CometD 2 Clustering with Oort

CometD 2 Scalability Clustering with Oort

The CometD distribution ships a clustering solution called Oort that enhances the scalability of a CometD-based system.
Instead of connecting to a single node (usually represented by a virtual or physical host), clients connect to multiple nodes so that the processing power needed to cope with the load is spread among multiple nodes, giving the whole system more scalability then using a single node.

Oort clustering is not a high availability clustering solution: if one of the nodes crashes, then all the clients will be disconnected and will reconnect to other nodes (with a new CometD handshake).
All the information built by one client with its server up to that point (for example, the state of an online chess game) is generally lost (unless - of course - the application has implemented some other way to retrieve that information).

Typical Infrastructure

A typical, but not the only, infrastructure to setup a Oort cluster is to have a load balancer in front of Oort nodes, so that clients can connect transparently to any node.
The load balancer should implement stickyness, and this may be based on the remote IP address or on CometD's BAYEUX_BROWSER cookie (see the Bayeux Specification, section 8.1), or could be based on some other mechanism supported by the load balancer.
DNS should be configured with a single host name / IP address pair (the one of the load balancer), so that in case of a node crash, clients will attempt to reconnect to the same host name, but the load balancer will notice that the node is crashed and direct the connection to another node. The second node will not know about this client, and upon receiving the connect request will send to the client the advice to handshake.

Terminology

In the following sections, the following terminology will be used: an Oort cluster is also referred to as "Oort cloud" (and therefore "cloud" is a synonym for "cluster"), and a Oort node is also referred to as "Oort comet" (and therefore "comet" is a synonym for "node").