From: Bruce Momjian Date: Wed, 22 Nov 2006 17:36:52 +0000 (+0000) Subject: Wording and term clarification for HA docs, per Markus Schiltknecht. X-Git-Tag: REL8_2_RC1~32 X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=6346355a9b98646154c3736f7fae642e7849b888;p=postgresql Wording and term clarification for HA docs, per Markus Schiltknecht. --- diff --git a/doc/src/sgml/high-availability.sgml b/doc/src/sgml/high-availability.sgml index 9e2adbaab2..bc672e5655 100644 --- a/doc/src/sgml/high-availability.sgml +++ b/doc/src/sgml/high-availability.sgml @@ -1,4 +1,4 @@ - + High Availability and Load Balancing @@ -49,13 +49,13 @@ meaning that a data-modifying transaction is not considered committed until all servers have committed the transaction. This guarantees that a failover will not lose any data and that all - load-balanced servers will return consistent results with little - propagation delay. Asynchronous updating has a delay between the - time of commit and its propagation to the other servers, opening - the possibility that some transactions might be lost in the switch - to a backup server, and that load balanced servers might return - slightly stale results. Asynchronous communication is used when - synchronous would be too slow. + load-balanced servers will return consistent results no matter + which server is queried. Asynchronous updating has a delay between + the time of commit and its propagation to the other servers, + opening the possibility that some transactions might be lost in + the switch to a backup server, and that load balanced servers + might return slightly stale results. Asynchronous communication + is used when synchronous would be too slow. @@ -110,7 +110,7 @@ https://forge.continuent.org/pipermail/sequoia/2006-November/004070.html Oracle RAC is a shared disk approach and just send cache invalidations to other nodes but not actual data. As the disk is shared, data is -only commited once to disk and there is a distributed locking +only committed once to disk and there is a distributed locking protocol to make nodes agree on a serializable transactional order. --> @@ -178,29 +178,29 @@ protocol to make nodes agree on a serializable transactional order. using two-phase commit ( and . - Pgpool is an example of this type of replication. + Pgpool and Sequoia are an example of this type of replication. - Synchonous Multi-Master Replication + Synchronous Multi-Master Replication - In synchonous multi-master replication, each server can accept + In synchronous multi-master replication, each server can accept write requests, and modified data is transmitted from the original server to every other server before each transaction commits. Heavy write activity can cause excessive locking, leading to poor performance. In fact, write performance is often worse than that of a single server. Read requests can - be sent to any server. Some implementations use cluster-wide - shared memory or shared disk to reduce the communication - overhead. Clustering is best for mostly read workloads, though - its big advantage is that any server can accept write requests - — there is no need to partition workloads between master - and slave servers, and because the data changes are sent from - one server to another, there is no problem with non-deterministic + be sent to any server. Some implementations use shared disk + to reduce the communication overhead. Synchronous multi-master + replication is best for mostly read workloads, though its big + advantage is that any server can accept write requests — + there is no need to partition workloads between master and + slave servers, and because the data changes are sent from one + server to another, there is no problem with non-deterministic functions like random(). @@ -222,9 +222,11 @@ protocol to make nodes agree on a serializable transactional order. For servers that are not regularly connected, like laptops or remote servers, keeping data consistent among servers is a - challenge. One simple solution is to allow each server to - modify the data, and have periodic communication compare - databases and ask users to resolve any conflicts. + challenge. Using asynchronous multi-master replication, each + server works independently, and periodically communicates with + the other servers to identify conflicting transactions. The + conflicts can be resolved by users or conflict resolution rules. + rules.