When calling singleRequest, how can one customize the execution context that is used by the connection pool?
I took a brief look at the code, and a call to singleRequest results in a message being sent to the PoolMasterActor, which in turn sends a message to the pool interface actor.
Is each connection blocking or non-blocking?
Which context is used for the connection pool? (I want to make sure that my HTTP requests don't block all the threads)
If you check out the singleRequest signature, it requires an implicit Materializer (and therefore an ActorSystem and its dispatchers) to run the underlying HTTP infrastructure - which is based on Akka Streams. More knowledge on how materializers spawn threads under-the-hood can be found in the docs, and this blogpost.
Going back to your questions:
The whole Akka-HTTP infrastructure is inherently non-blocking (as it's based on Akka Streams - which adheres to the Reactive Streams spec and is based on Akka Actors).
The threading used by the singleRequest call inherits from the ActorSystem dispatcher used down the line. Unless you do anything specific, you will end up using your system's default dispatcher. This is reasonable choice in many cases when you are writing an Akka HTTP client.
In case you really need your materializer to use a custom dispatcher you can achieve this by customizing your ActorMaterializerSettings, e.g.
implicit val materializer = ActorMaterializer(
ActorMaterializerSettings(actorSystem).withDispatcher("my-custom-dispatcher")
)
Related
Basic question: How does Spring Reactors WebClient achieve non blocking when compared to RestTemplate? Doesn't it have to block somewhere after it has dispatched the request to the external service (for example)? HTTP by nature is synchronous right? So the calling application has to wait for the response?
How does the thread know the context to react upon the response from the service?
There are several separate questions here.
How I/O operations are managed?
What's the threading model behind this runtime?
How does the application deal with the request/response model behind HTTP?
In the case of WebClient and project Reactor, the Netty event loop is used to queue/dispatch/process events. Each read/write operation is done is a non-blocking manner, meaning that no thread sits waiting for an I/O operation to complete. In this model, concurrency is not done through thread pools, but there's a small number of threads that process unit of work which should never block.
From a pure HTTP standpoint (i.e. if you were capturing the HTTP packets on the network), you'd see no big difference between a RestTemplate and a WebClient call. The HTTP transport itself doesn't support the backpressure concept. So the client still has to wait for the response - the difference here is that the application using that WebClient won't waste resources on waiting for that operation to complete - it will use them to process other events.
For more information on that, please check out the reactive programming introduction in the Reactor reference documentation and this talk given by Rossen Stoyanchev that explains things well if you're used to the typical Servlet container model.
I have some problem so can you help me. Is instance of AmqpTemplate class from RabbitMQ ( implementation of AMQP protocol) thread safe. Can it be accessed from multiple threads?
Thanks
AmqpTemplate is the interface, and RabbitTemplate is the implementation, and I assume by "thread-safe" you mean that its send/receive/sendAndReceive methods may be used concurrently. If so, then YES. The only state it maintains within instance variables are "converter" strategies for the Message and MessageProperties as well as default Exchange, Queue, and Routing Key settings (which are not even used if you invoke the methods that take those as arguments instead), and all of those are typically configured one time initially (e.g. via dependency injection). The template does not maintain any non-local state for any particular operation at runtime. With AMQP, the "Channel" is the instance that can only be used by one thread at a time, and the RabbitTemplate manages that internally such that each operation is retrieving a Channel to use within the scope of that operation. Multiple concurrent operations therefore lead to multiple instances of Channel being used, but that is not something you need to be worried about as an end-user of the template.
Hope that helps.
-Mark
In the project I'm currently working we're using WCF.
Company policy forces us to use async calls and the reason should be security.
I've asked why this is so much more secure but I don't get clear answers.
Can someone explain why this is so much secure?
They are not. The same security (authentication, encryption) mechanisms and considerations apply whether a call blocks until it gets a response or it uses a callback.
The only way someone may be confused into thinking that asynch calls are more "safe/secure", is they think that unhandled WCF exceptions will not bring down the main thread if they are asynchronous, as they will be raised inside the callback.
In this case, I would advice extreme caution when approaching the owner of this policy to avoid career-limiting consequences. Some people can get emotionally attached to their policies.
There is no point why an async call will be more secure than a sync call. I think you should talk to the owner of the policy for the same.
No they are not more or less secure than synchronous calls. The only difference is the client waits for a response on synchronous calls, whereas on async it is notified of a response.
Are they coming from the angle that synchronous calls leave the connection open longer or something?
Just exposing a WCF operation using an async signature (BeginBlah/EndBlah) doesn't actually affect the exposed operation at all. When you view the meta data, an operation like
[OperationContract(AsyncPattern=true)]
IAsyncResult BeginSomething(AsyncCallback, object)
void EndSomething(IAsyncResult)
...actually still ends up being represented as an operation called 'Something'. And actually this is one of the nice things about WCF: the client and server can differ in whether they choose to implement/consume an operation syncronously.
So if you are using generating WCF proxies (eg through Add Service Reference) then you will get syncronous versions of each operation whether they are implemented asyncronously or not unless you tick the little checkbox to generate the async overloads. And when you do you then get async versions of operations that might only be declared syncronously on the server.
All WCF is doing is, on both the client and server, giving you a choice about your threading model: do you want WCF to wait for the result, or are you going to signal it that you've finished. How the actual transport connection is managed is - to the best of my knowlege - totally unaffected. eg: For a NetTcpBinding the socket still stays open for the duration of the call, either way.
