I'm constrained to using the .NET 4.0 framework and Async CTP Extensions to do something like the following:
var dataTasks = _tasks.Select(t => t.GetData(keys));
var results = TaskEx.WhenAll(dataTasks).Result.ToList();
where _tasks is a list of objects that each provide a GetData method that returns a Task<Data>.
My problem is that one of the dataTasks is throwing an exception and tanking the whole. I'd like to be able to inspect the results of each dataTask and check the results, logging any exception and then continuing on with any valid results. I'm unsure now to go about it though. Any help would be greatly appreciated.
One way to do this would be to use a trivial ContinueWith() to change a potentially faulting Task<T> into a successful Task<Task<T>>. If you then use WhenAll().Result on that (or await WhenAll()), you'll get Task<T>[], which is exactly what you need:
var dataTasks = _tasks.Select(t => t.GetData(keys).ContinueWith(c => c));
Task<T>[] results = Task.WhenAll(dataTasks).Result;
TaskEx.WhenAll(dataTasks) is your combined task. It might be faulted.
When calling Result this cause the exception to be thrown. So don't do that. Examine the Exception or IsFaulted properties.
Related
I am attempting to put together a Cro service that has a react/whenever block consuming data "in the background" So unlike many examples of websocket usage with Cro, this has nothing to do with routes that may be accessed via the browser.
My use case is to consume message received via an MQTT topic and do some processing with them. At a later stage in development I might create a supply out of this data, but for now, when data is received it will be stored in a variable and dependant on certain conditions, be sent to another service via a http post.
My thought was to include a provider() in the Cro::HTTP::Server setup like so:
use Cro::HTTP::Log::File;
use Cro::HTTP::Server;
use Routes;
use DataProvider; # Here
my Cro::Service $http = Cro::HTTP::Server.new(
http => <1.1>,
host => ...,
port => ...,
application => [routes(), provider()], # Made this into an array of subs?
after => [
Cro::HTTP::Log::File.new(logs => $*OUT, errors => $*ERR)
]
);
And in the DataProvider.pm6:
use MQTT::Client;
sub provider() is export {
my $mqtt = MQTT::Client.new: server => 'localhost';
react {
whenever $mqtt.subscribe('some/mqtt/topic') {
say "+ topic: { .<topic> } => { .<message>.decode("utf8-c8") }";
}
}
}
This throws a bunch of errors:
A react block:
in sub provider at DataProvider.pm6 (DataProvider) line 5
in block <unit> at service.p6 line 26
Died because of the exception:
Invocant of method 'write' must be an object instance of type
'IO::Socket::Async', not a type object of type 'IO::Socket::Async'. Did
you forget a '.new'?
in method subscribe at /home/cam/raku/share/perl6/site/sources/42C762836A951A1C11586214B78AD34262EC465F (MQTT::Client) line 133
in sub provider at DataProvider.pm6 (DataProvider) line 6
in block <unit> at service.p6 line 26
To be perfectly honest, I am totally guessing that this is how I would approach the need to subscribe to data in the background of a Cro service, but I was not able to find any information on what might be considered the recommended approach.
Initially I had my react/whenever block in the main service.pm6 file, but that did not seem right. And needed to be wrapped in a start{} block because as I have just learned, react is blocking :) and cro was not able to actually start.
But following the pattern of how Routes are implemented seemed logical, but I am missing something. The error speaks about setting up a new method, but I'm not convinced that is the root cause. Routes.pm6 does not have a constructor.
Can anyone point me in the right direction please?
Thanks to all who have provided information, this has been a very valuable learning exercise.
The approach of passing additional sub routines, along side router() in the application parameter to Cro::HTTP::Server.new gave further trouble. (an array is not allowed, and broke routing)
Instead, I have moved the background work into a class of it's own, and given it a start and stop method more akin to Cro::HTTP::Server.
