fix: review comments

Author: maschnetwork

sdk/src/main/java/com/amazonaws/lambda/durable/DurableContext.java

@@ -50,21 +50,25 @@ private DurableContext(
                 durableConfig.getLoggerConfig().suppressReplayLogs());
     }
 
-    /** Creates a root context with the given contextId and registers the current thread. */
+    /**
+     * Creates a root context and registers the current thread for execution coordination.
+     *
+     * <p>The context itself always has a null contextId (making it a root context). The thread is registered with the
+     * ExecutionManager using the default {@link #ROOT_CONTEXT} identifier.
+     *
+     * @param executionManager the execution manager
+     * @param durableConfig the durable configuration
+     * @param lambdaContext the Lambda context
+     * @return a new root DurableContext
+     */
     static DurableContext createRootContext(
-            ExecutionManager executionManager, DurableConfig durableConfig, Context lambdaContext, String contextId) {
+            ExecutionManager executionManager, DurableConfig durableConfig, Context lambdaContext) {
         var ctx = new DurableContext(executionManager, durableConfig, lambdaContext, null);
-        executionManager.registerActiveThread(contextId, ThreadType.CONTEXT);
-        executionManager.setCurrentContext(contextId, ThreadType.CONTEXT);
+        executionManager.registerActiveThread(ROOT_CONTEXT, ThreadType.CONTEXT);
+        executionManager.setCurrentContext(ROOT_CONTEXT, ThreadType.CONTEXT);
         return ctx;
     }
 
-    /** Creates a root context with the default "Root" contextId and registers the current thread. */
-    static DurableContext createRootContext(
-            ExecutionManager executionManager, DurableConfig durableConfig, Context lambdaContext) {
-        return createRootContext(executionManager, durableConfig, lambdaContext, ROOT_CONTEXT);
-    }
-
     /**
      * Creates a child context without registering the current thread. Thread registration is handled by
      * ChildContextOperation, which registers on the parent thread before the executor runs and sets the context on the

sdk/src/main/java/com/amazonaws/lambda/durable/operation/ChildContextOperation.java

@@ -99,7 +99,12 @@ private void executeChildContext() {
         // E.g., root child context "1", nested child context "1-2", deeply nested "1-2-1".
         var contextId = getOperationId();
 
-        // Register child context thread before executor runs (prevents suspension)
+        // Thread registration is intentionally split across two threads:
+        // 1. registerActiveThread on the PARENT thread — ensures the child is tracked before the
+        //    parent can deregister and trigger suspension (race prevention).
+        // 2. setCurrentContext on the CHILD thread — sets the ThreadLocal so operations inside
+        //    the child context know which context they belong to.
+        // registerActiveThread is idempotent (no-op if already registered).
         registerActiveThread(contextId, ThreadType.CONTEXT);
 
         userExecutor.execute(() -> {