Re: DatabaseModifiedError, how to handle it correctly when searching?

> On Aug 30, 2021, at 7:15 PM, Olly Betts <[email protected]> wrote:
> 
> On Mon, Aug 30, 2021 at 06:50:48PM -0700, Yuan Fu wrote:
>>  int try_count = 0;
>>  while (true)
>>    {
>>      try
>>        {
>>          (do stuff)
>>          break;
>>        }
>>      catch (Xapian::DatabaseModifiedError &e)
>>        {
>>          if (try_count > 0) throw e;
>>          database.reopen();
>>          try_count++;
>>        }
>>    }
> 
> You should never get DatabaseModifiedError for a database you have a
> write lock on - i.e. a WritableDatabase (because what
> DatabaseModifiedError means is that a writer has committed modifications
> to the database underneath you in such a way that the operation you
> asked for can't be performed, and Xapian doesn't support concurrent
> writers so if you are the current writer this can't happen).
> 
> Also, note that if the writer is making a series of small commits in
> quick succession you could need to retry more than once.
> 
> It also often makes sense to retry a group of operations - e.g. if you
> perform a search and hit DatabaseModifiedError reading the documents
> in the results, you probably want to retry the search starting from
> get_mset() (even though that part worked) so that your results are from
> a consistent database state (this is the main reason that there isn't an
> auto-retry in this situation).  DatabaseModifiedError should be a rare
> occurrence so redoing extra in response to it shouldn't be costly.
> 
> There is a longer term plan to allow databases to keep old revisions
> which readers are using around, which will eventually provide a way to
> prevent this exception.

Thanks, that’s very helpful!

Yuan