My new approach:
service.pm6
use Cro::HTTP::Log::File;
use Cro::HTTP::Server;
use Routes;
use KlineDataSubscriber; # Moved mqtt functionality here
use Database;
my $dsn = "host=localhost port=5432 dbname=act user=.. password=..";
my $dbh = Database.new :$dsn;
my $mqtt-host = 'localhost';
my $subscriber = KlineDataSubscriber.new :$mqtt-host;
$subscriber.start; # Inspired by $http.start below
my Cro::Service $http = Cro::HTTP::Server.new(
http => <1.1>,
host => ...,
port => ...,
application => routes($dbh), # Basically back the way it was originally
after => [
Cro::HTTP::Log::File.new(logs => $*OUT, errors => $*ERR)
]
);
$http.start;
say "Listening at...";
react {
whenever signal(SIGINT) {
say "Shutting down...";
$subscriber.stop;
$http.stop;
done;
}
}
And in KlineDataSubscriber.pm6
use MQTT::Client;
class KlineDataSubscriber {
has Str $.mqtt-host is required;
has MQTT::Client $.mqtt = Nil;
submethod TWEAK() {
$!mqtt = MQTT::Client.new: server => $!mqtt-host;
await $!mqtt.connect;
}
method start(Str $topic = 'act/feed/exchange/binance/kline-closed/+/json') {
start {
react {
whenever $!mqtt.subscribe($topic) {
say "+ topic: { .<topic> } => { .<message>.decode("utf8-c8") }";
}
}
}
}
method stop() {
# TODO Figure how to unsubscribe and cleanup nicely
}
}
This feels much more "Cro idiomatic" to me, but I would be happy to be corrected.
More importantly, it works as expected and I feel is somewhat future proof. I should be able to create a supply to make real-time data available to the router, and push data to any connected web clients.
I also intend to have a http GET endpoint /status with various checks to ensure everything healthy
The root cause
The error speaks about setting up a new method, but I'm not convinced that is the root cause.
It's not about setting up a new method. It's about a value that should be defined instead being undefined. That typically means a failure to attempt to initialize it, which typically means a failure to call .new.
Can anyone point me in the right direction please?
Hopefully this question helps.
Finding information on a recommended approach
I am totally guessing that this is how I would approach the need to subscribe to data in the background of a Cro service, but I was not able to find any information on what might be considered the recommended approach.
It might be helpful for you to list which of the get-up-to-speed steps you've followed from Getting started with Cro, including the basics but also the "Learn about" steps at the end.
The error message
A react block:
in sub provider ...
Died because of the exception:
...
in method subscribe ...
The error message begins with the built in react construct reporting that it caught an exception (and handled it by throwing its own exception in response). A "backtrace" corresponding to where the react appeared in your code is provided indented from the initial "A react block:".
The error message continues with the react construct summarizing its own exception (Died because ...) and explains itself by reporting the original exception, further indented, in subsequent lines. This includes another backtrace, this time one corresponding to the original exception, which will likely have occurred on a different thread with a different callstack.
(All of Raku's structured multithreading constructs[1] use this two part error reporting approach for exceptions they catch and handle by throwing another exception.)
The first backtrace indicates the react line:
in sub provider at DataProvider.pm6 (DataProvider) line 5
use MQTT::Client;
sub provider() is export {
my $mqtt = MQTT::Client.new: server => 'localhost';
react {
The second backtrace is about the original exception:
Invocant of method 'write' must be an object instance of type
'IO::Socket::Async', not a type object of type 'IO::Socket::Async'. ...
in method subscribe at ... (MQTT::Client) line 133
This reports that the write method called on line 133 of MQTT::Client requires its invocant is an instance of type 'IO::Socket::Async'. The value it got was of that type but was not an instance, but instead a "type object". (All values of non-native types are either type objects or instances of their type.).
The error message concludes with:
Did you forget a '.new'?
This is a succinct hint based on the reality that 99 times out of a hundred the reason a type object is encountered when an instance is required is that code has failed to initialize a variable. (One of the things type objects are used for is to serve the role of "undefined" in languages like Perl.)
So, can you see why something that should have been an initialized instance of 'IO::Socket::Async' is instead an uninitialized one?
Footnotes
[1] Raku's constructs for parallelism, concurrency, and asynchrony follow the structured programming paradigm. See Parallelism, Concurrency, and Asynchrony in Raku for Jonathan Worthington's video presentation of this overall approach. Structured constructs like react can cleanly observe, contain, and manage events that occur anywhere within their execution scope, including errors such as error exceptions, even if they happen on other threads.
