docs: Improve A?synchronous Multimaster Replication descr.
The docs for sync and async multimaster replication were unclear about when to use it, and when it has benefits; this change clarifies that. Reported-by: juha-pekka.eloranta@reaktor.fi Discussion: https://postgr.es/m/156856543824.1274.12180817186798859836@wrigleys.postgresql.org Backpatch-through: 9.4
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@ -237,7 +237,8 @@ protocol to make nodes agree on a serializable transactional order.
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<listitem>
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<para>
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For servers that are not regularly connected, like laptops or
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For servers that are not regularly connected or have slow
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communication links, like laptops or
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remote servers, keeping data consistent among servers is a
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challenge. Using asynchronous multimaster replication, each
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server works independently, and periodically communicates with
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@ -256,9 +257,8 @@ protocol to make nodes agree on a serializable transactional order.
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In synchronous multimaster replication, each server can accept
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write requests, and modified data is transmitted from the
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original server to every other server before each transaction
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commits. Heavy write activity can cause excessive locking,
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leading to poor performance. In fact, write performance is
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often worse than that of a single server. Read requests can
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commits. Heavy write activity can cause excessive locking and
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commit delays, leading to poor performance. Read requests can
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be sent to any server. Some implementations use shared disk
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to reduce the communication overhead. Synchronous multimaster
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replication is best for mostly read workloads, though its big
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