Improve documentation

Author: vietj

src/main/asciidoc/index.adoc

@@ -4,6 +4,18 @@ Use virtual threads to write Vert.x code that looks like it is synchronous.
 
 You still write the traditional Vert.x code processing events, but you have the opportunity to write synchronous code for complex  workflows and use thread locals in such workflows.
 
+== Introduction
+
+One of the key advantages of Vert.x over many legacy application platforms is that it is almost entirely non-blocking (of kernel threads) - this allows it to handle a lot of concurrency (e.g. handle many connections, or messages) using a very small number of kernel threads, which allows it to scale very well.
+
+The non-blocking nature of Vert.x leads to asynchronous APIs. Asynchronous APIs can take various forms including callback style, promises or Rx-style. Vert.x uses futurese in most places (although, it also supports Rx).
+
+In some cases, programming using asynchronous APIs can be more challenging than using a direct synchronous style, in particular if you have several operations that you want to do in sequence. Also, error propagation is often more complex when using asynchronous APIs.
+
+Vertx virtual threads allows you to work with asynchronous APIs, but using a direct synchronous style that you're already familiar with.
+
+It does this by using Java 21 virtual threads. Virtual threads are very lightweight threads that do not correspond to underlying kernel threads. When they are blocked they do not block a kernel thread.
+
 == Usage
 
 To use the virtual threads with Vert.x add the following dependency to the _dependencies_ section of your build descriptor:
@@ -27,92 +39,67 @@ dependencies {
 }
 ----
 
-== Getting started
+[#_virtual_thread_verticle]
+== Virtual thread verticle
+
+A virtual thread verticle is a worker verticle that needs a *single* instance to run the application.When the verticle *awaits* for a future result, this verticle can still process events like an event-loop verticle.
 
 [source,java]
 ----
 {@link examples.VirtualThreadExamples#gettingStarted}
 ----
 
-== What this is about
-
-Async/Await for Vert.x
+=== Blocking in a virtual thread verticle
 
-== What this is not about
-
-Blocking on other JDK blocking constructs such as latches, locks, sleep, etc...
-
-NOTE: it remains possible to block on these constructs using `Async.await`
-
-== What you get
-
-By default, Vert.x dispatches events on an event-loop thread.
-
-[source,java]
-----
-{@link examples.VirtualThreadExamples#whatYouGet1}
-----
+You can block using the {@link io.vertx.virtualthreads.await.Async#await} method to suspend the current virtual thread until the awaited result is available.
 
-Using virtual threads with Vert.x requires to run application tasks on a virtual threads
+You can await on a Vert.x `Future`
 
 [source,java]
 ----
-{@link examples.VirtualThreadExamples#whatYouGet2}
+{@link examples.VirtualThreadExamples#awaitingFutures1}
 ----
 
-This project implements virtual threads with Vert.x with a race free model. Events are  dispatched to a virtual thread, when this virtual thread awaits an asynchronous result, the pending events are not dispatched until the virtual thread is resumed.
+or on a JDK `CompletionStage`
 
 [source,java]
 ----
-{@link examples.VirtualThreadExamples#whatYouGet3}
+{@link examples.VirtualThreadExamples#awaitingFutures2}
 ----
 
-When a virtual thread awaits a future, a new virtual thread can be started to handle new events and avoid blocking the appliction or potential self deadlocks, e.g. in the following example, awaiting the response does not prevent the timer to fire
+You can acquire the ownership of a `java.util.concurrent.locks`
 
 [source,java]
 ----
-{@link examples.VirtualThreadExamples#whatYouGet4}
+{@link examples.VirtualThreadExamples#awaitingLocks1}
 ----
 
-If you block a virtual thread without `{@link io.vertx.virtualthreads.await.Async`, your application then will behave like a regular worker and events will not be processed until the current task ends.
-
-== Verticles
+=== Field visibility
 
-Virtual thread verticles are actually worker verticles, however a single worker instance is enough to execute it.
+A virtual thread verticle can interact safely with fields before an `await` call. However, if you are reading a field before an `await` call and reusing the value after the call, you should keep in mind that this value might have changed.
 
 [source,java]
 ----
-{@link examples.VirtualThreadExamples#deployVerticle}
+{@link examples.VirtualThreadExamples#fieldVisibility1}
 ----
 
-== Supported primitives
-
-=== Futures
-
-You can await a Vert.x `Future`
+You should read/write fields before calling `await` to avoid this.
 
 [source,java]
 ----
-{@link examples.VirtualThreadExamples#awaitingFutures1}
+{@link examples.VirtualThreadExamples#fieldVisibility2}
 ----
 
-or a JDK `CompletionStage`
+NOTE: this is the same behavior with an event-loop verticle
 
-[source,java]
-----
-{@link examples.VirtualThreadExamples#awaitingFutures2}
-----
+=== Blocking without await
 
-=== Locks
+When your application blocks without `await`, the Vert.x scheduler cannot schedule events on the verticle, it behaves like a regular worker verticle, yet using a virtual thread.
 
