We have been using the Context object to in a long chain of async execution.
e.g.:
private void checkVehicle(final JsonObject cmd,
final RedisFleetStorage storage,
final Handler<AsyncResult<String>> handler) {
// omitted for brevity
// some async call to another verticle
storage.getVehicle(fleetId, vehicleId, result -> {
if (!result.succeeded()) {
LOG.error(String.format("Impossible to get vehicleStatus %s:%s", fleetId, vehicleId), result.cause());
handler.handle(Future.failedFuture("KO");
return;
}
// put vehicle in context for later use
final Vehicle vehicle = result.result();
LOG.info("vehicle details {}", vehicle);
context.put("vehicle", vehicle);
handler.handle(Future.succeededFuture());
});
}
As seen above, we put an object (vehicle) in the context and then access later in the execution.
But we suspect that the vehicle object it's altered by another execution. Is it possible? Can another event-loop change the object in the context?
A verticle instance handles all requests with the same event loop.
This why the Context object is not suited for storage of request specific data.
Related
Does anyone have any insights to which of the following two pseudo-patterns are the correct way of instantiating/utilizing reliable collections within a stateful service fabric service? Specifically, wondering if one approach is more performant, more memory-consuming or even more error prone.
Approach 1 (get instance from StateManager inside method):
public class MyService : IService {
public async Task<string> GetSomeValueAsync(string input){
var reliableDic = await StateManager.GetOrAddAsync<IReliableDictionary<string, string>>(StateKey);
var result = await reliableDic.TryGetValue(input);
return result.HasValue ? result.Value : null;
}
}
Approach 2 (store collection as member variable on class)
public class MyService : IService {
private bool _isInitialized;
private readonly object _lock = new object();
private IReliableDictionary<string, string> _dictionary;
private async Task Initialize(){
if (_isInitialized){
return;
}
lock(_lock){
if (_isInitialized){
return;
}
_dictionary = await StateManager.GetOrAddAsync<IReliableDictionary<string, string>>(StateKey);
_isInitialized = true;
}
}
public async Task<string> GetSomeValueAsync(string input){
await Initialize();
var result = await _dictionary.TryGetValue(input);
return result.HasValue ? result.Value : null;
}
}
So, approach 1 fetches the dictionary from the StateManager in each method while approach 2 does a lazy initialization check and then uses class members.
Most samples we see are using approach 1, but the idea behind approach 2 is to store the reliable dictionary in an instance field and avoid any StateManager.GetOrAddAsync hit in each method as well as centralize the handling of the StateKey which could be beneficial in larger services with many methods and potentially more reliable collections.
Would love to learn if there are any pitfalls or inefficiencies in either approach (obviously approach 2 is more verbose and uses more lines of code, but that is not the primary concern).
Actually there is no real reason to cache result of StateManager.GetOrAddAsync except saving memory allocations of Task object or making it available in places where having StateManager available isn't appropriate.
The reason for this is quite simple - the StateManager already caches the instance of IRealiableState for you. So it returns the same instance each time you do StateManager.GetOrAddAsync (here is the official answer from Microsoft).
You also can check it yourself with the very simple test (the c is true):
var q1 = stateManager.GetOrAddAsync<IReliableDictionary<string, string>>("MyDict")
.GetAwaiter().GetResult();
var q2 = stateManager.GetOrAddAsync<IReliableDictionary<string, string>>("MyDict")
.GetAwaiter().GetResult();
var c = q1 == q2;
In Kafka Streams' Processor API, can I pass processor context from init() as follows to other function and get the context back with state store in process()?
public void init(ProcessorContext context) {
this.context = context;
String resourceName = "config.properties";
ClassLoader loader = Thread.currentThread().getContextClassLoader();
Properties props = new Properties();
try(InputStream resourceStream = loader.getResourceAsStream(resourceName)) {
props.load(resourceStream);
}
catch(IOException e){
e.printStackTrace();
}
dataSplitter.timerMessageSource(props, context);//can I pass context like this?
this.context.schedule(1000);
// retrieve the key-value store named "patient"
kvStore = (KeyValueStore<String, PatientDataSummary>) this.context.getStateStore("patient");
//want to get the value of statestore filled by the called function timerMessageSource(), as the data to be put in statestore is getting generated in timerMessageSource()
//is there any way I can get that by using context or so
}
The usage of ProcessorContext is somewhat limited and you cannot call each method is provides at arbitrary times. Thus, it depend how you use it -- in general, you can pass it around as you wish (it will always be the same object throughout the live time of the processor).
If I understand your question correctly, you register a punctuation and use your dataSplitter within the punctuation callback and want to modify the store. That is absolutely possible -- you can either put the store into a class member similar to what you do with the context or use the context object to get the store within the punctuate callback.
