If I make this call in F#
let mailServers = task{
let! ms = lookupClient.QueryAsync(domain, QueryType.MX, QueryClass.IN, CancellationToken.None)
return ms
}
mailServers is a Task<IDnsQueryResponse>.
I would like to get at the IDnsQueryResponse value wrapped in the task. How can I change this async call to get the actual value?
In your example you already have the IDnsQueryResponse as ms within the task expression. Usually when you start working with Tasks you want to keep working with Tasks until all the work is done so you stay inside the task expression.
If you don't mind blocking the thread you can just call mailServers.Result to wait.
Related
I have a pool of tasks and I am trying to figure out the best way to be notified, through an event, when one is finished.
Since the tasks are quite varied, I don't want to add a piece of code inside the task itself since that would mean putting it in several places. These are long running tasks, I'm not waiting for them to complete anywhere, they're just getting started, do their work (minutes to days) and then they finish.
The ugly-but-could-work solution is to wrap each work task into another task that awaits for the work task to be complete and then sends an event, but I'm hoping there would be something more elegant.
In a comment you explained that you're starting your tasks like this:
Async.StartAsTask (runner.Start(), TaskCreationOptions.LongRunning, cancellationSource.Token)
Instead of doing that, start them like this:
startMyTask runner cancellationSource (fun() -> printfn "Task completed!")
Where:
let startMyTask (runner: RunnerType) (s: CancellationTokenSource) onDone =
let wrapper = async {
do! runner.Start()
onDone()
}
Async.StartAsTask (wrapper, TaskCreationOptions.LongRunning, s.Token)
So, I have a list of tasks which I want to schedule concurrently in a non-blocking fashion.
Basically, gather should do the trick.
Like
tasks = [ asyncio.create_task(some_task()) in bleh]
results = await asyncio.gather(*tasks)
But then, I also need a timeout. What I want is that any task which takes > timeout time cancels and I proceed with what I have.
I fould asyncio.wait primitive.
https://docs.python.org/3/library/asyncio-task.html#waiting-primitives
But then the doc says:
Run awaitable objects in the aws set concurrently and block until the condition specified by return_when.
Which seems to suggest that it blocks...
It seems that asyncio.wait_for will do the trick
https://docs.python.org/3/library/asyncio-task.html#timeouts
But how do i send in the list of awaitables rather than just an awaitable?
What I want is that any task which takes > timeout time cancels and I proceed with what I have.
This is straightforward to achieve with asyncio.wait():
# Wait for tasks to finish, but no more than a second.
done, pending = await asyncio.wait(tasks, timeout=1)
# Cancel the ones not done by now.
for fut in pending:
fut.cancel()
# Results are available as x.result() on futures in `done`
Which seems to suggest that [asyncio.wait] blocks...
It only blocks the current coroutine, the same as gather or wait_for.
In my application I have the following:
db2.CreateTable<CategoryGroup>();
db2.CreateTable<Category>();
db2.CreateTable<CategoryGroupSource>();
db2.CreateTable<CategorySource>();
db2.CreateTable<Phrase>();
db2.CreateTable<PhraseSource>();
db2.CreateTable<Score>();
db2.CreateTable<Setting>();
From what I understand there is an Async way to do this also:
database.CreateTableAsync<TodoItem>().Wait();
Can someone explain if there is any advantage in me using the Async way and do people normally always use the Async?
Also are there likely to be benefits if I use this type of Async query:
public Task<TodoItem> GetItemAsync(int id)
{
return database.Table<TodoItem>().Where(i => i.ID == id).FirstOrDefaultAsync();
}
When calling the methods on the main (UI) thread everything on the UI stops for as long as it takes that method to execute. If db2.CreateTable<CategoryGroup>() doesn't take up much time when doing it's thing, it shouldn't be a problem.
Doing a lot of time consuming actions straight after each other might affect your UI and make it freeze.
Calling the *Async variant of the method moves the work to a background thread, via the task API. Calling Wait() on that task, though, makes the current thread (in this case the UI thread) wait for the task to finish, and you're stuck with the same problem.
You should always await tasks: await database.CreateTableAsync<TodoItem>(). This will let it execute on a background thread and not make the current thread wait for it to finish. The next line in your code won't be executed until the Task is finished though. When you write the code, it makes the `Async variant look like it's behaving like the regular version.
