Re: [Postgres-xc-general] Cluster restart

[Michael P. <mic...@gm...>]

On Sat, Oct 5, 2013 at 9:00 PM, Stefan Lekov <ar...@er...> wrote:
> Hello,
>
> I'm new to the Postgres-XC project. In fact I am still considering if I
> should install it
> in order to try it as a replacement of my current database clusters
> (those are based
> around MySQL and its binary_log based replication).
Have you considered PostgreSQL as a potential solution before
Postgres-XC. Why do you especially need XC?

> Before actually starting the installation of postgres-xc I would like
> to know what is the procedure for restarting
> nodes. I have already read a few documents/mails regarding restoring or
> resyncing
> a failed datanode, however these documents does not answer my simple
> question:
>
> What should be the procedure for rebooting servers? For example I have a
> kernel
> updated pending (due to security reasons) - I'm installing the new
> kernel, but I have
> to reboot all machine. Theoretically all nodes (both coordinators and
> datanodes)
> are working on different physical servers or VMes. In a perfect scenario
> I would
> like to keep the system in production while I am restarting the servers
> one by one.
> However I am not sure what would be the effect of rebooting servers one
> by one.
If a node is restarted or facing an outage, all the transactions it
needs to be involved in will simply fail. In the case of Coordinator,
this has effect only for DDL. For Datanodes, this has effect as well
for DDL, but also for DML and SELECT of the node is needed for the
transaction.

> For purpose of example let me have four datanodes: A,B,C,D
>
> All servers are synced and are operating as expected.
> 1) Upgrade A, reboot A
> 2) INSERT/UPDATE/DELETE queries
> 3) A boots up and is successfully started
> 4) INSERT/UPDATE/DELETE queries
> 5) Upgrade B, reboot B
> ...
> ...
> As for the "Coordinators" nodes. How are those affected by temporary
> stopping
> and restarting the postgres-xc related services. What should be the load
> balancer
> in front of these servers in order to be able to both load-balance and
> fail-over if
> one of the Coordinators is offline either due to failed server or due to
> rebooting
> servers.
DDLs won't work. Applications will use one access point. In this case
no problems for your application, connect to the other Coordinators to
execute queries as long as they are not DDLs.

> I have no problem with relatively heavy operation of full restore of a
> datanode in
> event of failed server. Such restoration operation can be properly
> scheduled and
> executed, however I am interested how would postgres-xc react to simple
> scenarioa simple operation of restarting a server due to whatever
> reasons should
As mentioned above, transactions that will need it will simply fail.
You could always failover a slave for the outage period if necessary.
-- 
Michael