I have this the following scenario I need to achieve:
perform each network call for a list of request object with 1 second delay each
and I have this following implementation using rxjava2
emit an interval stream
emit an iterable stream
zip them to emit each item from the iterable source
which by far has no problem and I fully understand how it works, now I integrated the above to the following
map each item emitted from zip into a new observable that defer/postpone an observable source for a network call
each mapped-emitted observable will perform an individual network call for each request
which I ended up with the following code
Observable
.zip(Observable.interval(1, TimeUnit.SECONDS), Observable.fromIterable(iterableRequests), new BiFunction<Long, RequestInput, RequestResult>() {
#Override
public RequestResult apply(#NonNull Long aLong, #NonNull final RequestInput request) throws Exception {
return request;
}
})
.map(new Function<RequestResult, ObservableSource<?>>() {
#Override
public ObservableSource<?> apply(#NonNull RequestResult requestResult) throws Exception {
// map each requestResult into this observable and perform a new stream
return Observable
.defer(new Callable<ObservableSource<?>>() {
// return a postponed observable for each subscriber
})
.retryWhen(new Function<Observable<Throwable>, ObservableSource<?>>() {
// return throwable observable
})
}
})
.subscribe(new Observer<ObservableSource<?>>() {
//.. onSubscribe {}
//.. onError {}
//.. onComplete {}
#Override
public void onNext(ObservableSource<?> observableSource) {
// actual subscription for each of the Observable.defer inside
// so it will start to emit and perform the necessary operation
}
});
but the problem is, it executes the Observable.defer source, only ONCE, but keeps on iterating(by putting a Log inside the map operator to see the iteration).
Can anyone guide me please on how can I achieve what I want, I exhausted alot of papers, drawing alot of marble diagrams, just to see where Im at on my code,
I dont know if the diagram I created illustrate the thing that I want, if it does, I dont know why does the sample code dont perform as the diagram portraits
Any help would be greatly appreciated.
The first part is fine, but the map thingy is a bit unneeded, what you are doing is mapping each RequestResult to an Observable, and then manually subscribe to it at the Observer.onNext(), actually the defer is not necessary as you're creating separate Observable for each RequestResult with different data, defer will occur at each subscribe yoy do at onNext(), and the map occur as you observed for each emission of the zipped RequestResult.
what you probably need is simple flatMap() to map each RequestResult value to a separate Observable that will do the network request, and it will merge back the result for each request to the stream, so you'll just need to handle the final values emission for each request instead to subscribe manually to each Observable.
Just keep in mind that order might be lost, in case some requests might take longer than your delay between them.
Observable.zip(Observable.interval(1, TimeUnit.SECONDS), Observable.fromIterable(iterableRequests),
new BiFunction<Long, RequestInput, RequestResult>() {
#Override
public RequestResult apply(#NonNull Long aLong,
#NonNull final RequestInput request) throws Exception {
return request;
}
})
.flatMap(new Function<RequestResult, ObservableSource<?>>() {
#Override
public ObservableSource<?> apply(RequestResult requestResult) throws Exception {
return createObservableFromRequest(requestResult)
.retryWhen(new Function<Observable<Throwable>, ObservableSource<?>>() {
// return throwable observable
})
}
})
.subscribe(new Observer<ObservableSource<?>>() {
//.. onSubscribe {}
//.. onError {}
//.. onComplete {}
#Override
public void onNext(ObservableSource<?> observableSource) {
//do something with each network result request emission
}
});
I manage to make it work, as somewhere inside the Observable.defer, my retrofitclient was null,
retrofitClient.getApiURL().post(request); // client was null
my retrofitClient was null ( i looked somewhere in the code and I noticed i was not initialized, and I initialized it properly and made it work)
now can anybody tell me why Rx didnt throw an exception back to the original observable stream? theres no NullPointerException that occurred, Im confused
I am writing a server in netty, in which I need to make a call to memcached. I am using spymemcached and can easily do the synchronous memcached call. I would like this memcached call to be async. Is that possible? The examples provided with netty do not seem to be helpful.
I tried using callbacks: created a ExecutorService pool in my Handler and submitted a callback worker to this pool. Like this:
public class MyHandler extends ChannelInboundMessageHandlerAdapter<MyPOJO> implements CallbackInterface{
...
private static ExecutorService pool = Executors.newFixedThreadPool(20);
#Override
public void messageReceived(ChannelHandlerContext ctx, MyPOJO pojo) {
...
CallingbackWorker worker = new CallingbackWorker(key, this);
pool.submit(worker);
...
}
public void myCallback() {
//get response
this.ctx.nextOutboundMessageBuf().add(response);
}
}
CallingbackWorker looks like:
public class CallingbackWorker implements Callable {
public CallingbackWorker(String key, CallbackInterface c) {
this.c = c;
this.key = key;
}
public Object call() {
//get value from key
c.myCallback(value);
}
However, when I do this, this.ctx.nextOutboundMessageBuf() in myCallback gets stuck.
