Re: [Postgres-xc-developers] GTM/GTM Proxy invalid handles and failed to get snapshot issues

[Nikhil S. <ni...@st...>]

> Can GTM/proxy cleanup the pending list when it detects the death of a
> connection, just like GTM does its cleanup?
>
>
Well, the pending requests are not stored in a per client list. We could go
and look for this specific client in the list though and clean it up. But
remember that the client might have given a set of instructions to the
proxy and disappeared legitimately. So processing all requests in order
before processing the disconnection seems much cleaner.

Regards,
Nikhils


>
> On Tue, Aug 6, 2013 at 11:03 AM, Nikhil Sontakke <ni...@st...>wrote:
>
>>
>> A naive question,
>>> Can we also cleanup the pending list?
>>>
>>>
>> Umm, the processed commands are put onto the thr_processed_commands list
>> and freed up eventually. Is that what you were asking?
>>
>> Regards,
>> Nikhils
>>
>>>
>>>  On Tue, Aug 6, 2013 at 9:56 AM, Nikhil Sontakke <ni...@st...>wrote:
>>>
>>>> Hi,
>>>>
>>>> We have had a long standing issue with a GTM Proxy in the mix. It has
>>>> been reported here:
>>>>
>>>>
>>>> http://sourceforge.net/mailarchive/forum.php?thread_name=CAB7nPqQ51fabopxCOHPrRM%3D2WTJMikfZUPHNEW942DDHmED7bQ%40mail.gmail.com&forum_name=postgres-xc-developers
>>>>
>>>> I think I found the root cause:
>>>>
>>>> Here's how things work:
>>>>
>>>> The client connects to proxy. The proxy internally connects to the GTM.
>>>>
>>>> Some commands from clients are sent immediately to the GTM by the
>>>> proxy.
>>>>
>>>> Some commands are put in the pending list on the proxy for batch
>>>> processing later.
>>>>
>>>> Now, if the client goes away before processing pending commands, the
>>>> proxy is immediately telling the GTM so. That initiates a cleanup on the
>>>> GTM side and removes the XIDs associated with this client. After this
>>>> cleanup when the GTM receives a command like "MSG_SNAPSHOT_GET" or
>>>> "MSG_TXN_COMMIT_MULTI" etc. from the pending list from the proxy, then it's
>>>> going to get confused.
>>>>
>>>> A way to handle this is to put "disconnected" messages also onto the
>>>> pending list. We then process these "disconnected" messages LAST from the
>>>> pending list.
>>>>
>>>> I have tried this fix and run a number of tests (using pgbench) and
>>>> things seem to be ok. I do not see the invalid handle and snapshot errors
>>>> on the GTM side with this patch in place.
>>>>
>>>> PFA, patches for 1.0, 1.1 and head attached with this email.
>>>>
>>>> Regards,
>>>> Nikhils
>>>> --
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>>>> The Database Cloud
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>>>>
>>>
>>>
>>> --
>>> Best Wishes,
>>> Ashutosh Bapat
>>> EntepriseDB Corporation
>>> The Postgres Database Company
>>>
>>
>>
>>
>> --
>> StormDB - http://www.stormdb.com
>> The Database Cloud
>>
>
>
>
> --
> Best Wishes,
> Ashutosh Bapat
> EntepriseDB Corporation
> The Postgres Database Company
>



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