In an Akka Stream GraphStageLogic's postStop, is there a way to determine whether the stage has failed or completed without error? E.g. get a Try[Unit], that would be a Failure if failStage had been called or a Success if completeStage had been called.
failStage is a final method, so there is no way to hook into it
the documentation also doesn't say anything useful.
Seems there is not. So you need to monitor all of the node's own calls to completeStage, including inlet/outlet handlers.
Related
Protractor's documentation has a small summary of the control flow, where it says:
WebDriverJS (and thus, Protractor) APIs are entirely asynchronous ... WebDriverJS maintains a queue of pending promises, called the control flow, to keep execution organized
[http://www.protractortest.org/#/control-flow][1]
When does the "queue of pending promises" actually get emptied? i.e., when do the tasks execute? (Where can I get a list of API calls that result in the queue to actually get processed?)
Protractor is built on top of WebdriverJS and you are right their API's are entirely asynchronous and return Promises!
These promises are based on the ControlFlow class which handle the execution of asynchronous tasks. Each task is scheduled via ControlFlow execute() method, which will return a ManagedPromise that will be resolved with the task's result.
For Detailed explanation you should read this WebdriverJS Control Flow API doc
I'm having a difficult time trying to understand how the control flow in protractor work in relation to how JS event loop works. Here is what I know so far:
Protractor control flow stores commands that return promises in a queue. The first command will be at the front of the queue and the last command will be at the back. No command will be executed until the command in front of it has its promise resolved.
JS event loop stores asynchronous task (callbacks to be specific). Callbacks are not executed until all functions in the stack have completed and the stack is empty. Before running each callback, there is a check on whether the stack is empty or not.
so lets take this code for example. The code is basically clicking a search button and a api request is made. Then after data is returned, it checks whether the field that stores the returned data exists.
elem('#searchButton').click(); //will execute a api call to retrieve data
browser.wait(ExpectedConditions.presenceOf(elem('#resultDataField'),3000));
expect(elem('#resultDataField').isPresent()).toBeTruthy();
So with this code, I'm able to get it to work. But I don't know how it does it. How is the event loop applied in this scenario?
The core of the ControlFlow implementation is in runEventLoop_ (in Selenium's promise.js implementation).
As I understand it, the ControlFlow registers a call to runEventLoop_ with the JS event loop (e.g., with a 0-second timeout or somesuch). The call to runEventLoop_ can be thought of as a single iteration of a normal event loop. It registers code to actually run a scheduled task (i.e., actually do the work you queued up during your it). Once that task completes or fails (e.g., by hooking its async promise callbacks) the next iteration of runEventLoop_ is scheduled (see the calls to scheduleEventLoop in runEventLoop_).
There is some complexity when a callback ends up registering new promises (those need to be "inserted" before the old next event, this is accomplished by creating a "nested" control flow. Mostly you should never have to know this.)
I've implemented a custom workitemhandler which I want to complete only by an external REST call. Therefore the items executeWorkItem() method does NOT call manager.completeWorkItem(workItem.getId(), results); at the end, which is perfectly ok. I've also assigned a signal event to this workitem in my process, which is also called by an external REST call. Both things work as expected, but what I do not understand is that every time I signal the work item, it also automatically completes the work item, which leads to the problem that the process continuous with its regular path AND the signaled one. But the reason for the signal is to interrupt the process to follow ONLY the signaled path path.
The process image to this can be found here http://cl.ly/image/0F3L3E2w2l0j. In this example I signaled the "Fail Transfer" but the rest gets also executed even nothing completed the workitem.
I'm using jBPM 6.1 Final.
Thanks in advance for any help.
Nevermind, I found the reason for this behavior. The custom work item handler implemented
public void abortWorkItem(WorkItem workItem, WorkItemManager manager) {
manager.abortWorkItem(workItem.getId());
}
After removing manager.abortWorkItem(workItem.getId());, the process behaves as expected.
I am reading libpq reference. It has both of sync and async methods. Bu I discovered something strange.
When I see PQsendQuery function, it seems to send a query and return immediately. And I expected a callback function to get notified, but there was no such thing and the manual says to poll for data availability.
I don't understand why async method is written in polling way. Anyway, as libp is the official client implementation, I believe there should be a good reason for this design. What is that? Or am I missing correct callback stuffs mentioned somewhere else?
In the execution model of a mono-threaded program, the execution flow can't be interrupted by data coming back from an asynchronous query, or more generally a network socket. Only signals (SIGTERM and friends) may interrupt the flow, but signals can't be hooked to data coming in.
That's why having a callback to get notified of incoming data is not possible. The piece of code in libpq that would be necessary to emit the callback would never run if your code doesn't call it. And if you have to call it, that defeats the whole point of a callback.
There are libraries like Qt that provide callbacks, but they're architectured from the ground up with a main loop that acts as an event processor. The user code is organized in callbacks and event-based processing of incoming data is possible. But in this case the library takes ownership of the execution flow, meaning its mainloop polls the data sources. That just shifts the responsibility to another piece of code outside of libpq.
This page is describing how I can get be notified for async result fetch.
http://www.postgresql.org/docs/9.3/static/libpq-events.html#LIBPQ-EVENTS-PROC
PGEVT_RESULTCREATE
The result creation event is fired in response to any query execution
function that generates a result, including PQgetResult. This event
will only be fired after the result has been created successfully.
typedef struct {
PGconn *conn;
PGresult *result; } PGEventResultCreate; When a PGEVT_RESULTCREATE event is received, the evtInfo pointer should be cast to a
PGEventResultCreate *. The conn is the connection used to generate the
result. This is the ideal place to initialize any instanceData that
needs to be associated with the result. If the event procedure fails,
the result will be cleared and the failure will be propagated. The
event procedure must not try to PQclear the result object for itself.
When returning a failure code, all cleanup must be performed as no
PGEVT_RESULTDESTROY event will be sent.
I'm a newbie to WF and rather lost. Here's what I have so far:
I've created a workflow service app (xamlx), added needed variables
I've created a custom NativeActivity where I'm calling CreateBookmark from within Execute, which is between the Receive & Send activity for the service. (Ultimately this will actually do something besides creating the bookmark).
The bookmark gets created just fine, but after stepping out of the Execute method, nothing happens for one minute until the service times out, giving me that message "The request channel timed out while waiting for a reply after 00:00:59.9699970. Increase the timeout value passed to the call to Request or increase the SendTimeout value on the Binding. The time allotted to this operation may have been a portion of a longer timeout." (I tried posting an image of the xamlx, but as a newbie it won't let me; suffice it to say I'm getting from my Receive, into my custom native activity, but never getting as far as the SendReply).
I assume I'm missing something rather fundatmental, but I can't see what. I've originally tried using NativeActivity<T> to return what I want, but that behaves the same.
Found out what I was doing wrong: needed to use overload of CreateBookmark that has BookmarkOptions parameter and set it to BookmarkOptions.NonBlocking.
Strangely, I did not find one example anywhere that mentioned this.