Mule clusters consist of 2-8 server instances that act as a single unit. An active-active model allows all servers to support the application simultaneously, unlike an active-passive model where only one primary server is active. To ensure reliability, applications must have zero message loss tolerance and rely on a reliable ESB and connections between clustered nodes. Best practices include organizing steps to move messages transactionally between stores, using distributed stores, and implementing reliability patterns for high reliability apps.

1. MULE HIGH
AVAILABILITY

2. INTRODUCTION
A cluster is a set of Mule instances that acts as a unit. In other words, a
cluster is a virtual server composed of multiple nodes. The servers in a
cluster communicate and share information through a distributedshared
memory grid. This means that the data is replicated across memory in
different physicalmachines.

4. ABOUT
CLUSTERI
NG
A Mule ESB Cluster consists of two to eight Mule ESB server instances, or nodes, grouped together and
treated as a single unit. Thus, you can deploy, monitor, or stop all the nodes in a cluster as if they were a
single Mule server.
Mule uses an active-active model to cluster servers, rather than an active-passive model.
In an active-passive model, one server in a cluster acts as the primary, or active node, while the others
are secondary, or passive nodes. The application in such a model runs on the primary server, and only
ever runs on the secondary server if the first one fails. In this model, the processing power of the
secondary node(s) is mostly wasted in passive waiting for the primary node to fail.
In an active-active model, no one server in the cluster acts as the primary server; all servers in the cluster
support the application. This application in this model runs on all the servers, even splitting apart
message processing between nodes to expedite processing across nodes.

5. ABOUT QUEUES
You can set up a VM queue explicitly to load balance across nodes. Thus, if your
entire application flow contains a sequence of child flows, Mule can assign each
successive child flow to whichever node happens to be available at thetime.
Potentially, Mule can process a single message on multiple nodes as itpasses
through the entire application flow, as illustrated in nextfigure:

7. ABOUT HIGH-RELIABILITY
APPLICATIONS
A high-reliability application must feature thefollowing:
1. zero tolerance for message loss
2. a reliable underlying enterprise service bus(ESB)
3. highly reliable individualconnections

8. CLUSTER SUPPORT FOR
TRANSPORTS
 Mule supports three basic types of transports:
1. Socket-based transports read input sent to network sockets that Mule owns.
Examples include TCP, UDP, and HTTP[S].
2. listener-based transports read data using a protocol that fully supports
concurrent multiple accessors. Examples include JMS and VM.
3. resource-based transports read data from a resource that allows multiple
concurrent accessors, but does not natively coordinate their use of the resource. For
instance, suppose multiple programs are processing files in the same shared
directory by reading, processing, and then deleting the files. These programs must
use an explicit, application-level locking strategy to prevent the same file from
being processed more than once. Examples of resource-based transports include
File, FTP, SFTP, E-mail, and JDBC.

9. CLUSTERING AND RELIABLE
APPLICATIONS
High-reliability applications (ones that have zero tolerance for message loss) notonly
require the underlying ESB to be reliable, but that reliability needs to extend to
individual connections. Reliability Patterns give you the tools to build fully reliable
applications in your clusters.

10. CLUSTERING AND
NETWORKING
Toensure reliable connectivity between cluster nodes, all nodes of a cluster
should be located on the same LAN. Implementing a cluster with nodesacross
geographically separated locations, such as different datacenters connected
through a VPN, is possible but not recommended and notsupported.

11. CLUSTERING AND LOAD
BALANCING
When Mule clusters are used to serve TCP requests (where TCP includes
SSL/TLS, UDP, Multicast, HTTP, and HTTPS), some load balancing is needed to
distribute the requests among the clustered instances. There are various
software load balancersavailable.

12. CLUSTERING FOR HIGH
PERFORMANCE
If high performance is your primary goal (rather than reliability), you can configurea
Mule cluster or an individual application for maximum performance using a
performance profile.
Setting the performance profile has twoeffects:
1. It disables distributed queues, using local queues instead to preventdata
serialization/deserialization and distribution in the shared data grid.
2. It implements the object store without backups, to avoid replication.

13. BEST PRACTICES
There are a number of recommended practices related to clustering. Theseinclude:
• As much as possible, organize your application into a series of steps where eachstep
moves the message from one transactional store to another.
• If your application processes messages from a non-transactional transport,use
areliability pattern to move them to a transactional store such as a VM or JMSstore.
• Use transactions to process messages from a transactional transport. This ensuresthat
if an error is encountered, the message will bereprocessed.
• Use distributed stores such as those used with the VM or JMS transport – these stores
are available to an entire cluster. This is preferable to the non-distributed stores used
with transports such as File, FTP,and JDBC – these stores are read by a single node ata
time.
• Use the VM transport to get optimal performance. Use the JMS transportfor
applications where data needs to be saved after the entire clusterexits.
• Create the number of nodes within a cluster that best meets your needs.
• Implement reliability patterns to create high reliability applications.

14. CONCLUSION
• Currently you can create a cluster consisting of at least two servers and up toa
maximum of eight. However, each server must run in a different physical (or
virtual) machine.
• Tomaintain synchronization between the nodes in the cluster, Mule HA requires a
reliable network connection betweenservers.
• You must keep the following ports open in order to set up a Mule cluster: port
5701 and port 54327.
• Because new cluster member discovery is performed using multicast, you needto
enable the multicast IP: 224.2.2.3