This Java library leverages Solace's guaranteed delivery capabilities to provide simple Leader Election events to applications requiring fault tolerance. But it does require a Solace Message Broker is available for your application to connect to for this library to work.
If you are already using Solace, just pass your Solace session instance into the
FTMgr and listen for status events. If you are not using Solace there are several
great reasons why you might want to!
new FTMgr(solaceConnection)
.start(ftClusterName,
new FTEventListener() {
@Override
public void onActive(BytesXMLMessage m) {
// When started this way, m is always null
logger.info("STATE CHANGE TO ACTIVE");
}
@Override
public void onBackup() {
logger.info("STATE CHANGE TO BACKUP");
}
});
// ...This is a Maven project referencing all public libraries, so you will need either internet access to public repositories or cached libraries in your local repository. Building should be as simple as running the command:
mvn install
A convenience startup script is provided in the bin/ directory.
The executable requires a properties file with all variables defining connectivity to Solace (further documentation below). Other variables can be specified on the commandline and include:
bin/run-example.sh <propsfile> <solace-cluster-name>
props-file: Solace session properties file for connectingcluster-name: Solace FT cluster name
# Solace session properties
host = 192.168.56.199,192.168.56.200
vpn_name = default
username = default
After building the package can be run in place by leveraging MVN's understanding
of your classpath in the bin/run-example.sh script. For example:
bin/run-example.sh src/main/resources/localtest.properties myCluster01
The example simply connects to a Solace broker, binds to the FT-Cluster via a Solace exclusive-queue (and provisions it if it does not exist), and alerts whenever it changes state to MASTER or BACKUP.
This Java library leverages Solace's guaranteed delivery capabilities to provide simple Leader Election events to applications requiring fault tolerance, by creating an instance of the FTMgr and binding a listener to it to receive FT role event updates.
To use it you must have a Solace Message Broker available for your application to connect to for this library to work.
First you will need a connected Solace session. You can either create a new session just for solft by passing an
instance of Properties, or more likely you'd pass a shared Solace session, into your FTMgr constructor.
You use the FTMgr to join the FT-cluster and listen for events when cluster leadership state changes occur.
To listen for these events you must implement a simple FTEventListener interface and pass the listener into
the start() method when starting the FTMgr. All applications joining the same named cluster on the same
Solace broker will be part of the same FT-cluster, where a single application is elected leader until it unbinds
from the cluster for any reason. In that event, the cluster elects a new member and an FTEventListener.onActive(msg)
event is raised for that instance notifying it of this role change.
For Stateless Listeners, the message object will always be null.
With a Stateful Event Listener, an onActive(msg) event is received with the last output message sent by the previous
Active member. This can be used to synchronize your newly-elected Active member to that state.
To run a Stateful Event Listener you start the FTMgr instance by calling startStateful() rather than start() and
pass into it a topic subscription string that subscribes to all output from this application. Each instance must
be configured with this to support Stateful Listeners therefore must all be able to publish output messages on a
topic that can be subscribed to in this manner.
Here's an example.
Let's say I have an application that is setup to publish output messages to a topic hierarchy with an agreed structure. In our example, let's say the agreed structure is:
[SOURCE] / [SRC INST] / [DESTINATION] / [EVENT TYPE]
In our case, when my instance (#3) of app AKNA sends an OTD measurement to IGALUK the topic might look like this:
AKNA / 3 / IGALUK / OTD
This allows us to configure all the instances of AKNA to subscribe to all output from any of the instances (and NOT
from any other application instances) by starting our FTMgr instance with an output subscription like:
AKNA/>
The code example becomes:
new FTMgr(solaceConnection)
.startStateful(ftClusterName, "AKNA/>",
new FTEventListener() {
@Override
public void onActive(BytesXMLMessage m) {
// When started this way, m can contain output from the prior Active member
logger.info("STATE CHANGE TO ACTIVE with state: " + m.dump());
}
@Override
public void onBackup() {
logger.info("STATE CHANGE TO BACKUP");
}
});
// ...