[dna-dev] Non-XA compliant resource participation in distributed transactions

Thu May 15 10:09:44 EDT 2008

Well, I'm referring to federation as in DNA federation of multiple  
sources into a single repository, where those sources may be remote  
system or local systems.  (Ignore for now that a DNA repository may  
actually be hosted on a cluster, because each server in the cluster  
will have the same setup of connectors.)

A file system connector (or any other connector for that matter) can  
certainly be made to implement the XA interfaces.  But even if a file  
system connector correctly implements "transactional" behavior (even  
2PC) and ACID semantics (such that other file system connector  
instances accessing the same file system also respect those  
transactions), the operations on the file system are not truly ACID in  
a global sense.  Another client could get in and start changing the  
files on the file system without regard to those semantics, and now  
those changes may leak into the connectors.

So my question is whether it's a bad thing to implement a connector to  
a non-transactional system as if it is XA compliant, since the "XA- 
compliance" is really a misnomer?

On May 15, 2008, at 8:36 AM, Mark Little wrote:

> Do you mean federated within the same VM or across VMs?
>
> Mark.
>
>
> On 15 May 2008, at 14:21, Randall Hauch wrote:
>
>> Right.  Now I guess I have a question, tho, regarding the  
>> implementation of federation connectors to systems (e.g., file  
>> systems in particular) that don't have any natural support for  
>> transactions.  Is implementing the connector as if it is XA- 
>> compliant a bad thing to do?
>>
>> On May 15, 2008, at 3:59 AM, Mark Little wrote:
>>
>>> As I said in an earlier email, there are significant differences  
>>> between compensating transactions and ACID transactions (of which  
>>> XA is just one possible implementation). You shouldn't offer  
>>> compensating transactions opaquely to users if they register  
>>> multiple 1PC-aware resources because the semantics are different.  
>>> Using them without understanding the implications can be a PITA at  
>>> best and cause significant data inconsistencies at worse  
>>> (compensations can fail or take an arbitrary amount of time to  
>>> resolve). ACID transactions are a pretty good default. If users  
>>> have multiple 1PC-aware resources then it may be in their best  
>>> interests to convert them to 2PC rather than be lulled into some  
>>> false sense of security that somehow a compensating transaction  
>>> will give them the same guarantees.
>>>
>>> Mark.
>>>
>>>
>>>
>>> On 14 May 2008, at 22:28, Randall Hauch wrote:
>>>
>>>> I agree that what you are suggesting is still transactional  
>>>> (except for the last source, which is not transactional).  But we  
>>>> will likely need to supporting several non-XA sources (e.g., file  
>>>> systems, applications, etc.) within an XA system.  And, if we  
>>>> know the individual changes to each node for each source, we  
>>>> shouldn't we also be able to use compensating transactions to  
>>>> "rollback" changes to the non-XA system?
>>>>
>>>> I think there's value to supporting both.  I just think the odds  
>>>> of having more than one non-XA source is pretty high.
>>>>
>>>> On May 14, 2008, at 4:22 PM, John P. A. Verhaeg wrote:
>>>>
>>>>> The big difference here seems to be the exposure of the updates  
>>>>> before the distributed transaction is committed.  What I'm  
>>>>> suggesting is still XA-compliant, while compensating  
>>>>> transactions are not.
>>>>>
>>>>> Randall Hauch wrote:
>>>>>> This is the behavior allowed by JBoss Transactions (Arjuna),  
>>>>>> and it seems useful.  However, I wonder if we can do  
>>>>>> compensating transactions, is there still an advantage to  
>>>>>> supporting n-1 XA plus 1 non-XA?
>>>>>>
>>>>>> Cheers,
>>>>>>
>>>>>> Randall
>>>>>>
>>>>>> On May 14, 2008, at 4:10 PM, John P. A. Verhaeg wrote:
>>>>>>
>>>>>>> Seems like we ought to strive to support n-1 XA resources for  
>>>>>>> distributed transactions, allowing for the last participant in  
>>>>>>> a transaction (thus, this participants updates can't be done  
>>>>>>> in parallel with the other resources) to not support XA  
>>>>>>> transactions, but still participate in a distributed  
>>>>>>> transaction with other XA-compliant resources.  Unlike  
>>>>>>> compensating transactions, the non-XA participant would appear  
>>>>>>> transactional and none of the updates would be visible until  
>>>>>>> the entire transaction was complete.  It seems like this could  
>>>>>>> open up many more possible configurations for customers.
>>>>>>> _______________________________________________
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>>>>>>
>>>>>>
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>>> ----
>>> Mark Little
>>> mlittle at redhat.com
>>>
>>> JBoss, a Division of Red Hat
>>> Registered Address: Red Hat UK Ltd, Amberley Place, 107-111  
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>>
>
> ----
> Mark Little
> mlittle at redhat.com
>
> JBoss, a Division of Red Hat
> Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod  
> Street, Windsor, Berkshire, SI4 1TE, United Kingdom.
> Registered in UK and Wales under Company Registration No. 3798903  
> Directors: Michael Cunningham (USA), Charlie Peters (USA), Matt  
> Parsons (USA) and Brendan Lane (Ireland).
>