Expand on requirements of Snapshot (tc39#64)

Author: legendecas

README.md

@@ -296,6 +296,9 @@ runWhenIdle(() => {
 });
 ```
 
+A detailed explanation of why `AsyncContext.Snapshot` is a requirement can be
+found in [SNAPSHOT.md](./SNAPSHOT.md).
+
 # Examples
 
 ## Determine the initiator of a task
@@ -431,7 +434,7 @@ const traceContext = new AsyncContext.Variable();
 traceContext.run("trace-id-a", userAction);
 traceContext.run("trace-id-b", userAction);
 
-runWhenIdle();
+scheduler.runWhenIdle();
 // The userTask will be run with the trace context it was enqueued with.
 // => 'trace-id-a'
 // => 'trace-id-b'

SNAPSHOT.md

@@ -0,0 +1,95 @@
+# Requirements of `AsyncContext.Snapshot`
+
+`AsyncContext.Snapshot` presents two unique requirements:
+
+- It does not expose the value associated with any `Variable` instances.
+- It captures _all_ `Variable`s' current value and restores those values
+  at a later time.
+
+The above requirements are essential to decouple a queueing implementation
+from the consumers of `Variable` instances. For example, a scheduler can queue
+an async task and take a snapshot of the current context:
+
+```typescript
+// The scheduler doesn't access any AsyncContext.Variable.
+const scheduler = {
+  queue: [],
+  postTask(task) {
+    // Each callback is stored with the context at which it was enqueued.
+    const snapshot = new AsyncContext.Snapshot();
+    queue.push({ snapshot, task });
+  },
+  runWhenIdle() {
+    const queue = this.queue;
+    this.queue = [];
+    for (const { snapshot, task } of queue) {
+      // All tasks in the queue would be run with the current context if they
+      // hadn't been wrapped with the snapshot.
+      snapshot.run(task);
+    }
+  }
+};
+```
+
+In this example, the scheduler can propagate values of `Variable`s but doesn't
+have access to any `Variable` instance. They are not coupled with a specific
+consumer of `Variable`. A consumer of `Variable` will not be coupled with a
+specific scheduler as well.
+
+A consumer like a tracer can use `Variable` without knowing how the scheduler
+is implemented:
+
+```typescript
+// tracer.js
+const asyncVar = new AsyncContext.Variable();
+export function run(cb) {
+  // Create a new span and run the callback with it.
+  const span = {
+    startTime: Date.now(),
+    traceId: randomUUID(),
+    spanId: randomUUID(),
+  };
+  asyncVar.run(span, cb);
+}
+
+export function end() {
+  // Get the current span from the AsyncContext.Variable and end it.
+  const span = asyncVar.get();
+  span?.endTime = Date.now();
+}
+```
+
+The `Snapshot` API enables user-land queueing implementations to be cooperate
+with any consumers of `Variable`. For instances, a queueing implementation can
+be:
+
+- A user-land Promise-like implementation,
+- A user multiplexer that multiplexes an IO operation with a batch of async
+  tasks.
+
+Without an API like `Snapshot`, a queueing implementation would have to be built
+on top of the built-in `Promise`, as it is the only way to capture the current
+`Variable` values and restore them later. This would limit the implementation
+of a user-land queueing.
+
+```typescript
+const scheduler = {
+  queue: [],
+  postTask(task) {
+    const { promise, resolve } = Promise.withResolvers();
+    // Captures the current context by `Promise.prototype.then`.
+    promise.then(() => {
+      task();
+    });
+    // Defers the task execution by resolving the promise.
+    queue.push(resolve);
+  },
+  runWhenIdle() {
+    // LIMITATION: the tasks are not run synchronously.
+    for (const cb of this.queue) {
+      cb();
+    }
+    this.queue = [];
+  }
+};
+```