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The Shibboleth IdP uses terracotta for clustering (idp session state, etc.). The terracotta configuration file is /usr/local/shibboleth-idp/conf/tc-config.xml.
Unfortunately, the terracotta software is not robust in all situations. It can have problems recovering if both nodes are restarted at the same time (e.g. following a power outage). Also, if the server loses contact with a client, e.g. if the client takes too long doing a garbage collection, the server can declare the client dead, and then refuse its subsequent reconnection attempts.
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