms
The fact that addresses can be used in different ways suggests that message routing can be accomplished in different ways. Before going into the specifics of the different routing mechanisms, it would be good to first define what is meant by the term routing:
In a network built of multiple, interconnected routers 'routing' determines which connection to use to send a message directly to its destination or one step closer to its destination.
Each router serves as the terminus of a collection of incoming and outgoing links. Some of the links are designated for message routing, and others are designated for link routing. In both cases, the links either connect directly to endpoints that produce and consume messages, or they connect to other routers in the network along previously established connections.
1.4.4.2.1. Message Routed
Message routing occurs upon delivery of a message and is done based on the address in the message’s to field.
When a delivery arrives on an incoming message-routing link, the router extracts the address from the delivered message’s to field and looks the address up in its routing table. The lookup results in zero or more outgoing links onto which the message shall be resent.
Message routing can also occur without an address in the message’s to field if the incoming link has a target address. In fact, if the sender uses a link with a target address, the to field shall be ignored even if used.
1.4.4.2.2. Link Routed
Link routing occurs when a new link is attached to the router across one of its AMQP connections. It is done based on the target.address field of an inbound link and the source.address field of an outbound link.
Link routing uses the same routing table that message routing uses. The difference is that the routing occurs during the link-attach operation, and link attaches are propagated along the appropriate path to the destination. What results is a chain of links, connected end-to-end, from source to destination. It is similar to a virtual circuit in a telecom system.
Each router in the chain holds pairs of link termini that are tied together. The router then simply exchanges all deliveries, delivery state changes, and link state changes between the two termini.
The endpoints that use the link chain do not see any difference in behavior between a link chain and a single point-to-point link. All of the features available in the link protocol (flow control, transactional delivery, and so on) are available over a routed link-chain.
1.4.4.3. Message Settlement
Messages may be delivered with varying degrees of reliability.
- At most once
- At least once
- Exactly once
The reliability is negotiated between the client and server during link establishment. The router handles all levels of reliability by treating messages as either pre-settled or unsettled.
| Delivery | Handling |
|---|---|
| Pre-settled | If the arriving delivery is pre-settled (that is, fire and forget), the incoming delivery shall be settled by the router, and the outgoing deliveries shall also be pre-settled. In other words, the pre-settled nature of the message delivery is propagated across the network to the message’s destination. |
| Unsettled | Unsettled delivery is also propagated across the network. Because unsettled delivery records cannot be discarded, the router tracks the incoming deliveries and keeps the association of the incoming deliveries to the resulting outgoing deliveries. This kept association allows the router to continue to propagate changes in delivery state (settlement and disposition) back and forth along the path which the message traveled. |
1.4.5. Security
AMQ Interconnect uses the SSL/TLS protocol and related certificates and SASL protocol mechanisms to encrypt and authenticate remote peers. Router listeners act as network servers and router connectors act as network clients. Both connection types may be configured securely with SSL/TLS and SASL.
The router Policy module is an optional authorization mechanism enforcing user connection restrictions and AMQP resource access control.
1.5. Document conventions
In this document, sudo is used for any command that requires root privileges. You should always exercise caution when using sudo, as any changes can affect the entire system.
For more information about using sudo, see The sudo Command.
