Switching from write through to write behind automatically

Hi,
We are considering a Coherence solution to protect a customer facing application from outages due to database failures. This is for a financial company and the monetary value of each transaction is large and we want to provide 100% guarantee against data loss while not incurring any outages. We want to provide a write-through persistence to the database through Coherence which can switch to a write-behind automatically at runtime if the database persistence fails. Is this doable automatically and would it solve the problem I am trying to solve without losing any inflight transactions? Are there any real customer cases that were successful in achieving this using Coherence?
Thanks
Sairam
Edited by: SKR on Feb 16, 2012 3:14 PM
Edited by: SKR on Feb 16, 2012 3:15 PM

SKR wrote:
Jonathan.Knight wrote:
Hi Sairam
I know you can change the write-delay in JMX for a cache using write-behind but I pretty certauin you cannot make a write-through cache suddenly become a write-behind cache.
I'm not sure why you think changing from write-through to write-behind will allow you to guarantee 100% no data loss - do you mean no loss of updates to the DB or no loss of data in the cache cluster? There are certainly scenarios that can occur where you can loose data from either the cluster or the DB that write-through or write-behind will not save you from. Presumably you want to use write-behind to allow for the DB to go down, although you will still need to configure Coherence to properly retry failed write-behind calls CacheStore behaviour on failure. What happens to your data if you are using write-behind and you loose a partition from you cluster (i.e. you loose a physical machine or two or more JVMs in a short space of time) - you have data loss - you cannot guarantee against this you can only mitigate it and have a recovery policy/procedure.
JKJK,
Thanks for your reply. I must have explained the scenario better. What we are trying to do is to have our transactions commit to the database synchronously using write-through, so that during normal operation, the data will be committed, persisted and durable in the database. But our RW database becomes a single point of failure and if some problem occurs to the database during the peak load time, we run the risk of an outage till we fix the database problem or failover to the standby (We don't have RAC architecture or automatic failover and the manual switchover takes about 10 - 15 mins minimum). We want to avoid this by providing a cache-only operation mode during such a failure, where the customers can continue to transact and the writes will get queued in the cache. I do understand that losing both the database and the cache or losing the primary and the backup in the cache would result in a data loss. But I am assuming such a dual failure is rare.
We do not want to run write-behind all the time but only during the database failure window. From what you mentioned, it seems the runtime switching from write-through to write-behind is not available as an option.
SairamHi Sairam,
I would suggest that you configure write-behind to have a fairly short write-delay, and you only return a confirmation to the client
- either after the write-behind succeeded (you can use a backing map listener to listen for the removal of the decoration which meant that the entry was dirty)
- or if the database went down (noticeable from the failure), then it is up to you whether you send a confirmation which also mentions that it is not persisted to disk yet, or not at all
Best regards,
Robert

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