High availability is an important aspect associated with any server implementation that ensures a certain degree of operational continuity when unplanned downtime events impact parts of a system. JMS 1.1 does not provide this; instead, most of the time it is vendor specific.
The idea behind failover is to stop single-point-of-failure in a system. As the broker is the middle man storing and forwarding the messages, if that server goes down, the entire message flow of the system will go down no matter what other servers and functions are involved. In order to make a robust messaging system, it is mandatory to have a failover mechanism.
In order to achieve this, a few instances of message broker servers are set up and running in the system, while the system (generally) uses one broker. If that broker goes down, it automatically switches to the second broker and continues messaging. If the second broker fails, it will try the next one and so on. Thus, the whole system will not have any downtime.
Follow the instructions below to set up a failover mechanism using EI Message Broker profile and the Integrator runtime in WSO2 EI:
Setting up the broker instances
Set up the broker instances as follows.
Setting up the Broker instances in a single machine
- Make an exact copy of EI Message Broker runtime in three different locations and rename them as MB1, MB2 and MB3. Those will be the 3 separate MB instances.
<EI_HOME>/wso2/broker/conf/carbon.xmland define an offset of 1.
- Navigate to
<EI_HOME>/wso2/broker/conf/carbon.xmlin MB2 and define an offset of 2.
- Navigate to
<EI_HOME>/wso2/broker/conf/carbon.xmlin MB3 and define an offset of 3.
- The 3 MB instances are ready. Start each instance of MB by running one of the following commands:
The broker instances will start on ports 5673, 5674 and 5675 respectively.
Setting up Broker instances in different machines
- Make an exact copy of that folder in three separate machines. Let's assume MB1 is in the machine with IP1, MB2 in the machine with IP2 and MB3 in the machine with IP3.
- Start all the above instances by running one of the following commands:
Running the broker instances
Start the Integrator runtime in WSO2 EI in the default port (port offset 0). This is possible if you used the single machine setup above, as those servers were started with different port offsets. If you are using different machines, use another machine to start the Integrator runtime. Instructions can be found here.
It is not possible to start multiple WSO2 products with their default configurations simultaneously in the same environment. Since all WSO2 products use the same port in their default configuration, there will be port conflicts.
There is only a single difference to enable failover across the three brokers we have setup. That is when specifying the following:
For example, if you hope to use a single machine broker setup as described above,
If you hope to use the broker setup made across several machines as described above,
The parameters used above are described in detail below:
The broker list defines the various brokers that can be used for this connection. A minimum of one broker URL is required. Additional URLs are semi-colon(';') delimited.
Broker URL format
Option Default Description retries 1 The number of times to retry connecting to this Broker ssl false Use ssl on the connection connecttimeout 30000 How long in (milliseconds) to wait for the connection to succeed connectdelay none How long in (milliseconds) to wait before attempting to reconnect
Brokerlist failover option
Method Description singlebroker This will only use the first broker in the list. roundrobin This method tries each broker in turn. nofailover [New in 0.5] This method disables all retry and failover logic. onetime This method ensures that XA transactions do not failover from one node another. That is, if the connection fails before all messages are committed to the broker node, the XA transaction will be canceled. The client will then reconnect to the next available broker in the list and resend all the messages from the beginning. See Handling Distributed Transactions for more information
The current default is to use the singlebroker method when only one broker is present, and the roundrobin method with multiple brokers. The method value in the URL may also be any valid class on the classpath that implements the FailoverMethod interface.
Option Default Description cyclecount 1 The number of times to loop through the list of available brokers before failure.
Testing the setup
When sending messages to a queue (you can configure SOAP UI to send messages continuously one after the other) using the setup described above, kill MB1. You will notice that failover happens at the Integrator instance to the second broker i.e MB2. The logs at the Integrator side will indicate that failover occurred.
If there were previous messages at MB1, they will not be delivered. Usually, the solution for that (and many other problems) is broker clustering. For information on the advantages of clustering, see Clustering the Broker Profile.