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The IdP is a stateful application. That is it maintains operational information, between requests, that is required to answer subsequent requests. The IdP keeps this information in memory (for various reasons cookies can not cannot be used as they are in some web applications). Therefore, in order to achieve high-availability this information must be shared amongst all IdP nodes. By default the Shibboleth team recommends V2 release assumed (and documentsdocumented) the use of Terracotta as the mechanism for doing this. This is still supported, but only on Java 6 because the Terracotta solution was not ported to support Java 7.
At present, the only options for clustering in-memory state with Java 7 are third party extensions, such as the memcached plugin. An alternative that relies on client-side state is mentioned below.
Like most applications that use this approach, each IdP node keeps the state it creates in memory in a form readily usable by the node but uses a more compact form when making it available to other nodes. Therefore, any load balancing solution used should route all subsequent requests to the same node that serviced the initial request. This prevents the IdP nodes from constantly reading/writing information to/from this more compact form (an expensive process). This is generally known as session affinity load balancing.
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The Shibboleth team strongly discourages this approach.
Hardware
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-Based Clustering
This is done by using specially dedicated hardware to intercept and route traffic the various nodes in a cluster (so the hardware basically becomes a switch in front of the nodes). This hardware is then given the host name for all the services provided by the clusters behind it.
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When it comes to using LDAP (most common case) for attribute resolution and authentication, you may also have a look at the multiple LDAP configuration hints.
Stateless Clustering
Finally, if you're willing to give up some features and (in many cases) do some custom development, it's possible to deploy the IdP in a configuration that does not rely on shared runtime state. See this topic for more information.
Proxy Clustering
Further finally, if you front-end your server with Apache and are willing to give up some features, it's possible to cluster an IdP using Apache rewrite and proxy directives . See this topic for more information.