postgres-xc-general Mailing List for Postgres-XC

Hi, Michael

Thank you for answer.

 >> I've understood that it's safe to execute against datanode on higher
 >> version of Postgres-XC 1.0.1.
 >> Is this right?
 >>
 > Yes. The read queries are made MVCC-safe when launched directly to
 > Datanodes, so you can have a direct access to the statistics of datanodes
 > by directly connecting to them or monitor them with things like 
Nagios. But
 > you have to be aware that write queries are not blocked in 1.0.X releases
 > when launched directly to Datanodes and this is extremely dangerous 
as you
 > might break data consistency in cluster. Use only read queries!
 >
Alright.
I'll be careful of breaking data consistency.

 > The code on master is made safer and blocks write queries when an
 > application connects to a Datanode that tries to launch a query that may
 > change Database, like a Postgres HOT standby would do.
 >
It's good news.
I'll check the master code.

Thank you again.

regards,
--------
Tomonari Katsumata

On Thu, Oct 11, 2012 at 9:12 PM, Tomonari Katsumata <
kat...@po...> wrote:

> Hi,
>
> When I use Postgres-XC 1.0.0 and execute a query against datanode,
> I get below WARNING messages.
> ------------------------------------
> Do not have a GTM snapshot available
> ------------------------------------
>
> When I use Postgres-XC 1.0.1 and execute a query against datanode,
> I don't any WARNING messages.
>
> I think it's because of below changes.
> ---
> ref.)Release Note
> - Fix snapshot management on slave nodes and on Datanodes accessed
> directly by applications (Michael Paquier)
>

> A slave node session acquires its snapshot directly from WAL like
> PostgreSQL does. Also, a Datanode is now able to get a global snapshot
> directly from GTM, avoiding WARNING messages that a snapshot is not
> available.
>
> Queries interacting with remote nodes running on slave nodes are blocked.
> Such queries need a global snapshot and a new transaction ID, but
> sessions on
> slave nodes cannot acquire that.
> ---
>
> I've understood that it's safe to execute against datanode on higher
> version of Postgres-XC 1.0.1.
> Is this right?
>
Yes. The read queries are made MVCC-safe when launched directly to
Datanodes, so you can have a direct access to the statistics of datanodes
by directly connecting to them or monitor them with things like Nagios. But
you have to be aware that write queries are not blocked in 1.0.X releases
when launched directly to Datanodes and this is extremely dangerous as you
might break data consistency in cluster. Use only read queries!

The code on master is made safer and blocks write queries when an
application connects to a Datanode that tries to launch a query that may
change Database, like a Postgres HOT standby would do.
-- 
Michael Paquier
http://michael.otacoo.com

Hi,

When I use Postgres-XC 1.0.0 and execute a query against datanode,
I get below WARNING messages.
------------------------------------
Do not have a GTM snapshot available
------------------------------------

When I use Postgres-XC 1.0.1 and execute a query against datanode,
I don't any WARNING messages.

I think it's because of below changes.
---
ref.)Release Note
- Fix snapshot management on slave nodes and on Datanodes accessed
directly by applications (Michael Paquier)

A slave node session acquires its snapshot directly from WAL like
PostgreSQL does. Also, a Datanode is now able to get a global snapshot
directly from GTM, avoiding WARNING messages that a snapshot is not
available.

Queries interacting with remote nodes running on slave nodes are blocked.
Such queries need a global snapshot and a new transaction ID, but
sessions on
slave nodes cannot acquire that.
---

I've understood that it's safe to execute against datanode on higher
version of Postgres-XC 1.0.1.
Is this right?

---------
Tomonari Katsumata