Personally, I'd probably move all the methods into a task and just await that. That way you're not returning to the UI thread between each task to execute the next one:
await Task.Run(() =>
{
db2.CreateTable<CategoryGroup>();
db2.CreateTable<Category>();
db2.CreateTable<CategoryGroupSource>();
db2.CreateTable<CategorySource>();
db2.CreateTable<Phrase>();
db2.CreateTable<PhraseSource>();
db2.CreateTable<Score>();
db2.CreateTable<Setting>();
}
In this case you're making the database do all it's work on a background thread (and not freezing the UI while it's doing it). It then returns the result to the UI thread to enable you to update UI.
public Task<TodoItem> GetItemAsync(int id)
{
return database.Table<TodoItem>().Where(i => i.ID == id).FirstOrDefaultAsync();
}
I'm trying to capture output written from each task as it is executed. The code below works as expected when running Gradle with --max-workers 1, but when multiple tasks are running in parallel this code below picks up output written from other tasks running simultaneously.
The API documentation states the following about the "getLogging" method on Task. From what it says I judge that it should support capturing output from single tasks regardless of any other tasks running at the same time.
getLogging()
Returns the LoggingManager which can be used to control the logging level and standard output/error capture for this task. https://docs.gradle.org/current/javadoc/org/gradle/api/Task.html
graph.allTasks.forEach { Task task ->
task.ext.capturedOutput = [ ]
def listener = { task.capturedOutput << it } as StandardOutputListener
task.logging.addStandardErrorListener(listener)
task.logging.addStandardOutputListener(listener)
task.doLast {
task.logging.removeStandardOutputListener(listener)
task.logging.removeStandardErrorListener(listener)
}
}
Have I messed up something in the code above or should I report this as a bug?
It looks like every LoggingManager instance shares an OutputLevelRenderer, which is what your listeners eventually get added to. This did make me wonder why you weren't getting duplicate messages because you're attaching the same listeners to the same renderer over and over again. But it seems the magic is in BroadcastDispatch, which keeps the listeners in a map, keyed by the listener object itself. So you can't have duplicate listeners.
Mind you, for that to hold, the hash code of each listener must be the same, which seems surprising. Anyway, perhaps this is working as intended, perhaps it isn't. It's certainly worth an issue to get some clarity on whether Gradle should support listeners per task. Alternatively raise it on the dev mailing list.
I have a spring MVC app where a user can kick off a Report generation via button click. This process could take few minutes ~ 10-20 mins.
I use springs #Async annotation around the service call so that report generation happens asynchronously. While I pop a message to user indicating job is currently running.
Now What I want to do is, if another user (Admin) can kick off Report generation via the button which should cancel/stop currently running #Async task and restart the new task.
To do this, I call the
.. ..
future = getCurrentTask(id); // returns the current task for given report id
if (!future.isDone())
future.cancel(true);
service.generateReport(id);
How can make it so that "service.generateReport" waits while the future cancel task kills all the running threads?
According to the documentation, after i call future.cancel(true), isDone will return true as well as isCancelled will return true. So there is no way of knowing the job is actually cancelled.
I can only start new report generation when old one is cancelled or completed so that it would not dirty data.
From documentation about cancel() method,
Subsequent calls to isCancelled() will always return true if this method returned true
Try this.
future = getCurrentTask(id); // returns the current task for given report id
if (!future.isDone()){
boolean terminatedImmediately=future.cancel(true);
if(terminatedImmediately)
service.generateReport(id);
else
//Inform user existing job couldn't be stopped.And to try again later
}
Assuming the code above runs in thread A, and your recently cancelled report is running in thread B, then you need thread A to stop before service.generateReport(id) and wait until thread B is completes / cancelled.
One approach to achieve this is to use Semaphore. Assuming there can be only 1 report running concurrently, first create a semaphore object acccessible by all threads (normally on the report runner service class)
Semaphore semaphore = new Semaphore(1);
At any point on your code where you need to run the report, call the acquire() method. This method will block until a permit is available. Similarly when the report execution is finished / cancelled, make sure release() is called. Release method will put the permit back and wakes up other waiting thread.
semaphore.acquire();
// run report..
semaphore.release();