On 4/6/23 23:16, Tatsuo Ishii wrote:
>>>> But, I heard PgPool is still affected by Split brain syndrome.
>>> Can you elaborate more? If more than 3 pgpool watchdog nodes (the
>>> number of nodes must be odd) are configured, a split brain can be
>>> avoided.
>> Split brain is a hard situation to avoid. I suppose OP is talking about
>> PostgreSQL split brain situation. I'm not sure how PgPool's watchdog would
>> avoid that.
> Ok, "split brain" means here that there are two or more PostgreSQL
> primary serves exist.
>
> Pgpool-II's watchdog has a feature called "quorum failover" to avoid
> the situation. To make this work, you need to configure 3 or more
> Pgpool-II nodes. Suppose they are w0, w1 and w2. Also suppose there
> are two PostgreSQL servers pg0 (primary) and pg1 (standby). The goal
> is to avoid that both pg0 and pg1 become primary servers.
>
> Pgpool-II periodically monitors PostgreSQL healthiness by checking
> whether it can reach to the PostgreSQL servers. Suppose w0 and w1
> detect that pg0 is healthy but pg1 is not, while w2 thinks oppositely,
> i.e. pg0 is unhealthy but pg1 is healthy (this could happen if w0, w1,
> pg0 are in a network A, but w2 and pg1 in different network B. A and B
> cannot reach each other).
>
> In this situation if w2 promotes pg1 because w0 seems to be down, then
> the system ends up with two primary servers: split brain.
>
> With quorum failover is enabled, w0, w1, and w2 communicate each other
> to vote who is correct (if it cannot communicate, it regards other
> watchdog is down). In the case above w0 and w1 are majority and will
> win. Thus w0 and w1 just detach pg1 and keep on using pg0 as the
> primary. On the other hand, since wg2 looses, and it gives up
> promoting pg1, thus the split brain is avoided.
>
> Note that in the configuration above, clients access the cluster via
> VIP. VIP is always controlled by majority watchdog, clients will not
> access pg1 because it is set to down status by w0 and w1.
And this concept is quite old. (It's also what Windows clustering uses.)
--
Born in Arizona, moved to Babylonia.
