Traffic Server scales from a single node to multiple nodes that form a cluster allowing you to improve system performance and reliability. This chapter discusses the following topics:

Understanding Traffic Server Clusters

A Traffic Server cluster consists of multiple Traffic Server nodes. The nodes in a cluster share configuration information and can form a single logical cache. Traffic Server detects the addition and deletion of nodes in the cluster automatically and can detect when a node is unavailable. Traffic Server uses its own protocol for clustering, which is multicast for node location and heartbeat, but unicast for all data exchange within the cluster. Traffic Server has two clustering modes:

Management-only mode; refer to Management-Only Clustering below.
Full-clustering mode; refer to Full Clustering

Management-Only Clustering

In management-only clustering mode, Traffic Server cluster nodes share configuration information. You can administer all the nodes at the same time. Traffic Server uses a multicast management protocol to provide a single system image of your Traffic Server cluster. Information about cluster membership, configuration, and exceptions is shared across all nodes, and the traffic_manager process automatically propagates configuration changes to all the nodes.

Full Clustering

In full-clustering mode, as well as sharing configuration information, a Traffic Server cluster distributes its cache across its nodes into a single, virtual object store, rather than replicating the cache node by node. Traffic Server can provide an enormous aggregate cache size and can maximize cache hit rate by storing objects only once across the entire cluster.

A fully clustered Traffic Server maps objects to specific nodes in the cluster. When a node receives a request, it checks to see if the request is a hit somewhere in the cluster. If the request is a hit on a different node, the node handling the request obtains the object from the hit node and serves it to the client. Traffic Server uses its own communication protocol to obtain an object from sibling cluster nodes.

If a node fails or is shut down and removed, Traffic Server removes references to the missing node on all nodes in the cluster.

Full clustering recommends a dedicated network interface for cluster communication to get better performance.

Enable Clustering Mode

Before you put a node into a Cluster, please make sure the following thing checked:

you are using the samy Operation System:
- Using the same distrobution, ie: RHEL 5.5
- Have same kernel, ie: 2.6.18-194.17.1.el5
- The same ARCH, ie: x86_64
install with the same version of Traffic Server
the same hardware
on the same switch or same vlan.

Traffic Server does not apply the clustering mode change to all the nodes in the cluster. You must change the clustering mode on each node individually. You may following these instructions:

setup the same cluster name, or proxy name, e.g. MyCluster:
```
traffic_line -s proxy.config.proxy_name -v MyCluster
```
enable cluster mode:
```
traffic_line -s proxy.local.cluster.type -v 1
traffic_line -s proxy.config.cluster.ethernet_interface -v eth0
```
eth0 should be replaced by your real interface, we recommends a dedicated interface here. Refer to proxy.local.cluster.type for a full description.
enable:
```
traffic_line -x
```
restart:
```
traffic_line -L
```
process traffic_server and traffic_manager need to restart after the change of 'proxy.local.cluster.type' and 'proxy.config.cluster.ethernet_interface'

Traffic Server will join into the Cluster in about 10 seconds and you can run traffic_line -r proxy.process.cluster.nodes to check the hosts in cluster or checkout the cluster.config in the configuration directory.

After a successful join of the cluster, all changes of global configurations on any node, will take effect on all node now.

Deleting Nodes from a Cluster

To delete a node from the Traffic Server cluster, just roll back proxy.config.cluster.type to the default value 3 and reload.

Administrators's Guide - Overview