So, to get to the point, I really struggle to imagine how this could possibly make any difference to the security envelope of a WCF service. If a service is exposed using an async pattern, and is genuinely implemented in an async way (async for outbound IO, or queues work via the thread pool or something) then there's probably an argument that it would be harder to DOS the service (by exhausting the pool of WCF IO threads), but that'd be about it.
See Syncronous and Asyncronous Operations in MSDN
NB: If you are sharing the contract interface between the client and server then obviously the syncronisity of the two ends match (because they are both using the same interface type), but that's just a limitation of using a shared interface. If you made another equivilent interface, differing only by the async pattern, you could still create a ChannelFactory against it just fine.
I agree with the other answers - definitely not more secure.
Fire up Fiddler and watch a synchronous request vs. an asynchronous request. You'll basically see the same type of traffic (although the sync may send and receive more data since it's probably a postback). But you can intercept both of those requests, manipulate them, and resend them and cause havoc on your server.
Fiddler's a great tool, by the way. It's an eye-opener in terms of what kind of data and how much data you're sending to the server.
Using techniques as hinted at in:
http://msdn.microsoft.com/en-us/library/system.servicemodel.servicecontractattribute.callbackcontract.aspx
I am implementing a ServerPush setup for my API to get realtime notifications from a server of events (no polling). Basically, the Server has a RegisterMe() and UnregisterMe() method and the client has a callback method called Announcement(string message) that, through the CallbackContract mechanisms in WCF, the server can call. This seems to work well.
Unfortunately, in this setup, if the Server were to crash or is otherwise unavailable, the Client won't know since it is only listening for messages. Silence on the line could mean no Announcements or it could mean that the server is not available.
Since my goal is to reduce polling rather than immediacy, I don't mind adding a void Ping() method on the Server alongside RegisterMe() and UnregisterMe() that merely exists to test connectivity of to the server. Periodically testing this method would, I believe, ensure that we're still connected (and also that no Announcements have been dropped by the transport, since this is TCP)
But is the Ping() method necessary or is this connectivity test otherwise available as part of WCF by default - like serverProxy.IsStillConnected() or something. As I understand it, the channel's State would only return Faulted or Closed AFTER a failed Ping(), but not instead of it.
2) From a broader perspective, is this callback approach solid? This is not for http or ajax - the number of connected clients will be few (tens of clients, max). Are there serious problems with this approach? As this seems to be a mild risk, how can I limit a slow/malicious client from blocking the server by not processing it's callback queue fast enough? Is there a kind of timeout specific to the callback that I can set without affecting other operations?
Your approach sounds reasonable, here are some links that may or may not help (they are not quite exactly related):
Detecting Client Death in WCF Duplex Contracts
http://tomasz.janczuk.org/2009/08/performance-of-http-polling-duplex.html
Having some health check built into your application protocol makes sense.
If you are worried about malicious clients, then add authorization.
The second link I shared above has a sample pub/sub server, you might be able to use this code. A couple things to watch out for -- consider pushing notifications via async calls or on a separate thread. And set the sendTimeout on the tcp binding.
HTH
I wrote a WCF application and encountered a similar problem. My server checked clients had not 'plug pulled' by periodically sending a ping to them. The actual send method (it was asynchronous being a server) had a timeout of 30 seconds. The client simply checked it received the data every 30 seconds, while the server would catch an exception if the timeout was reached.
Authorisation was required to connect to the server (by using the built-in feature of WCF that force the connecting person to call a particular method first) so from a malicious client perspective you could easily add code to check and ban their account if they do something suspicious, while disconnecting users who do not authenticate.
As the server I wrote was asynchronous, there wasn't any way to really block it. I guess that addresses your last point, as the asynchronous send method fires off the ping (and any other sending of data) and returns immediately. In the SendEnd method it would catch the timeout exception (sometimes multiple for the client) and disconnect them, without any blocking or freezing of the server.
Hope that helps.
You could use a publisher / subscriber service similar to the one suggested by Juval:
http://msdn.microsoft.com/en-us/magazine/cc163537.aspx
This would allow you to persist the subscribers if losing the server is a typical scenario. The publish method in this example also calls each subscribers on a separate thread, so a few dead subscribers will not block others...
Few methods in my WCF service are quite time taking - Generating Reports and Sending E-mails.
According to current requirement, it is required so that Client application just submits the request and then do not wait for the whole process to complete. It will allow user to continue doing other operations in client applications instead of waiting for the whole process to finish.
I am in a doubt over which way to go:
AsyncPattern = true OR
IsOneWay=true
Please guide.
It can be both.
Generally I see no reason for WCF operation to not be asynchronous, other than developer being lazy.
You should not compare them, because they are not comparable.
In short, AsyncPattern=True performs asynchronous invocation, regardless of whether you're returning a value or not.
OneWay works only with void methods, and puts a lock on your thread waiting for the receiver to ack it received the message.
I know this is an old post, but IMO in your scenario you should be using IsOneWay on the basis that you don't care what the server result is. Depending on whether you need to eventually notify the client (e.g. of completion or failure of the server job) then you might also need to look at changing the interface to use SessionMode=required and then using a Duplex binding.
Even if you did want to use asynchronous 2-way communication because your client DID care about the result, there are different concepts:
AsyncPattern=true on the Server - you would do this in order to free up server resources, e.g. if the underlying resource (?SSRS for reporting, Mail API etc) supported asynchronous operations. But this would benefit the server, not the client.
On the client, you can always generate your service reference proxy with "Generate Asynchronous Operations" ticked - in which case your client won't block and the callback will be used when the operation is complete.