-You can lock a `java.util.concurrent.locks`
+This use case is not precluded, however the verticle should be deployed with several instances to deliver the aimed concurrency.
 
-[source,java]
-----
-{@link examples.VirtualThreadExamples#awaitingLocks1}
-----
 
-== Thread local support
+=== Thread local support
 
 Thread locals are only reliable within the execution of a context task.
 

src/main/java/examples/VirtualThreadExamples.java

@@ -23,71 +23,47 @@
 public class VirtualThreadExamples {
 
   public void gettingStarted(Vertx vertx) {
-    Async async = new Async(vertx);
-    async.run(v -> {
-      // Run on a Vert.x a virtual thread
-      HttpClient client = vertx.createHttpClient();
-      HttpClientRequest req = Async.await(client.request(HttpMethod.GET, 8080, "localhost", "/"));
-      HttpClientResponse resp = Async.await(req.send());
-      int status = resp.statusCode();
-      Buffer body = Async.await(resp.body());
-    });
-  }
 
-  public void whatYouGet1(HttpClientRequest request) {
-    request
-      .send()
-      .onSuccess(response -> {
-      // Set the buffer for handlers
-      response.handler(buffers -> {
+    AbstractVerticle verticle = new AbstractVerticle() {
+      @Override
+      public void start() {
+        HttpClient client = vertx.createHttpClient();
+        HttpClientRequest req = Async.await(client.request(
+          HttpMethod.GET,
+          8080,
+          "localhost",
+          "/"));
+        HttpClientResponse resp = Async.await(req.send());
+        int status = resp.statusCode();
+        Buffer body = Async.await(resp.body());
+      }
+    };
 
-      });
-    });
+    // Run the verticle a on virtual thread
+    vertx.deployVerticle(verticle, new DeploymentOptions()
+      .setWorker(true)
+      .setInstances(1)
+      .setWorkerOptions(new VirtualThreadOptions()));
   }
 
-  public void whatYouGet2(HttpClientRequest request)  {
-    Thread.startVirtualThread(() -> {
-      CompletableFuture<HttpClientResponse> fut = new CompletableFuture<>();
-      request.send().onComplete(ar -> {
-        if (ar.succeeded()) {
-          fut.complete(ar.result());
-        } else {
-          fut.completeExceptionally(ar.cause());
-        }
-      });;
-      try {
-        HttpClientResponse response = fut.get();
-        // As we get the response the virtual thread, there is a window of time where the event-loop thread has already sent buffers and we lost these events
-        response.handler(buffer -> {
+  private int counter;
 
-        });
-      } catch (Exception e) {
-        // Ooops
-      }
-    });
+  public void fieldVisibility1() {
+    int value = counter;
+    value += Async.await(getRemoteValue());
+    // the counter value might have changed
+    counter = value;
   }
 
-  public void whatYouGet3(HttpClientRequest request) {
-    Future<HttpClientResponse> fut = request.send();
-    HttpClientResponse response = Async.await(fut);
-    // Buffer events might be in the queue and if they are, they will be dispatched next
-    response.handler(buffer -> {
-
-    });
+  public void fieldVisibility2() {
+    counter += Async.await(getRemoteValue());
   }
 
-  public void whatYouGet4(Vertx vertx, HttpClientRequest request) {
-    Promise<HttpClientResponse> promise = Promise.promise();
-    vertx.setTimer(100, id -> promise.tryFail("Too late"));
-    request.send().onComplete(promise);
-    try {
-      HttpClientResponse response = Async.await(promise.future());
-    } catch (Exception timeout) {
-      // Too late
-    }
+  private Future<Buffer> callRemoteService() {
+    return null;
   }
 
-  private Future<Buffer> callRemoteService() {
+  private Future<Integer> getRemoteValue() {
     return null;
   }
 
@@ -115,7 +91,6 @@ public void start() {
       .setWorkerOptions(new VirtualThreadOptions()));
   }
 
-
   public void awaitingFutures1(HttpClientResponse response) {
     Buffer body = Async.await(response.body());
   }
@@ -133,7 +108,7 @@ public void awaitingLocks1(Lock theLock) {
     }
   }
 
-  public void threadLocalSupport1(Lock theLock, String userId, HttpClient client) {
+  public void threadLocalSupport1(String userId, HttpClient client) {
     ThreadLocal<String> local = new ThreadLocal();
     local.set(userId);
     HttpClientRequest req = Async.await(client.request(HttpMethod.GET, 8080, "localhost", "/"));