Document Concurrently Modified Exception and CASMismatch Exception is similar in nature, What is the state of mutation operation in both the case?


#1

I looked into various documentation around durability requirement and kind of exception that can occur during such operation. i would like to know more about DocumentConcurrentlyModifiedException, Even though everywhere it is said that it should be treated in similar way as CASMismatchException, is it true in all cases?

  1. When this exception occurs, the original mutation operation has not updated the managed cache nor persisted the change in master node, is it a valid assumption? If not then it cannot be considered similar to CASMismatchException.
  2. When exactly this exception is thrown including failover scenarios?


This link talks about CASMismatchException, where a user can retry the operation as it clear that the mutation operation is not persisted to disk or managed cache.

http://developer.couchbase.com/documentation/server/4.0/sdks/python-2.0/crud.html
Based on Sample provided in the page.

  1. Why Application A will be getting DocumentConcurrentlyModified exception?
  2. Did the changes done by Application A is persisted in master node?
  3. Should application B be getting CASMismatchException, in this scenario rather than a durability error for Application A?

Imagine that the application A mutates document D and gets a CAS returned. Then it starts polling. Slightly afterward, application B also mutates document D and the CAS value changes again. The managed cache on the server performs deduplication, or the document can be replicated between poll cycles. Therefore, the CAS returned to application A might never be observed since it already changed to the CAS from application B.

To detect this scenario, once the CAS on the master changes the SDK will raise a DocumentConcurrentlyModifiedException. How to react depends on the nature of the application. If from the semantics the latter operation subsumes the previous one, ignoring the error might be acceptable. If not, application A needs to fetch the document again, perform its changes or raise an error. In general, error handling is very similar to a CASMismatchException.