Microsoft.Azure.Mobile Client - Handling Server Error using custom IMobileServiceSyncHandler - Xamarin Forms

Question

I have implemented the Azure - Offline Sync based on the documentation / Sample provided by Microsoft Sample in my Xamarin Forms Application.

In the sample / documentation provided, they are using the default Service Handler.

// Simple error/conflict handling. A real application would handle the various errors like network conditions,server conflicts and others via the IMobileServiceSyncHandler.

Since I need to implement a retry logic for 3 times if the Pull / Push fails. As per the documentation I have created a custom Service Handler(IMobileServiceSyncHandler).

Please find my code logic here.

public class CustomSyncHandler : IMobileServiceSyncHandler
{
    public async Task<JObject> ExecuteTableOperationAsync(IMobileServiceTableOperation operation)
    {
        MobileServiceInvalidOperationException error = null;
        Func<Task<JObject>> tryExecuteAsync = operation.ExecuteAsync;

        int retryCount = 3;
        for (int i = 0; i < retryCount; i++)
        {
            try
            {
                error = null;
                var result = await tryExecuteAsync();
                return result;
            }
            catch (MobileServiceConflictException e)
            {
                error = e;
            }
            catch (MobileServicePreconditionFailedException e)
            {
                error = e;
            }
            catch (MobileServiceInvalidOperationException e)
            {
                error = e;
            }
            catch (Exception e)
            {
                throw e;
            }

            if (error != null)
            {
                if(retryCount <=3) continue;
                else
                {
                     //Need to implement
                     //Update failed, reverting to server's copy.
                }
            }
        }
        return null;
    }

    public Task OnPushCompleteAsync(MobileServicePushCompletionResult result)
    {
        return Task.FromResult(0);
    }
}

But I am not sure how to handle / revert server copy in case all the 3 retry failed.

In the TODO sample they where reverting it based on the MobileServicePushFailedException. But which is available when we implement IMobileServiceSyncHandler. More over if we include custom IMobileServiceSyncHandler it wont execute the code after PushAsync / PullAsync. Even the try catch wont fire in case any exception.

        try
        {
            await this.client.SyncContext.PushAsync();

            await this.todoTable.PullAsync(
                //The first parameter is a query name that is used internally by the client SDK to implement incremental sync.
                //Use a different query name for each unique query in your program
                "allTodoItems",
                this.todoTable.CreateQuery());
        }
        catch (MobileServicePushFailedException exc)
        {
            if (exc.PushResult != null)
            {
                syncErrors = exc.PushResult.Errors;
            }
        }

        // Simple error/conflict handling. A real application would handle the various errors like network conditions,
        // server conflicts and others via the IMobileServiceSyncHandler.
        if (syncErrors != null)
        {
            foreach (var error in syncErrors)
            {
                if (error.OperationKind == MobileServiceTableOperationKind.Update && error.Result != null)
                {
                    //Update failed, reverting to server's copy.
                    await error.CancelAndUpdateItemAsync(error.Result);
                }
                else
                {
                    // Discard local change.
                    await error.CancelAndDiscardItemAsync();
                }

                Debug.WriteLine(@"Error executing sync operation. Item: {0} ({1}). Operation discarded.", error.TableName, error.Item["id"]);
            }
        }
    }

Note

In my application I am only trying to achieve retry for 3 time in case any server error. I am not looking for to resolve conflicts. Thant is the reason I haven't added the code for the same.

If someone came across similar issues and resolved it please help.

Stez.

Answer 1

You say you aren't trying to resolve conflicts, but you need to resolve them one way or another (without telling the user what's going on, perhaps) by accepting the server version of the object or updating the client operation. Otherwise it will just keep telling you about the same conflict each time it retries the operation.

You need to have a subclass of the Microsoft.WindowsAzure.MobileServices.Sync.MobileServiceSyncHandler class, which overrides OnPushCompleteAsync() in order to handle conflicts and other errors. Let's call the class SyncHandler:

public class SyncHandler : MobileServiceSyncHandler
{
    public override async Task OnPushCompleteAsync(MobileServicePushCompletionResult result)
    {
        foreach (var error in result.Errors)
        {
            await ResolveConflictAsync(error);
        }
        await base.OnPushCompleteAsync(result);
    }

    private static async Task ResolveConflictAsync(MobileServiceTableOperationError error)
    {
        Debug.WriteLine($"Resolve Conflict for Item: {error.Item} vs serverItem: {error.Result}");

        var serverItem = error.Result;
        var localItem = error.Item;

        if (Equals(serverItem, localItem))
        {
            // Items are the same, so ignore the conflict
            await error.CancelAndUpdateItemAsync(serverItem);
        }
        else // check server item and local item or the error for criteria you care about
        {
            // Cancels the table operation and discards the local instance of the item.
            await error.CancelAndDiscardItemAsync();
        }
    }
}

Include an instance of this SyncHandler() when you initialize your MobileServiceClient:

await MobileServiceClient.SyncContext.InitializeAsync(store, new SyncHandler()).ConfigureAwait(false);

Read up on the MobileServiceTableOperationError to see other conflicts you can handle as well as its methods to allow resolving them.

Answer 2

The exception carries with it a copy of the server version. In my implementation of IMobileServiceSyncHandler I therefore just return error.Value and this seems to work.

A more extensive example of this kind of logic can be found in this MSDN blog.

The same author has another example where he shows how you can resolve the conflict in favour of the server copy or the client copy, here.