I have a Blazor Server application which listens to messages on a message broker and updates the UI with the new messages as they arrive. The gist of the component looks like this below, except of course the data is much more complex.
#page "/"
#implements IDisposable
#inject MessageBroker MessageBroker
<h1>#message?.Data</h1>
#code
{
private Message message;
protected override void OnInitialized()
{
MessageBroker.OnMessage += OnMessage;
}
private async void OnMessage(object sender, Message m)
{
message = m;
await InvokeAsync(StateHasChanged);
}
public void Dispose()
{
MessageBroker.OnMessage -= OnMessage;
}
}
The MessageBroker service is listening to messages over AMQP and invoking the OnMessage event. I find that there can significant delays between the time when the message is received and the UI updates. This is exacerbated when there are numerous clients connected via their browser.
I'm trying to track down the source of these delays and one suspect is the BuildRenderTree method which I understand is called each time StateHasChanged is invoked.
Is there anyway to see how long this method is taking? Or if there are other Blazor methods which might be responsible for the delays?
Related
I was reading about Spring Integration's FluxMessageChannel here and here, but I still don't understand exactly what are the differences between using a DirectChannel and FluxMessageChannel when using Project Reactor. Since the DirectChannel is stateless and controlled by its pollers, I'd expect the FluxMessageChannel to not be needed. I'm trying to understand when exactly should I use each and why, when speaking on Reactive Streams applications that are implemented with Spring Integration.
I currently have a reactive project that uses DirectChannel, and it seems to work fine, even the documentation says:
the flow behavior is changed from an imperative push model to a reactive pull model
I'd like to understand when to use each of the channels and what is the exact difference when working with Reactive Streams.
The DirectChannel does not have any poller and its implementation is very simple: as long as a message is sent to it, the handler is called. In the same caller's thread:
public class DirectChannel extends AbstractSubscribableChannel {
private final UnicastingDispatcher dispatcher = new UnicastingDispatcher();
private volatile Integer maxSubscribers;
/**
* Create a channel with default {#link RoundRobinLoadBalancingStrategy}.
*/
public DirectChannel() {
this(new RoundRobinLoadBalancingStrategy());
}
Where that UnicastingDispatcher is:
public final boolean dispatch(final Message<?> message) {
if (this.executor != null) {
Runnable task = createMessageHandlingTask(message);
this.executor.execute(task);
return true;
}
return this.doDispatch(message);
}
(There is no executor option for the DirectChannel)
private boolean doDispatch(Message<?> message) {
if (tryOptimizedDispatch(message)) {
return true;
}
...
protected boolean tryOptimizedDispatch(Message<?> message) {
MessageHandler handler = this.theOneHandler;
if (handler != null) {
try {
handler.handleMessage(message);
return true;
}
catch (Exception e) {
throw IntegrationUtils.wrapInDeliveryExceptionIfNecessary(message,
() -> "Dispatcher failed to deliver Message", e);
}
}
return false;
}
That's why I call it " imperative push model". The caller is this case is going to wait until the handler finishes its job. And if you have a big flow, everything is going to be stopped in the sender thread until a sent message has reached the end of the flow of direct channels. In two simple words: the publisher is in charge for the whole execution and it is blocked in this case. You haven't faced any problems with your solution based on the DirectChannel just because you didn't use reactive non-blocking threads yet like Netty in WebFlux or MongoDB reactive driver.
The FluxMessageChannel was really designed for Reactive Streams purposes where the subscriber is in charge for handling a message which it pulls from the Flux on demand. This way just after sending the publisher is free to do anything else. Just because it is already a subscriber responsibility to handle the message.
I would say it is definitely OK to use DirectChannel as long as your handlers are not blocking. As long as they are blocking you should go with FluxMessageChannel. Although don't forget that there are other channel types for different tasks: https://docs.spring.io/spring-integration/docs/current/reference/html/core.html#channel-implementations
I'am new to Vaadin and java web applications in general.
