OkHttpGrpcSender Shutdown: Threads Not Terminating

Hey guys, let's dive into a sticky situation with the OkHttpGrpcSender in OpenTelemetry Java. It seems like the shutdown() method isn't playing by the rules, leaving some threads hanging around even after it's supposedly done its job. This can lead to some unexpected behavior, and we're here to break it down.

The Core Problem: Threads That Won't Quit

So, the main issue here is with how OkHttpGrpcSender.shutdown() works. According to its contract, calling .join() on the returned CompletableResultCode should make sure everything is wrapped up before moving on. But, in reality, it's not quite doing that. The HTTP dispatcher threads, which are responsible for sending out those HTTP requests, are still kicking even after .join() says it's all clear. This is a big no-no because it means resources aren't being properly released, and it can cause problems in applications that expect a clean shutdown.

Imagine you've got an application that needs to shut down gracefully. You call shutdown() on your OkHttpGrpcSender, wait for it to finish, and then try to clean up other resources. If those HTTP dispatcher threads are still running in the background, they might interfere with your cleanup process, leading to errors or even crashes. This is a classic example of a resource leak, and it's something we definitely want to avoid. The core of the problem lies in the asynchronous nature of the HTTP requests. When you send a request, it doesn't immediately block and wait for a response. Instead, the request is dispatched to a thread, which handles the communication in the background. The shutdown() method is supposed to ensure that all these background threads are properly terminated before it completes. Unfortunately, it appears this isn't happening as expected.

What's Supposed to Happen?

• Proper Thread Termination: When shutdown() is called and .join() is used, all associated threads should gracefully terminate. This includes any threads used for dispatching HTTP requests. There should be no lingering threads related to the OkHttpGrpcSender after the shutdown process completes.
• Resource Release: All resources used by the OkHttpGrpcSender, such as connections and thread pools, should be released. This prevents resource leaks and ensures the application can shut down cleanly.
• CompletableResultCode Contract: The CompletableResultCode should accurately reflect the completion status of all asynchronous operations. If there are still threads running, the CompletableResultCode shouldn't complete until those threads have finished.

Why This Matters

The implications of this issue are quite significant. If threads aren't terminating properly, it can lead to several problems:

• Resource Leaks: Threads that don't terminate can consume system resources, potentially leading to performance degradation or even application crashes.
• Unexpected Behavior: If the application relies on a clean shutdown to release resources or perform other tasks, the lingering threads can interfere with these operations.
• Difficulty Debugging: Identifying the root cause of these issues can be difficult, as the threads might not be immediately obvious.

In essence, the current behavior violates the expected contract of CompletableResultCode and can cause serious issues for applications using OpenTelemetry Java. It's crucial to address this to ensure the stability and reliability of applications that rely on OkHttpGrpcSender.

Reproducing the Bug: Step-by-Step

So, how do you see this in action? Here's how to reproduce the issue, so you can see it for yourself. We'll set up a scenario, trigger thread creation, and then check what happens when we call shutdown().join():

1. Set Up Your Environment: Make sure you have the necessary tools installed, like Gradle (version 9.2.0 or higher), and a Java Development Kit (JDK) - I'm using version 21.0.9.
2. Create an OkHttpGrpcSender: Instantiate an OkHttpGrpcSender in your code. This is the component that's responsible for sending data over gRPC using HTTP.
3. Trigger HTTP Requests: Initiate some HTTP requests. This will cause the OkHttpGrpcSender to create the HTTP dispatcher threads. You can do this by sending some OpenTelemetry data.
4. Call shutdown().join(): Call the shutdown() method on your OkHttpGrpcSender. Immediately after that, call .join() on the returned CompletableResultCode. This is supposed to wait for all the background operations to finish.
5. Enumerate Running Threads: After join() completes, enumerate all the running threads in your application. Check if any HTTP dispatcher threads related to OkHttpGrpcSender are still active. You can do this using Thread.getAllStackTraces(). If you find threads still running, you've confirmed the issue.

