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The term High Availability (HA) usually refers to having a number of instances of a service such as a Message Broker available so that should a service unexpectedly fail, or requires to be shutdown for maintenance, users may quickly connect to another instance and continue their work with minimal interuption. HA is one way to make a overall system more resilient by eliminating a single point of failure from a system.
HA offerings are usually categorised as Active/Active or Active/Passive. An Active/Active system is one where all nodes within the cluster are usuaully available for use by clients all of the time. In an Active/Passive system, one only node within the cluster is available for use by clients at any one time, whilst the others are in some kind of standby state, awaiting to quickly step-in in the event the active node becomes unavailable.
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