On Sat, 2008-12-13 at 00:00 +0100, Markus Wanner wrote: > Hi, > > Fujii Masao wrote: > > I'd like to define the meaning of "synch rep" again. "synch rep" means: > > > > (1) Transaction commit waits for WAL records to be replicated to the standby > > before the command returns a "success" indication to the client. > > > > (2) The standby has (can read) all WAL files indispensable for recovery. > > Let me point out that - very much like the original Postgres-R algorithm > - this guarantees committed transactions to be durable and consistent > (no late aborts of conflicting transactions), but it does not guarantee > that a transaction committed on one node is immediately visible on the > other node. In that sense, it is not synchronous as commonly understood, > because it does not "operate with all their parts in synchrony" [1], as > implied by the term "synchronous". This might (and often has in the > past) lead to confusion.
You're right that neither the data transfer nor data availability is entirely synchronous, but data transfer is synchronous at time of *commit*: it is recorded on multiple nodes at the same time. The term "synchronous replication" is already well used in the industry to mean synchronous commit, so I don't think we should change the name now. The project here is also known to everybody as "synch rep". * Oracle Data Guard calls it "synchronous redo transport" * MS Exchange calls it "synchronous replication" * MS SQL Server has "Database Mirroring", "Log Shipping" and "Replication". "Database Mirroring" provides synchronous mechanism, with "Replication" meaning data transfer to other databases, publish&subscribe. * DB2 HADR provides "synchronous replication" * MySQL call it "synchronous replication" What is confusing is that "replication" itself is a much abused term and is used to describe technologies for HA, DR and data movement. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