I tried to use event bus from google, this is my setup
Event bus singleton class:
public class Bus {
protected static final EventBus eventBus = new EventBus(); //single instance
public static EventBus getBus() {
return eventBus;
}
}
Place where i post my event:
public class MainMenuElementMain extends HorizontalLayout {
Bus eventbus = new Bus();
public MainMenuElementMain(String sectionTitle) {
ClickedMainMenuElement event = new ClickedMainMenuElement(sectionTitle);
eventbus.getBus().post(event);
}
}
Place where i handle the event:
#Subscribe
public void menuElementClicked (ClickedMainMenuElement event) {
add.(someComponent);
}
Everything is O.K. up to the time when i post the event from other web browser, than i got one of those two errors:
IllegalStateException: Can't move a node from one state tree to another
or
IllegalStateException: Cannot access state in VaadinSession or UI without locking the session.
I did some debugging and it seems that each time I refresh the browser or open new tab, the new instance of #Subscribe is register into the Event buss. In conclusion when i post the event all of the #Subscribe methods, from different sessions are called, which cause the error that i mentioned above.
It seems that my eventBus is a singleton sheared through all the sessions, What would be the best approach to make one instance of Event Buss for one session ?
The code below is a Web API that prints on behalf of a SPA. For brevity I've omitted using statements and the actual printing logic. That stuff all works fine. The point of interest is refactoring of the printing logic onto a background thread, with the web api method enqueuing a job. I did this because print jobs sent in quick succession were interfering with each other with only the last job printing.
It solves the problem of serialising print jobs but raises the question of how to detect shutdown and signal the loop to terminate.
namespace WebPrint.Controllers
{
public class LabelController : ApiController
{
static readonly ConcurrentQueue<PrintJob> queue = new ConcurrentQueue<PrintJob>();
static bool running = true;
static LabelController()
{
ThreadPool.QueueUserWorkItem((state) => {
while (running)
{
Thread.Sleep(30);
if (queue.TryDequeue(out PrintJob job))
{
this.Print(job);
}
}
});
}
public void Post([FromBody]PrintJob job)
{
queue.Enqueue(job);
}
}
public class PrintJob
{
public string url { get; set; }
public string html { get; set; }
public string printer { get; set; }
}
}
Given the way I acquire a thread to servicing the print queue, it is almost certainly marked as a background thread and should terminate when the app pool tries to exit, but I am not certain of this, and so I ask you, dear readers, for your collective notion of best practice in such a scenario.
Well, I did ask for best practice.
Nevertheless, I don't have long-running background tasks, I have short-running tasks. They arrive asynchronously on different threads, but must be executed serially and on a single thread because the WinForms printing methods are designed for STA threading.
Matt Lethargic's point about possible job loss is certainly a consideration, but for this case it doesn't matter. Jobs are never queued for more than a few seconds and loss would merely prompt operator retry.
For that matter, using a message queue doesn't solve the problem of "what if someone shuts it down while it's being used" it merely moves it to another piece of software. A lot of message queues aren't persistent, and you wouldn't believe the number of times I've seen someone use MSMQ to solve this problem and then fail to configure it for persistence.
This has been very interesting.
http://thecodelesscode.com/case/156
I would look at your architecture at a higher level, doing 'long running tasks' such as printing should probably live outside of you webapi process entirely.
If this we myself I would:
Create a windows service (or what have you) that has all the printing logic in it, the job of the controller is then to just talk to the service either by http or some kind of queue MSMQ, RabbitMQ, ServiceBus etc.
If via http then the service should internally queue up the print jobs and return 200/201 to the controller as soon as possible (before printing happens) so that the controller can return to the client efficiently and release it's resources.
If via a queuing technology then the controller should place a message on the queue and again return 200/201 as quick as possible, the service can then read the messages at it's own rate and print one at a time.
Doing it this way removes overhead from your api and also the possibility of losing print jobs in the case of a failure in the webapi (if the api crashes any background threads may/will be effected). Also what if you do a deployment at the point of someone printing, there's a high chance the print job will fail.