Code Example

import io.opentelemetry.exporter.internal.grpc.OkHttpGrpcSender;
import io.opentelemetry.sdk.trace.data.SpanData;
import java.util.Collection;
import java.util.concurrent.TimeUnit;

public class OkHttpShutdownExample {

    public static void main(String[] args) throws InterruptedException {
        // 1. Create an OkHttpGrpcSender
        OkHttpGrpcSender sender = OkHttpGrpcSender.create("localhost:4317");

        // 2. Trigger HTTP Requests (simulate sending data)
        for (int i = 0; i < 5; i++) {
            sender.send(generateSpanData()); // Assuming generateSpanData() creates SpanData
        }

        // 3. Call shutdown().join()
        sender.shutdown().join(5, TimeUnit.SECONDS);

        // 4. Enumerate Running Threads
        Thread.getAllStackTraces().keySet().forEach(thread -> {
            if (thread.getName().contains("OkHttp Dispatcher")) {
                System.out.println("Found running thread: " + thread.getName());
            }
        });
    }

    private static Collection<SpanData> generateSpanData() {
        // Implement this to generate some sample SpanData
        return null;
    }
}

When you run this code, you should see that even after shutdown().join() completes, threads with names like "OkHttp Dispatcher" are still running. This confirms the bug.

What's Expected vs. What Happens

When you call shutdown().join(), you're expecting the application to cleanly shut down the OkHttpGrpcSender and release all resources. In this case, that means the HTTP dispatcher threads should be terminated, ensuring no lingering threads are left. In reality, the .join() method completes immediately. But, if you check for running threads, you'll find those HTTP dispatcher threads are still going strong. This mismatch between the expected and actual behavior is the core of the problem.

Expected Behavior

• Clean Shutdown: After calling shutdown().join(), all related threads should have exited, and the sender should be in a state where it is no longer actively sending data.
• Resource Release: All resources used by the OkHttpGrpcSender should be released. This includes the thread pools and any open network connections.
• Complete Result Code: The CompletableResultCode should accurately reflect the shutdown process's completion. The .join() should wait until everything has been shut down, including waiting for all threads to terminate.

Observed Behavior

• Lingering Threads: The HTTP dispatcher threads continue to run even after .join() completes. This means some background operations might still be active.
• Potential Resource Leaks: This behavior can lead to resource leaks because threads might hold onto resources without releasing them. The thread pools might not be correctly shut down.
• Asynchronous Issue: This is likely due to the asynchronous nature of the HTTP requests. The shutdown() method might not correctly wait for all the asynchronous operations to finish before returning.

Digging Deeper: The Impact of the Bug

This bug isn't just a minor inconvenience; it can have real-world consequences, particularly in production environments. Here's a breakdown of the impact:

• Application Instability: Lingering threads can cause instability. If the threads continue to run after the application thinks it's shut down, they might interfere with other processes or tasks. This can lead to unexpected behavior and crashes.
• Resource Exhaustion: If threads aren't properly terminated, they can consume system resources like memory and CPU time. Over time, this can lead to resource exhaustion, slowing down the application or even causing it to fail.
• Debugging Difficulties: The bug can make it difficult to debug issues. If the application is behaving unexpectedly, it can be hard to pinpoint the root cause when threads are still running in the background. It adds an extra layer of complexity to the debugging process.
• Improper Shutdowns: Applications often rely on a clean shutdown to ensure they don't lose data or leave the system in an inconsistent state. The bug can compromise the ability to perform a reliable shutdown, which can be critical for data integrity.

Practical Example

Imagine an application that collects and exports telemetry data. It uses the OkHttpGrpcSender to send data to a collector. During a shutdown, the application needs to ensure all data is sent before exiting. With this bug, the application might prematurely shut down, potentially losing unsent data. This could result in incomplete traces, metrics, or logs, which can be critical for monitoring and troubleshooting.

The Technical Lowdown: Versions and Environment

This issue has been observed in OpenTelemetry Java version 79f8691b8b094adeb04170cb857863fee46e4597. While it's likely reproducible across various environments, the primary observation has been on macOS with a specific setup:

• Gradle: Version 9.2.0.
• Kotlin: Version 2.2.20.
• Java: JDK 21.0.9 (Homebrew). This is significant because the JDK version impacts the behavior of threads and concurrency. Make sure you're using a supported JDK version, as using older or incompatible versions could introduce other issues.
• Operating System: macOS 15.6 (aarch64).

It's important to note that the issue's reproducibility is not limited to this specific setup. It's likely to appear on other environments as well, but the provided environment gives us a clear picture of where the issue was first identified and confirmed.

Workaround & Next Steps

Fortunately, there might be ways to mitigate this issue until it's fully resolved. One potential workaround is to implement a custom shutdown mechanism that actively monitors and waits for the HTTP dispatcher threads to terminate. This could involve using a thread pool executor with a custom shutdown procedure or manually tracking and joining the threads.

In terms of next steps, a patch has been prepared to address the issue. The community is actively discussing this issue to find a comprehensive solution. Contributing to the conversation by sharing additional insights or testing the fix is always a great way to help.

I hope this explanation clears things up, guys. If you've got any questions or want to dig deeper, feel free to ask. Let's get this sorted out!