Edge case for conflict resolution when adding entities that were already successfully sent to the server

[nvrnight]

Very few of my users ended up running across this issue.

I use PushAsync on the EF DB Context on the client devices. I suspect that maybe my user's device succeeded in uploading the entities, but failed save the result locally that it succeeded. Then every push afterwards ends up with the same result of conflict, even though my conflict resolution code was in place. I looked at the source code for the server library for the table Create endpoint and it always returns a Conflict response whenever the entity already exists.

I am currently working around this issue by modifying the Kind column on the data sync operation from Add to Update after my conflict resolution is performed for this scenario, it succeeds on the next push.

[adrianhall]

There is not enough information to suggest a path forward here. Do you have the code somewhere on github I could reference?

You can update your conflict provider to say "if the key fields are the same, then just ignore it - server wins".

[nvrnight]

There is not enough information to suggest a path forward here. Do you have the code somewhere on github I could reference?

You can update your conflict provider to say "if the key fields are the same, then just ignore it - server wins".

I will try and provide a github repo example of the problem next week.

In this scenario the client believes that it has not successfully uploaded the entity yet, so it's Kind is Create(0 iirc) still and is calling the server's Create API endpoint. The Create API endpoint will then return a Conflict result, the client will receive this result and store the server's 409 response on its DataSyncOperation record. If your conflict resolution runs and makes the client version match the server version, it's still going to fail on the next run because it calls the Create API again which just continues to return a 409 response. It's an infinite cycle of conflict failures.

[adrianhall]

So - item created on server; queue entry says create.

I'll try and set up a scenario today for this - may need some assistance validating the scenario.

[adrianhall]

I think this is actually a fairly easy fix. Looking at OperationsQueueManager.cs#318 - I just need to add:

                // The client entity is the winner, so we need to update the operation and re-queue it.
                if (operation.Kind == OperationKind.Add)
                {
                    // The server has an entity and the client is winning, so we need to replace the entity on the server.
                    operation.Kind = OperationKind.Replace;
                }

Running tests now.

[adrianhall]

My test addition and change worked, so I'm doing the PR now.

It would be really great if you could also test the current version before I build it into a release. Let me know if it solves your issue.