Resolving the Empty Data Issue in AWS AppSync GraphQL Subscriptions

Learn how to fix the common issue of receiving `null` data in AWS AppSync GraphQL subscriptions, specifically when dealing with item mutation and subscriptions.
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This video is based on the question https://stackoverflow.com/q/72725898/ asked by the user 'Jonathan Holvey' ( https://stackoverflow.com/u/3803805/ ) and on the answer https://stackoverflow.com/a/72726088/ provided by the user 'Mhamd Bahja' ( https://stackoverflow.com/u/19377508/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding the Problem of Empty Data in AWS AppSync Subscriptions

If you’re working with AWS AppSync and utilizing GraphQL subscriptions for real-time data updates, you may confront an issue where your subscription returns null data. Recently, this problem has been reported by developers who have been away from their applications for a while. The common scenario involves using a mutation like pushItems that triggers an onItemChange subscription, but instead of the expected data, the subscribed client receives an error indicating a null value for a non-nullable type.

What’s the underlying cause, and how can you fix this? Let's dive into it.

The Error Message

When encountering this problem, you may see an error message that resembles the following:

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This error suggests that the id property within the Item object is returning null. Despite sending a mutation with valid input (like an item ID), the issue persists—leading to confusion about whether AppSync has changed in its handling of subscription data over the years.

Dissecting the GraphQL Schema

Your GraphQL schema plays a crucial role in defining how data flows through your AppSync service. Here’s a portion of the relevant schema you provided:

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From the schema, the onItemChange subscription is expected to return a single Item.

The Root Cause: Mismatch in Data Types

The core of the problem lies in the mismatch between what your mutation returns and the expected output of your subscription. In particular, the mutation pushItems is returning an array of items, while the subscription is set up to return a single item. This inconsistency leads AppSync to return null for fields that are expected to be non-nullable (ID in this case).

The Solution

To resolve this issue, you need to ensure that the structure of the data returned from your mutation matches what the subscription defines. To do this, you need to update the onItemChange subscription to expect an array of items rather than a single item. Modify your subscription definition as follows:

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Minimally Updated GraphQL Schema

Here’s what your entire updated GraphQL schema might look like:

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Testing Your Solution

After updating your schema, you can test to ensure everything is functioning as expected. Perform these steps in the AWS AppSync web console:

Subscribe to the Subscription:

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Send a Mutation:

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Check if the subscribed client now successfully receives the updated data without any null values.

Conclusion

When working with AWS AppSync and GraphQL subscriptions, it’s important to maintain alignment between the data structures used in mutations and subscriptions. If you encounter a null value for a non-nullable type, revisit your GraphQL schema to ensure consistency. By implementing the changes discussed, you should be able to resolve the Empty Data issue and continue using your real-time subscriptions without further interruptions.

Employ these solutions to not only fix the present issues in your application but also as a preventive measure for future projects. Happy coding!