My 2 cents worth
I believe that the desired behavior is not something that should be done within a Controller.
public interface IPrintAgent {
void Enqueue(PrintJob job);
void Cancel();
}
The above abstraction can be implemented and injected into the controller using the frameworks IDependencyResolver
public class LabelController : ApiController {
private IPrintAgent agent;
public LabelController(IPrintAgent agent) {
this.agent = agent;
}
[HttpPost]
public IHttpActionResult Post([FromBody]PrintJob job) {
if (ModelState.IsValid) {
agent.Enqueue(job);
return Ok();
}
return BadRequest(ModelState);
}
}
The sole job of the controller in the above scenario is to queue the job.
Now with that aspect out of the way I will focus on the main part of the question.
As already mentioned by others, there are many ways to achieve the desired behavior
A simple in memory implementation can look like
public class DefaultPrintAgent : IPrintAgent {
static readonly ConcurrentQueue<PrintJob> queue = new ConcurrentQueue<PrintJob>();
static object syncLock = new Object();
static bool idle = true;
static CancellationTokenSource cts = new CancellationTokenSource();
static DefaultPrintAgent() {
checkQueue += OnCheckQueue;
}
private static event EventHandler checkQueue = delegate { };
private static async void OnCheckQueue(object sender, EventArgs args) {
cts = new CancellationTokenSource();
PrintJob job = null;
while (!queue.IsEmpty && queue.TryDequeue(out job)) {
await Print(job);
if (cts.IsCancellationRequested) {
break;
}
}
idle = true;
}
public void Enqueue(PrintJob job) {
queue.Enqueue(job);
if (idle) {
lock (syncLock) {
if (idle) {
idle = false;
checkQueue(this, EventArgs.Empty);
}
}
}
}
public void Cancel() {
if (!cts.IsCancellationRequested)
cts.Cancel();
}
static Task Print(PrintJob job) {
//...print job
}
}
which takes advantage of async event handlers to process the queue in sequence as jobs are added.
The Cancel is provided so that the process can be short circuited as needed.
Like in Application_End event as suggested by another user
var agent = new DefaultPrintAgent();
agent.Cancel();
or manually by exposing an endpoint if so desired.
I have implemented a simple Rest service by which I'd like to test deferredResult from Spring. While am I getting texts in that order:
TEST
TEST 1
TEST AFTER DEFERRED RESULT
I am very interested why in a browser (client) I need to wait that 8 seconds. Isn't that deferedResult shouldn't be non-blocking and run a task in the background? If no, how to create a rest service which will be non-blocking and run tasks in the background without using Java 9 and reactive streams?
#RestController("/")
public class Controller {
#GetMapping
public DeferredResult<Person> test() {
System.out.println("TEST");
DeferredResult<Person> result = new DeferredResult<>();
CompletableFuture.supplyAsync(this::test1)
.whenCompleteAsync((res, throwable) -> {
System.out.println("TEST AFTER DEFERRED RESULT");
result.setResult(res);
});
System.out.println("TEST 1");
return result;
}
private Person test1() {
try {
Thread.sleep(8000);
} catch (InterruptedException e) {
e.printStackTrace();
}
return new Person("michal", 20);
}
}
class Person implements Serializable {
private String name;
private int age;
}
DeferredResult is a holder for a WebRequest to allow the serving thread to release and serve another incoming HTTP request instead of waiting for the current one's result. After setResult or setError methods will be invoked - Spring will release that stored WebRequest and your client will receive the response.
DeferredResult holder is a Spring Framework abstraction for Non-blocking IO threading.
Deferred result abstraction has nothing with background tasks. Calling it without threading abstractions will cause the expected same thread execution. Your test1 method is running in the background because of CompletableFuture.supplyAsync method invocation that gives the execution to common pool.
The result is returned in 8 seconds because the whenCompleteAsync passed callback will be called only after test1 method will return.
You cannot receive the result immediately when your "service call logic" takes 8 seconds despite you are performing it in the background. If you want to release the HTTP request - just return an available proper object (it could contain a UUID, for example, to fetch the created person later) or nothing from the controller method. You can try to GET your created user after N seconds. There are specific HTTP response codes (202 ACCEPTED), that means the serverside is processing the request. Finally just GET your created object.
The second approach (if you should notify your clientside - but I will not recommend you to do it if this is the only reason) - you can use WebSockets to notify the clientside and message with it.
