Background:

• I'm using mongodb 4.2.0 
• I have deployed a mongo cluster which contain 5 configs, 3 querys and 3 shards. Each shard consist of 4 replicas and 1 arbiter.
• All members are set on VMs.
• ReplicaSets writeConcernMajorityJournalDefault flag is true.

The Test:

Before implementing the cluster on the production environment, I've conducted several "stress tests". I have created a simple script that performs many inserts to the cluster and returns the amount of successful inserts.

When I run and stop the script everything is just fine. The number of inserts_count is identical to the count of documents in the collection.

BUT, When I run the script and then PowerOff the Primary member, I'm facing a hitch. My script's insert_count is bigger (10-20) than the count of documents in my collection. I assume that I'm losing data.

I got successful insert acknowledge even though my replicaSet is set writeConcernMajorityJournalDefault true.

Raising the primary doesn't help to retrieve the lost data.

I think the data was still in memory!

Conclusion:

I believe that there is some malfunction with the journaling setting.

P.S:

I tried to insert with _

_ parameters.
Same results

Regards,
Mark Berg