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Add witness server information

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Ian Barwick
2015-03-12 11:42:28 +09:00
parent a0a2f87d32
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README.md
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@@ -238,9 +238,26 @@ Example log output (at default log level):
Witness server
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Note that by default the witness server will run on port 5499, to facilitate
easier setup of the witness server instance on an existing replication cluster
server running a standby or master node.
In a situation caused e.g. by a network interruption between two
data centres, it's important to avoid a "split-brain" situation where
both sides of the network assume they are the active segment and the
side without an active master unilaterally promotes one of its standbys.
To prevent this situation happening, it's essential to ensure that one
network segment has a "voting majority", so other segments will know
they're in the minority and not attempt to promote a new master. Where
an odd number of servers exists, this is not an issue. However, if each
network has an even number of nodes, it's necessary to provide some way
of ensuring a majority, which is where the witness server becomes useful.
This is not a fully-fledged standby node and is not integrated into
replication, but it effectively represents the "casting vote" when
deciding which network segment has a majority. A witness server can
be set up using `repmgr witness create` (see below for details) and
can run on a dedicated server or an existing node. Note that it only
makes sense to create a witness server in conjunction with running
`repmgrd`; the witness server will require its own `repmgrd` instance.
Monitoring
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