On the client side javascript I have
stomp.subscribe("/topic/path", function (message) {
console.info("message received");
});
And on the server side
public class Controller {
private final MessageSendingOperations<String> messagingTemplate;
ï¼ Autowired
public Controller(MessageSendingOperations<String> messagingTemplate) {
this.messagingTemplate = messagingTemplate;
}
#SubscribeMapping("/topic/path")
public void subscribe() {
LOGGER.info("before send");
messagingTemplate.convertAndSend(/topic/path, "msg");
}
}
From this setup, I am occasionally (around once in 30 page refreshes) experiencing message dropping, which means I can see neither "message received" msg on the client side nor the websocket traffic from Chrome debugging tool.
"before send" is always logged on the server side.
This looks like that the MessageSendingOperations is not ready when I call it in the subscribe() method. (if I put Thread.sleep(50); before calling messagingTemplate.convertAndSend the problem would disappear (or much less likely to be reproduced))
I wonder if anyone experienced the same before and if there is an event that can tell me MessageSendingOperations is ready or not.
The issue you are facing is laying in the nature of clientInboundChannel which is ExecutorSubscribableChannel by default.
It has 3 subscribers:
0 = {SimpleBrokerMessageHandler#5276} "SimpleBroker[DefaultSubscriptionRegistry[cache[0 destination(s)], registry[0 sessions]]]"
1 = {UserDestinationMessageHandler#5277} "UserDestinationMessageHandler[DefaultUserDestinationResolver[prefix=/user/]]"
2 = {SimpAnnotationMethodMessageHandler#5278} "SimpAnnotationMethodMessageHandler[prefixes=[/app/]]"
which are invoked within taskExecutor, hence asynchronously.
The first one here (SimpleBrokerMessageHandler (or StompBrokerRelayMessageHandler) if you use broker-relay) is responsible to register subscription for the topic.
Your messagingTemplate.convertAndSend(/topic/path, "msg") operation may be performed before the subscription registration for that WebSocket session, because they are performed in the separate threads. Hence the Broker handler doesn't know you to send the message to the session.
The #SubscribeMapping can be configured on method with return, where the result of this method will be sent as a reply to that subscription function on the client.
HTH
Here is my solution. It is along the same lines. Added a ExecutorChannelInterceptor and published a custom SubscriptionSubscribedEvent. The key is to publish the event after the message has been handled by AbstractBrokerMessageHandler which means the subscription has been registered with the broker.
#Override
public void configureClientInboundChannel(ChannelRegistration registration) {
registration.interceptors(new ExecutorChannelInterceptorAdapter() {
#Override
public void afterMessageHandled(Message<?> message, MessageChannel channel, MessageHandler handler, Exception ex) {
SimpMessageHeaderAccessor accessor = SimpMessageHeaderAccessor.wrap(message);
if (accessor.getMessageType() == SimpMessageType.SUBSCRIBE && handler instanceof AbstractBrokerMessageHandler) {
/*
* Publish a new session subscribed event AFTER the client
* has been subscribed to the broker. Before spring was
* publishing the event after receiving the message but not
* necessarily after the subscription occurred. There was a
* race condition because the subscription was being done on
* a separate thread.
*/
applicationEventPublisher.publishEvent(new SessionSubscribedEvent(this, message));
}
}
});
}
A little late but I thought I'd add my solution. I was having the same problem with the subscription not being registered before I was sending data through the messaging template. This issue happened rarely and unpredictable because of the race with the DefaultSubscriptionRegistry.
Unfortunately, I could not just use the return method of the #SubscriptionMapping because we were using a custom object mapper that changed dynamically based on the type of user (attribute filtering essentially).
I searched through the Spring code and found SubscriptionMethodReturnValueHandler was responsible for sending the return value of subscription mappings and had a different messagingTemplate than the autowired SimpMessagingTemplate of my async controller!!
So the solution was autowiring MessageChannel clientOutboundChannel into my async controller and using that to create a SimpMessagingTemplate. (You can't directly wire it in because you'll just get the template going to the broker).
In subscription methods, I then used the direct template while in other methods I used the template that went to the broker.