How Stuff Works

Documentation Overview

The following diagrams and notes are my understanding of part of the ServiceMix architecture based on a study of the source code – PS

Contents

• NMR flow types
  • st
  • seda
  • jms
  • jca

NMR Flow types

The flow type determines the mechanism by which the Normalized Message Router (NMR) Broker sends messages from one BC/SE to another.

Flow implementation classes

• JBIContainer "flowName" attribute determines the flow type used

• There is only 1 flow per Broker (and 1 Broker per JBIContainer)

• Flow name/class mappings are in the ServiceMix JAR. See META-INF/services/org/servicemix/jbi/nmr/flow

• There are a number of flow implementations. For example: STFlow, SedaFlow, JMSFlow, JCAFlow

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Broker send sequence diagram

• The NMR Broker calls sendExchangePacket to deliver a message

• The flow implementation of abstract doSend Method determines how the message sending mechanism works

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STFlow

STFlow = "Straight Through" Flow. This is the simplest mechanism.

There is no staging or buffering.

sequence diagram

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SedaFlow

SEDA = "Staged Event Driven Architecture" Flow.

SEDA decomposes a complex, event-driven application into a set of stages connected by queues.

SedaFlow / SedaQueue classes

• There is 1 SedaFlow instance and a number of SedaQueue instances (1 per component)

• The SedaFlow "enqueues" an ME to send a message

• The SedaQueue instance "dequeues" the ME and processes it

• Each SedaQueue holds a concurrent HashMap of MessageExchanges (MEs) to the component. Enqueue/Dequeue is simply put/get of this HashMap

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sequence diagram

• SedaQueue instances are lazilly created as needed and referenced in a queueMap

• Each SedaQueue instance runs as JCA1.5 asynchronous Work thread polling the HashMap at fixed intervals

• Having enqueued the ME, the SedaFlow doSend Method may return

• When the SedaQueue Worker (run Method) finds (dequeues) an ME, it processes it in another JCA Worker thread created solely for that purpose

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JMSFlow

Uses JMS for message routing among a network of remote containers.

"remote" just means separate. In fact, it makes no difference if the JMSFlow containers are on the same or separate machines.

JMSFLow classes and JMS Destinations

• There is one single Topic across all containers. This is created during JMSFlow init. A ConsumerAdvisor configured for this Topic allows container to be "aware" of each other. Special messages sent to this Topic allow broadcasting of you components to other containers in the network.

• There is one Queue per container. This is created during JMSFlow init. This Queue is for sending request MEs to.

• There is one Queue per sender component. This is created when a component is activated. These Queue is for sending response MEs to.

• The JMSFlow is a JMS MessageListener (ie it implements onMessage Method) for the Topic and all Queues

• The JMSFlow is also an ActiveMQ ConsumerAdvisoryEventListener for the Topic. This means all JMSFlow instances will be aware of remote container startup/shutdown

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Notification of remote component state changes

• TODO fix diagram

• I have shown a "local" and "remote" JMSFLow instance for clarity although they may be the same instance

• A container needs to be made aware when the state of a remote container's components changes. JMSFlow uses an ActiveMQ ConsumerAdvisoryEvent to detect a change in status of a remote Consumer of the Topic.

• • If remote Topic Consumer activated - eg container started
    • Information of all (local) components are re-broadcast to the Topic so the new container can lern about them

• • If remote Topic Consumer is deactivates - eg container shutdown
    • All (local) knowledge of the remote container's components is removed

• Conversely, a remote container needs to be made aware if there is some state change with any component of the local container. JMSFlow is a JMS MessageListener on the Topic

• • If a component state changes a ComponentPacketEvent (which container component state information) is serialized and sent as a JMS ObjectMessage to the Topic.
    • All remove JMSFlows receive this message and adjust their knowledge of our component accordingly
  • If the component is becoming activated, then a Queue is created (for remote containers to send response MEs to)

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JMSFlow send ME sequence diagram

• I have shown a "local" and "remote" JMSFLow instance for clarity although they may be the same instance

• The ME is serialized and sent as a JMS ObjectMessage

• Requests are sent to a generic Queue on the remote container

• Responses are sent back to a specific Queue for the component that sent the request exchange (because the sender may have done a synchronous send and is awaiting a response)

• The JMSFlow is a MessageListener for both Queues

• When the MessageListener's onMessage is triggered the ObjectMessage content is deserialized. If the deserialized Object is a ME then it is routed to the destination component.

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JCAFlow

• TODO

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