So, overall, my question is: how to do async memcached calls in Netty?
There are two problems here: a small-ish issue with the way you're trying to code this, and a bigger one with many libraries that provide async service calls, but no good way to take full advantage of them in an async framework like Netty. That forces users into suboptimal hacks like this one, or a less-bad, but still not ideal approach I'll get to in a moment.
First the coding problem. The issue is that you're trying to call a ChannelHandlerContext method from a thread other than the one associated with your handler, which is not allowed. That's pretty easy to fix, as shown below. You could code it a few other ways, but this is probably the most straightforward:
private static ExecutorService pool = Executors.newFixedThreadPool(20);
public void channelRead(final ChannelHandlerContext ctx, final Object msg) {
//...
final GetFuture<String> future = memcachedClient().getAsync("foo", stringTranscoder());
// first wait for the response on a pool thread
pool.execute(new Runnable() {
public void run() {
String value;
Exception err;
try {
value = future.get(3, TimeUnit.SECONDS); // or whatever timeout you want
err = null;
} catch (Exception e) {
err = e;
value = null;
}
// put results into final variables; compiler won't let us do it directly above
final fValue = value;
final fErr = err;
// now process the result on the ChannelHandler's thread
ctx.executor().execute(new Runnable() {
public void run() {
handleResult(fValue, fErr);
}
});
}
});
// note that we drop through to here right after calling pool.execute() and
// return, freeing up the handler thread while we wait on the pool thread.
}
private void handleResult(String value, Exception err) {
// handle it
}
That will work, and might be sufficient for your application. But you've got a fixed-sized thread pool, so if you're ever going to handle much more than 20 concurrent connections, that will become a bottleneck. You could increase the pool size, or use an unbounded one, but at that point, you might as well be running under Tomcat, as memory consumption and context-switching overhead start to become issues, and you lose the scalabilty that was the attraction of Netty in the first place!
And the thing is, Spymemcached is NIO-based, event-driven, and uses just one thread for all its work, yet provides no way to fully take advantage of its event-driven nature. I expect they'll fix that before too long, just as Netty 4 and Cassandra have recently by providing callback (listener) methods on Future objects.
Meanwhile, being in the same boat as you, I researched the alternatives, and not being too happy with what I found, I wrote (yesterday) a Future tracker class that can poll up to thousands of Futures at a configurable rate, and call you back on the thread (Executor) of your choice when they complete. It uses just one thread to do this. I've put it up on GitHub if you'd like to try it out, but be warned that it's still wet, as they say. I've tested it a lot in the past day, and even with 10000 concurrent mock Future objects, polling once a millisecond, its CPU utilization is negligible, though it starts to go up beyond 10000. Using it, the example above looks like this:
// in some globally-accessible class:
public static final ForeignFutureTracker FFT = new ForeignFutureTracker(1, TimeUnit.MILLISECONDS);
// in a handler class:
public void channelRead(final ChannelHandlerContext ctx, final Object msg) {
// ...
final GetFuture<String> future = memcachedClient().getAsync("foo", stringTranscoder());
// add a listener for the Future, with a timeout in 2 seconds, and pass
// the Executor for the current context so the callback will run
// on the same thread.
Global.FFT.addListener(future, 2, TimeUnit.SECONDS, ctx.executor(),
new ForeignFutureListener<String,GetFuture<String>>() {
public void operationSuccess(String value) {
// do something ...
ctx.fireChannelRead(someval);
}
public void operationTimeout(GetFuture<String> f) {
// do something ...
}
public void operationFailure(Exception e) {
// do something ...
}
});
}
You don't want more than one or two FFT instances active at any time, or they could become a drain on CPU. But a single instance can handle thousands of outstanding Futures; about the only reason to have a second one would be to handle higher-latency calls, like S3, at a slower polling rate, say 10-20 milliseconds.
One drawback of the polling approach is that it adds a small amount of latency. For example, polling once a millisecond, on average it will add 500 microseconds to the response time. That won't be an issue for most applications, and I think is more than offset by the memory and CPU savings over the thread pool approach.
I expect within a year or so this will be a non-issue, as more async clients provide callback mechanisms, letting you fully leverage NIO and the event-driven model.
How to communicate user defined objects and user defined (checked) exceptions between Service and UI in JavaFX2?
The examples only show String being sent in to the Service as a property and array of observable Strings being sent back to the UI.
Properties seem to be defined only for simple types. StringProperty, IntegerProperty, DoubleProperty etc.
Currently I have a user defined object (not a simple type), that I want Task to operate upon and update with the output data it produced. I am sending it through the constructor of Service which passes it on through the constructor of Task. I wondered about the stricture that parameters must be passed in via properties.