You seem to be fine now but when I first saw this I made this https://github.com/jonathanstowe/Cro-MQTT which turns the MQTT client into a first class Cro service.
I haven't released it yet but it may be instructive.
i have a problem that i don't exactly know how to solve. I'm implementing an E2E test in which using selenium i need to click in a Link and check that is sending me to the right URL.
Here starts the problem...
There are 3 possibilities, mix of 2 types of links, just one type of link or the other type of link. No problems with the situations in which there are both types of links but when there is just one type when it searches for the identifier we use for the links that are not in page it gives me a timeoutException. This is not a failure because it's a posible situation but i will like to know if there is a way in which to check that if it finds no links it asserts that the exception is thrown.
I though using a runCatching (or try catch) wait for the link to appear and if it doesn't appear the test asserts that when i look for the element the timeout exception is thrown again.
It smells a bit for me this way of doing it and i don't know if it's correctly done.
EDIT: Im ussing AssertK and JUnit5 for testing.
EDIT 2: I've done this, i dont know if it a correct way of doing it
runCatching {
driver.waitFor(numberOfWidgetsToBeMoreThan(BrowserSelector.cssSelector(OFFERS_WITH_PRICE_AND_DATE), 0), ofMillis(2000))
}.onFailure {
assertThrows<WaitTimeoutException> {
findLink(OFFERS_WITH_PRICE_AND_DATE)
}
}.onSuccess {
val widget = findLink(OFFERS_WITH_PRICE_AND_DATE)
widget.click()
assertThat(driver.url).contains(NO_DATE_TEXT)
}
I'm not sure I understood your problem correctly, but you can use assertFails to assert that a piece of code throws an exception:
#Test
fun test() {
val exception = assertFails {
// some code that should throw
}
// some more assertions on the type of exception etc. may go here
}
This code section i am getting error any ideas ?
public IEnumerable<LOBinfo> getLobinfo()
{
// var obj = from n in lobj.LOBinfoes select n;
return lobj.LOBinfoes.Select(m=>m).ToList();
// return obj.ToList();
}
I am not even using USING keyword ?
This issue been for a while and i referred many articles in stackoverflow itself but things looking bad for me .
Thank you for your suggestions
Underlying provider failed to open means, that at some point a database could not be reached, due to wrong connection settings for example or a previous failure etc...
The code you show here is allright, except for the fact you write a useless .Select you can drop the .Select(m=>m) and just leave the .ToList there.
Further, to find out what is causing your crash post the exact error message as well as the inner exception. (and the inner exception's inner exception and so on...)
Next to that the part "lobj.LOBinfoes" is probably a repository or something? You might as well post the code of that and the code of your data access object as well.
I have a dependency with parameters constructor. When I call the action more than 1x, it show this error:
Error activating IValidationPurchaseService
More than one matching bindings are available.
Activation path:
1) Request for IValidationPurchaseService
Suggestions:
1) Ensure that you have defined a binding for IValidationPurchaseService only once.
public ActionResult Detalhes(string regionUrl, string discountUrl, DetalhesModel detalhesModel)
{
var validationPurchaseDTO = new ValidationPurchaseDTO {...}
KernelFactory.Kernel.Bind<IValidationPurchaseService>().To<ValidationPurchaseService>()
.WithConstructorArgument("validationPurchaseDTO", validationPurchaseDTO)
.WithConstructorArgument("confirmPayment", true);
this.ValidationPurchaseService = KernelFactory.Kernel.Get<IValidationPurchaseService>();
...
}
I'm not sure what are you trying to achieve by the code you cited. The error is raised because you bind the same service more than once, so when you are trying to resolve it it can't choose one (identical) binding over another. This is not how DI Container is supposed to be operated. In your example you are not getting advantage of your DI at all. You can replace your code:
KernelFactory.Kernel.Bind<IValidationPurchaseService>().To<ValidationPurchaseService>()
.WithConstructorArgument("validationPurchaseDTO", validationPurchaseDTO)
.WithConstructorArgument("confirmPayment", true);
this.ValidationPurchaseService = KernelFactory.Kernel.Get<IValidationPurchaseService>();
With this:
this.ValidationPurchaseService = new ValidationPurchaseService(validationPurchaseDTO:validationPurchaseDTO, confirmPayment:true)
If you could explain what you are trying to achieve by using ninject in this scenario the community will be able to assist further.
