From: | John R Pierce <pierce(at)hogranch(dot)com> |
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To: | pgsql-general(at)postgresql(dot)org |
Subject: | Re: Passing master tag around in a multi-site master-slave system |
Date: | 2012-06-13 23:19:03 |
Message-ID: | 4FD91FE7.1000000@hogranch.com |
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Lists: | pgsql-general |
On 06/13/12 3:25 PM, Gauthier, Dave wrote:
> One master, 4 slaves. Can only write to the master (over WAN). No
> write transaction can be committed until it's duplicated at all the
> slave sites. (this, so far, is I think a standard
> requirement/request). Now, the "master" token can get passed from one
> site to the other depending on the viability of the communications
> between the sites. If site A was the master but went down, the
> remaing 4 should be smart enough to detect this and decide who becomes
> the new master. If site A became isolated, it ought to detect that it
> can't communicate with the other sites and that it needs to put itself
> into read_only mode.
what if A (initial master) and B are isolated from C, D, E (maybe a
transpacific link gets broken, so continents are isolated). does C,D,E
decide its a quorum and promotes one to master, while A decides they
lost too many peers so demotes itself to offline? or does A assume that
B is sufficient slave, and retains master status, unknowingly while C
has been promoted too?
all schemes like this are frigging complicated.
oh yeah, and a 5-way synchronous commit across a global WAN is going to
be SLOW!
--
john r pierce N 37, W 122
santa cruz ca mid-left coast
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