Also if an exception is thrown during Task's operation, How would it be passed from Service to the UI? I see only a getException() method, no traditional throw/catch.
Properties http://docs.oracle.com/javafx/2/binding/jfxpub-binding.htm
Service and Task http://docs.oracle.com/javafx/2/threads/jfxpub-threads.htm
Service javadocs http://docs.oracle.com/javafx/2/api/javafx/concurrent/Service.html#getException()
"Because the Task is designed for use with JavaFX GUI applications, it
ensures that every change to its public properties, as well as change
notifications for state, errors, and for event handlers, all occur on
the main JavaFX application thread. Accessing these properties from a
background thread (including the call() method) will result in runtime
exceptions being raised.
It is strongly encouraged that all Tasks be initialized with immutable
state upon which the Task will operate. This should be done by
providing a Task constructor which takes the parameters necessary for
execution of the Task. Immutable state makes it easy and safe to use
from any thread and ensures correctness in the presence of multiple
threads."
But if my UI only touches the object after Task is done, then it should be ok, right?
Service has a signature Service<V> the <V> is a generic type parameter used to specify the type of the return object from the service's supplied task.
Let's say you want to define a service which returns a user defined object of type Foo, then you can do it like this:
class FooGenerator extends Service<Foo> {
protected Task createTask() {
return new Task<Foo>() {
protected Foo call() throws Exception {
return new Foo();
}
};
}
}
To use the service:
FooGenerator fooGenerator = new FooGenerator();
fooGenerator.setOnSucceeded(new EventHandler<WorkerStateEvent>() {
#Override public void handle(WorkerStateEvent t) {
Foo myNewFoo = fooGenerator.getValue();
System.out.println(myNewFoo);
}
});
fooGenerator.start();
If you want to pass an input value into the service each time before you start or restart it, you have to be a little bit more careful. You can add the values you want to input to the service as settable members on the service. These setters can be called from the JavaFX application thread, before the service's start method is invoked. Then, when the service's task is created, pass the parameters through to the service's Task's constructor.
When doing this it is best to make all information passable back and forth between threads immutable. For the example below, a Foo object is passed as an input parameter to the service and a Foo object based on the input received as an output of the service. But the state of Foo itself is only initialized in it's constructor - the instances of Foo are immutable and cannot be changed once created and all of it's member variables are final and cannot change. This makes it much easier to reason about the program, as you never need worry that another thread might overwrite the state concurrently. It seems a little bit complicated, but it does make everything very safe.
class FooModifier extends Service<Foo> {
private Foo foo;
void setFoo(Foo foo) { this.foo = foo; }
#Override protected Task createTask() {
return new FooModifierTask(foo);
}
private class FooModifierTask extends Task<Foo> {
final private Foo fooInput;
FooModifierTask(Foo fooInput) { this.fooInput = fooInput; }
#Override protected Foo call() throws Exception {
Thread.currentThread().sleep(1000);
return new Foo(fooInput);
}
}
}
class Foo {
private final int answer;
Foo() { answer = random.nextInt(100); }
Foo(Foo input) { answer = input.getAnswer() + 42; }
public int getAnswer() { return answer; }
}
There is a further example of providing input to a Service in the Service javadoc.
To return a custom user exception from the service, just throw the user exception during the service's task call handler. For example:
class BadFooGenerator extends Service<Foo> {
#Override protected Task createTask() {
return new Task<Foo>() {
#Override protected Foo call() throws Exception {
Thread.currentThread().sleep(1000);
throw new BadFooException();
}
};
}
}
And the exception can be retrieved like this:
BadFooGenerator badFooGenerator = new BadFooGenerator();
badFooGenerator.setOnFailed(new EventHandler<WorkerStateEvent>() {
#Override public void handle(WorkerStateEvent t) {
Throwable ouch = badFooGenerator.getException();
System.out.println(ouch.getClass().getName() + " -> " + ouch.getMessage());
}
});
badFooGenerator.start();
I created a couple of executable samples you can use to try this out.
Properties seem to be defined only for simple types. StringProperty, IntegerProperty, DoubleProperty etc. Currently I have a user defined object (not a simple type), that I want Task to operate upon and update with the output data it produced
If you want a property that can be used for your own classes try SimpleObjectProperty where T could be Exception, or whatever you need.
Also if an exception is thrown during Task's operation, How would it be passed from Service to the UI?
You could set an EventHandler on the Task#onFailedProperty from the UI with the logic with what to do on failure.
But if my UI only touches the object after Task is done, then it should be ok, right?
If you call it from your UI you are sure to be on the javaFX thread so you will be OK. You can assert that you're on the javaFX thread by calling Platform.isFxApplicationThread().