Propagate thread local stuff

This project provides a common mechanism for propagating thread-local data across system thread boundaries. This is useful for logging/tracing/debugging in concurrent or asynchronous systems.

Usage

1. Instrument your concurrency/flow primitives with ThreadContext.runnable(Runnable) or ThreadContent.callable(Callable).
2. Register your callables with ThreadContext.savers.

ThreadLocal

com.lucidchart:thread-context:<version>

For example, suppose there is a task that is scheduled on a thread pool in 10 sequential tasks. You can track a ThreadLocal through each of the tasks.

import com.github.threadcontext.ContextManager;
import com.github.threadcontext.PropagatingExecutorService;
import com.github.threadcontext.ThreadLocalSaver;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

final class Main {

    // Step 1: instrument thread pool
    private static final ExecutorService executorService = new PropagatingExectorService(Executors.newFixedThreadPool(8));

    // Step 2: register ThreadLocal
    private static final ThreadLocalSaver example = new ThreadLocal<String>();
    static {
        ThreadContext.savers.add(new ThreadLocalSaver<>(example));
    }

    private static void countdown(int n) {
        if (n > 0) {
            executorService.submit(() -> {
                System.out.println(String.format("%d %s", n, example.get()));
                work(n - 1);
            });
        }
    }

    public void main(String[] args) {
        example.set("example1");
        countdown(10); // these tasks print example1
        example.set("example2");
        countdown(20); // these tasks print example
    }
}

MDC

com.lucidchart:thread-context-slf4j:<version>

You can also track MDC (Mapped Diagnostic Context) values.

ThreadContext.savers.add(new MdcSingleSaver("example"));

Now, the MDC example key will be propagated across thread boundaries.

Instrumentation notes

When instrumenting a thread pool or other construct, wrap tasks at their "creation" origin with ThreadContent.runnable(Runnable) or ThreadContent.callable(Callable). When called, the objects produced from these will load the saved context and undo that when finished. Look at PropogatingExecutorService for an example.