Your KernelFactory probably returns the same kernel (singleton) on each successive call to the controller. Which is why you add a similar binding every time you hit the URL that activates this controller. So it probably works the first time and starts failing after the second time.
What's the best practice for handling exceptions in NHibernate?
I've got a SubjectRepository with the following:
public void Add(Subject subject)
{
using (ISession session = HibernateUtil.CurrentSession)
using (ITransaction transaction = session.BeginTransaction())
{
session.Save(subject);
transaction.Commit();
}
}
And a Unit Test as follows:
[Test]
public void TestSaveDuplicate()
{
var subject = new Subject
{
Code = "En",
Name = "English"
};
_subjectRepository.Add(subject);
var duplicateSubject = new Subject
{
Code = "En",
Name = "English1"
};
_subjectRepository.Add(duplicateSubject);
}
I got to the point of handling the error generated by the unit test and got a bit stuck. This fails as expected, though with a GenericADOException, I was expecting a ConstraintViolationException or something similar (there is a uniqueness constraint on the subject code at database level).
The ADOException wraps a MySQL Exception that has a sensible error message but I don't want to start breaking encapsulation by just throwing the inner exception. Particularly as MySQL isn't finalised as the back end for this project.
Ideally I'd like to be able to catch the exception and return a sensible error to the user at this point. Are there any documented best practice approaches to handling NHibernate Exceptions and reporting back up to the user what went wrong and why?
Thanks,
Matt
I would handle it in the Add method as such:
public void Add(Subject subject)
{
using (ISession session = HibernateUtil.CurrentSession)
using (ITransaction transaction = session.BeginTransaction())
{
try
{
session.Save(subject);
transaction.Commit();
}
catch (Exception ex)
{
transaction.Rollback();
// log exception
throw;
}
}
}
In the catch block, you should first rollback the transaction and log the exception. Then your options are:
Rethrow the same exception, which is what my version does
Wrap it in your own exception and throw that
Swallow the exception by doing nothing, which is very rarely a good idea
You don't have any real options for handling the exception in this method. Assuming that the UI calls this method, it should call it in its own try..catch and handle it by displaying a meaningful error message to the user. You can make your unit test pass by using the ExpectedException(type) attribute.
To answer your question directly, you should create your own "sensible error" by extending Exception and throw that with the original exception as its InnerException. That's the exception wrapping technique I listed in (2).
All the Nhibernate exceptions are non recoverable, you could revisit the design of the app/data layer if you are trying to recover from nhibernate exceptions .
You can also Take a look at spring.net 's exception translation implementaion
Also you manually handling transactions on exceptions is tedious and error prone, take a look at nhibernate's contextual sessions .
Spring.net also has some nice helpers around nhibernate .
The general question is going to be, what do you want to tell the user, and who is the user?
If the user will sometimes be another computer (i.e., this is a web service), then you would want to use the appropriate mechanism to return a SOAP Fault or HTTP error.
If the user will sometimes be a UI of some sort, then you may want to display a message to the user, but what would you tell the user so he can do something about it? For instance, most web sites will say, "sorry, we had an unexpected error", no matter what the reason. That's because there's usually nothing the user could do about the error.
But in either case, the choice of how to tell "the user" is a matter for the Presentation layer (UI tier), not for the DAL. You should possibly wrap exceptions from the DAL in another exception type, but only if you're going to change the message. You don't need your own exception class, unless your callers would do something different if it's a data access exception rather than some other kind.
I'd probably validate the input before saving the object; that way you can implement whatever validation you like (e.g. check the length of the Subject Code as well as the fact that there aren't any duplicates), and pass back meaningful validation errors back to the user.
The logic is as follows; exceptions are used to indicate exceptional circumstances that your program doesn't cater for. A user entering a duplicate Subject Code in your example above is something your program should be catering for; so, rather than handling an exception because a DB constraint gets violated (which is an exceptional event and shouldn't be occurring), you'd want to handle that scenario first and only attempt to save the data when you know that the data you're saving is correct.
The advantage with implementing all validation rules in your DAL is that you can then ensure that the data going into your DB is valid as per your business processes in a consistent manner, rather than relying on the constraints in your DB to catch those for you.