Overview

State Replication is the simpler of Talon's 2 HA models. With State Replication Talon replicates changes to state and the outbound messages emitted by your application's message handlers. In the event of failover to a backup your application's state is available at the same point where processing left off, and the engine will retransmit any outbound messages that were left in doubt as a result of the failure. 

Coding For State Replication

A basic state replication application is straightforward. A quick way to get started is to use the nvx-talon-sr-processor-archetype described in Maven Archetype Quick Starts. The general flow for creating a state replication application involves:

• Modeling Messages and State
• Declaring a main class annotated for StateReplication
• Providing Talon with a state factory for creating your application state
• Writing message handlers to perform business logic. 

A Basic App

Model Application State

The steps outlined below assume that you have already modeled some messages and state for your application to use. See the Modeling Message and State sections to get started. 

Annotate App Main for State Replication

Provide a StateFactory

An AEPEngine is application state agnostic: it deals with your application an plain old java object graph with a single root. Given the root object for your application's state the underlying store will track changes made to fields on the root (and its descendants) and replicate or persist those changes. As such, an AepEngine needs to be able to create a new application state during when your application is initialized. This is done by finding a method on your main application class anotated with @AppStateFactoryAccessor. The state factory accessor returns a newly initialized set of state for the engine to manage. As messages are processed the engine will pass the relevant state root back into the application to be operated upon. 

As your application makes changes to this root object (setting fields etc), the engine will monitor the root and replicate deltas to backup members or disk. 

Inject Message Sender

If your application will send messages, it can add an injection point for the underlying AEPEngine to inject a message sender.

Declare Message Handlers

When working with an application using StateReplication, the underlying AepEngine will pass in the root object for your application state along with the message. Outbound message sends and state changes are managed by Talon as an atomic unit of work. 

Configuring the Engine

Register Object Factories

When working with state replication both the ADM and message and state object factories need to be registered with the runtime. Registering the state factory allows the underlying state replication machinery to deserialize replicated state objects based on the ids encoded in the replication stream. The message factories allow are used for deserializing replicated outbound messages as well as messages received from message buses. The state factories can be declared in your config.xml or programmatically:

Enable Storage

To actually achieve high availability storage must be configured for the application. The primary means of storage is for Talon apps is through clustered replication to a backup instance. Talon also logs state changes to a disk based transaction log as a fallback mechanism. Storage and persistence can be enabled in the application's configuration xml. 

Enabling clustering allows 2 applications of the same name to discover one another and form an HA cluster. When one or more instances of an application connect to one another one instance is elected as the primary via a leadership election algorithm. The primary member will establish messaging connections and begin invoking message handlers in your application. 

Enabling persistence causes the replication stream that is sent to backup instances to also be logged locally on disk to a transaction log file. The transaction log file can be used to recover application state from a cold start, and is also used to initialize new cluster members that connect to it so when clustering is enabled, persistence must be enabled as well. 

There are many configuration knobs that can be used to customize the store's behavior and performance. See DDL Config Reference for a listing of configuration knobs for storage.

Unsolicited Senders

For applications that use state replication with an unsolicited sender (i.e. those not done from a message handler) such as the Feeder app, the application should additionally configure the engine to replicate unsolicited sends so that state and messages are replicated for the sender. The Feeder is such an application, so it additionally sets this value to true:

Creation of Application State

Application state is always created by the AEP engine and is created in one of three different ways:

1. By a primary engine when it receives the first message of a flow, in which case it utilizes the application provided IApplicationStateFactory.
2. By a backup engine when application state is replicated from a primary.
3. Explicitly by an application on an AepMessagingPrestartEvent handler.

Two constraints need to be met when the state is created explicitly by the application.

1. For performance reasons, the state creation is not thread safe. Thus, the state must be created before the engine's messaging has been started.
2. The state needs to be created after the engine has bound to the applications's ODS store. This needs to be done so that the created state is attached to the opened store.

The first constraint implies that the state must be created before the engine has started messaging. The second constraint implies that the state needs to be created during or after start (since the engine opens the store during start()) i.e. the only time the state can be explicitly created by the application is between start() and the receipt of the messaging started event. The messaging prestart event is such a point in the flow.

Interacting With Application State

From a message handler

The below snippet comes from the Enricher message handler. With a message handler, application state should be retrieved using the Engine's getApplicationState(MessageView) accessor, which retrieves the state associated with the message's flow. In the Ticker sample, the flow is defined by the hashCode of the symbol and is set by the Feeder. Flows define the order in which messages are processed in the system. Flows essentially partition traffic into ordered parallel processed streams and, therefore, enable parallelized, concurrent message processing.
Under the covers, the Engine ensures that both messages and state are replicated to the backup to prevent loss or duplication in the event of a failure.

Unsolicited Sends

The Feeder operates slightly differently. Because it is the origin point of the traffic, it can't retrieve its state from the Engine using a MessageView, but as mentioned previously, Creation of State can only be done from an engine thread. It must therefore create its state from within an AepMessagingPrestartEvent handler. If the Feeder were to operate with multiple flows (and consequently multiple application state instances, it would need to create them all here.

The sample application is simple and operates with a single flow based on the symbol. For simplicity, we use the hashcode of the symbol name to map this to an integer id representing the flow. The sender creates the state and stores it in a private variable _state for use during runtime. The created state is not threadsafe, so in this context, it can only be used by the feeder's thread sending its messages.

Limitations and Upcoming Enhancements

See State Graph Limitations for a discussion of current state graph limitations.