Скачать или смотреть Resolving Swift NIO EventLoopFuture Chain Not Completing Issue

Learn how to troubleshoot and fix the `EventLoopFuture` chain not completing in your Swift NIO application. Get insights into managing asynchronous operations effectively.
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This video is based on the question https://stackoverflow.com/q/65779808/ asked by the user 'Jerry' ( https://stackoverflow.com/u/491754/ ) and on the answer https://stackoverflow.com/a/65782634/ provided by the user 'Johannes Weiss' ( https://stackoverflow.com/u/55925/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Troubleshooting Swift NIO EventLoopFuture Chain Not Completing

If you're developing a service using Swift's Vapor framework and have encountered an issue where your EventLoopFuture chain doesn't complete, you're not alone. Many developers face similar challenges when dealing with asynchronous programming in Swift, particularly with the Swift NIO framework. In this guide, we’ll explore the problem and provide a comprehensive solution.

Understanding the Problem

You have a service that calls external API methods to retrieve data. In this particular scenario, you're experiencing an issue where:

All callbacks in your EventLoopFuture chain execute as expected, but the final EventLoopFuture does not complete.

Consequently, your application hangs indefinitely.

To illustrate, consider the service calling a method to obtain a session ID, followed by a check to see if a person is a member of a specific group. Although the interim calls return reasonable values, the last call in the chain fails to complete, leaving you puzzled.

Key Code Snippet

Here’s a simplified version of the code that demonstrates the problem:

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Analyzing the Solution

After thorough investigation, we found that the primary cause of the hanging was located in the way the code handled the remoteApi.shutdown() method. Here are the specific issues and how to resolve them.

Identify Blocking Operations

The problem arises from the blocking nature of remoteApi.shutdown(). When a blocking operation is called on the EventLoop, it halts the execution of any subsequent operations.

Possible Solutions:

Use Non-Blocking Calls: If shutdown() can be made asynchronous, try refactoring it to ensure it does not block the EventLoop. If not, consider moving this operation to a different thread or properly executing it after other futures are complete.

Wrap Blocking Code: If you must call a blocking method, consider running the operation in a different queue or using a DispatchQueue to prevent blocking the event loop.

Here's how you might modify the code:

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Conclusion

The hanging nature of the EventLoopFuture chain can often stem from blocking operations in your code. By identifying these operations and ensuring you use non-blocking calls or properly manage thread execution, you can resolve such issues and allow your Swift NIO services to operate smoothly.

If the issue persists despite these changes, it may be worth revisiting the configurations of your API calls and reviewing their error handling. Happy coding!