Confluence Data Center is a clustered solution that is available from Confluence 5.6. If your cluster is running Confluence 5.4 or earlier, refer to Confluence Cluster Issues as there are some significant differences to the underlying clustering technology. 

Resources

Confluence Data Center - a general overview of Data Center

Confluence Data Center Technical Overview - a more technical overview including Data Center works and the infrastructure you'll need to achieve performance at scale and high availability.

Data Center Troubleshooting - a good first port of call for troubleshooting information for Confluence Data Center. 

Data Center Articles

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  "Exception bootstrapping cluster:Shared home directory is not configured correctly" Error during Confluence Data Center startup
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  Cannot find "external_id" column when trying to upgrade to a Confluence CDC license after upgrading from a pre-5.5 Confluence Clustered installation
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  Cluster Panic due to Multicast Traffic Communication Problem
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  Hazelcast CANNOT start on this node. No matching network interface found.
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  How to capture Java Heap dumps before Full GC in Confluence
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  How to create a support zip via command line when Confluence is down
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  How to suppress cluster warning messages in the Confluence log files
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  List of REST APIs available to configure SSO on Confluence DC
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  Multicast communication works only one-way
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  Recovering from a Data Center cluster split-brain
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  Starting Confluence node fails with 'Port [5801] is already in use and auto-increment is disabled. Hazelcast cannot start' error

Cluster Edition Articles

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  'Multicast address does not correspond with cluster name' during Cluster Configuration
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  'UnicastUdpSocket failed to set receive buffer size to 1428 packets' Error Due to Coherence Tuning
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  Cluster communication problems: Member has left cluster, or Member has been forcefully evicted from cluster, or A potential communication problem has been detected
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  Cluster Nodes Can Connect to the Database, but Fail to Make a Cluster
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  Clustered Instance Does Not Start with java.util.NoSuchElementException (No server id found)
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  Clustered instance does not start with Unicast listening address (not allowed to create a new cluster)
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  Confluence will not start due to fatal error in Confluence cluster
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  How to Configure a Cluster Without Multicast Traffic
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  How to move the shared-home folder in Confluence Data Center
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  How to suppress cluster warning messages in the Confluence log files
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  Performance Problems due to Cluster Communication
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  Search Results and Recently Updated Macro Are Inconsistent across Confluence Cluster Nodes
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  Unable to Start Node due to no Cluster License
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  Unable to Upgrade Confluence Clustered due to Clustered Edition Distribution