[cowboy]

Simple Persistence
Simple persistence is generally based on network characteristics, like source IP address. It can also include the destination port, to give the load balancer a bit more capacity in terms of simultaneously applications supported. Best practices avoid simple persistence to avoid reoccurrence of the mega-proxy problem which had a tendency to overwhelm application instances. Network load balancing uses a form of simple persistence.

SSL Session ID Persistence
SSL Session ID persistence became necessary when SSL was broadly accepted as the de facto means of securing traffic in flight for web applications. Because SSL sessions need to be established and are very much tied to a session between client and server, failing to “stick” SSL-secured sessions results in renegotiation of the session, which takes a noticeable amount of time and annoys end-users. To avoid unnecessary renegotiation, load balancers use the SSL Session ID to ensure sessions are properly routed to the application instance to which they first connected.

Cookie Persistence
Cookie persistence is a technique invented by F5 (shameless plug) that uses the HTTP cookie header to persist connections across a session. Most application servers insert a session id into responses that is used by developers to access data stored in the server session (shopping carts, etc… ). This value is used by load balancing services to enable persistence. This technique avoids the issues associated with simple persistence because the session id is unique.

Universal Persistence
Universal persistence is the use of any piece of data (network, application protocol, payload) to persist a session. This technique requires the load balancer to be able to inspect and ultimately extract any piece of data from a request or response. This technique is the basis for application-specific persistence solutions addressing popular applications like SIP, WTS, and more recently, VMware View.

SIP, WTS, Username Persistence
Session Initiation Protocol (SIP) and Windows Terminal Server (WTS) persistence are application-specific persistence techniques that use data unique to a session to persist connections. Username persistence is a similar technique designed to address the needs of VDI – specifically VMware View solutions – in which sessions are persisted (as one might expect) based on username. When a type of persistence becomes very commonly used it is often moved from being a customized, universal persistence implementation to a native, productized persistence profile. This improves performance and scalability by removing the need to inspect and extract the values used to persist sessions from the data flow and results in an application-specific persistence type, such as SIP or WTS.

Hash Persistence
Hash persistence is the use of multiple values within a request to enable persistence. To avoid problems with simple persistence, for example, a hash value may be created based on Source IP, Destination IP, Destination Port. While not necessarily unique to every session, this technique results in a more even distribution of load across servers.