Mongo WriteConflictException in log file
this does not seem to affect the application, because I am not seeing any special errors and everything seems to work fine.


Write conflict exceptions (WCEs) are an indication from the WiredTiger storage engine that multiple internal writers are trying to update the same data. WiredTiger uses optimistic concurrency control (more specifically Multi-Version Concurrency Control aka MVCC) rather than pessimistic (or lock-based) document updates.

WCEs are an expected (and mostly internal) aspect of this approach: when the storage engine detects conflicts between two concurrent write operations, one will incur a write conflict and be transparently retried. The WCEs are logged for diagnostic purposes but are not exceptions that your application needs to handle. 


  Should I be concerned about this?


A small number of write conflict exceptions can be expected and should not have any impact aside from some possible (and generally negligible) latency for internal retries. Your redacted log lines show low retries and do not appear to be concerning in terms of WCEs.

If you are seeing updates with tens or hundreds of write conflict exceptions (or significantly slow execution), it would be worth reviewing the data model and updates in question for potential improvements in efficiency. For example, if a high number of write conflicts was being logged when incrementing a counter in a popular document, you might consider coalescing  N updates of  {$inc: 1} into  a single update of {$inc: N}.

The MongoDB Concurrency FAQ is also worth reviewing for more information.