As far as I understand, RPC is a client-server model while the client sends some requests to the server side and get some results back. Then, is Java servlet also a kind of RPC which uses HTTP protocol? Am I right?
Here is the very first sentence of the wikipedia article on RPC:
In computer science, a remote procedure call (RPC) is an inter-process communication that allows a computer program to cause a subroutine or procedure to execute in another address space (commonly on another computer on a shared network) without the programmer explicitly coding the details for this remote interaction.1 That is, the programmer writes essentially the same code whether the subroutine is local to the executing program, or remote.
So, Servlets would be an RPC mechanism if you could invoke a servlet from a client using
SomeResult r = someObject.doSomething();
That's not the case at all. To invoke a servlet, you need to explicitely send a HTTP request and encode parameters in the way the servlet expects them, then read and parse the response.
Related
While discussing asynchronous messaging on page 67 of the Microservices Patterns book by Chris Richardson (2019), the author writes:
Synchronous—The client expects a timely response from the service and might even block while it waits.
Asynchronous - The client doesn’t block, and the response, if any, isn’t necessarily sent immediately
Given that, it seems that moving from "synchronous" to "asynchronous" communication actually just swaps one synchronous service (e.g., Service A) with a different synchronous service (e.g., a listening port on the message broker like Active MQ, Kafka, IBM MQ, AWS Kinesis, etc.).
That's because the client, presumably, must still block (or at least use 1 thread or connection from a pool) while communicating with the broker, instead of communicating directly with Service A--especially since the client probably expects a broker response (e.g., SUCCESS) for reliability purposes.
Is that analysis correct?
Yes, your analysis is correct.
Working on your case, the broker's client library provides the asynchronous functionality to the caller code (ServiceA for example), which means that it doesn't block the ServiceA's thread until the operation is finished, but it lets you provide a callback that will be invoked (with the results of the async operation) when it is finished.
Now the question is: who will invoke that callback? Well, some code from the broker's client library, which runs on a thread that presumably does some periodic checks to see if the operation is finished (or any other logic that will eventually emit this result).
So yes, there has to be some background thread that does some synchronous work to grab those results.
if HTTP is connection-less, how does ASP.net response property, HttpResponse.IsClientConnected detect client is connected or not?
HTTP is not "connection-less" - you still need a connection to receive data from the server; more correctly, HTTP is stateless. Applications running on-top of HTTP will most likely actually be stateful, but HTTP itself is not.
"Connectionless" can also refer to a system using UDP as the transport instead of TCP. HTTP primarily runs over TCP and pretty much every real webserver expects, and returns, TCP messages instead of UDP. You might see HTTP-like traffic in UDP-based protocols like UPnP, but because you want your webpage to be delivered reliably, TCP will always be used instead of UDP.
As for IsClientConnected, when you access that property it calls into the current HttpWorkerRequest which is an abstract class implemented by the current host environment.
IIS7+ implements it such that if it previously received a TCP disconnect message (that sets a field) the method would now return false.
The ISAPI implementation (IIS 6) instead calls into a function within IIS that informs the caller if the TCP client on the current request/response context is still connected, though presumably it works on the same basis: when the webserver receives a TCP timeout, disconnect or connection-reset message it sets a flag and lets execution continue instead of terminating the response-generator thread.
Here's the relevant source code:
HttpResponse.IsClientConnected: http://referencesource.microsoft.com/#System.Web/HttpResponse.cs,80335a4fb70ac25f
IIS7WorkerRequest.IsClientConnected: http://referencesource.microsoft.com/#System.Web/Hosting/IIS7WorkerRequest.cs,1aed87249b1e3ac9
ISAPIWorkerRequest.IsClientConnected: http://referencesource.microsoft.com/#System.Web/Hosting/ISAPIWorkerRequest.cs,f3e25666672e90e8
It all starts with an HTTP request. Inside it, you can, for example, spawn worker threads, that can outlive the request itself. Here is where IsClientConnected comes in handy, so that the worker thread knows that the client has already received the response and disconnected or not.
From the introduction on gRPC:
In gRPC a client application can directly call methods on a server application on a different machine as if it was a local object, making it easier for you to create distributed applications and services. As in many RPC systems, gRPC is based around the idea of defining a service, specifying the methods that can be called remotely with their parameters and return types. On the server side, the server implements this interface and runs a gRPC server to handle client calls. On the client side, the client has a stub that provides exactly the same methods as the server.
The above paragraph talks about a client and a server, with the former being the one who is invoking methods to the other. What am I wondering is: can the server-end of the connection invoke methods that have been registered on the client?
No, a server cannot invoke calls on the client. gRPC works with HTTP, and HTTP has not had such semantics in the past.
There has been discussion as to various ways to achieve such a feature, but I'm unaware of any work having started or general agreement on a design. gRPC does support bidirectional streaming, which may get you some of what you need. With bidirectional streaming the client can respond to messages from server, but the client still calls the server and only one type of message can be sent for that call.
The protocol does not implement it, but you may pretend this situation.
Define a server method that returns a stream of a ServerRequest message:
import "google/protobuf/any.proto";
service FullDuplex {
rpc WaitRequests (google.protobuf.Any) returns (stream ServerRequest);
}
message ServerRequest {
float someValue = 1;
float anotherAnother = 1;
}
ServerRequest may be an Oneof, so your may receive different types of server requests.
If you need that your client sends back a response for each request, you may create a stream from your client to the server, but you will need to implement a logic in your server side that triggers a timeout waiting for that response.
service FullDuplex {
rpc WaitRequests (stream ClientResponse) returns (stream ServerRequest);
}
What you can do is start a HTTP server in both processes and use clients at each end to initiate communication. There's a bit of boilerplate involved and you have to design a simple handshaking protocol (one end registers with the other, advertising its listen address) but it's not too much work.
I'm having issues with the NetworkAccessManager.get method. When i make two http-connections, the second connection fails with the error "99: The bound addres is already in use".
I start the second connection in the finish-slot of the first connection. Maybe multiple async http-connections are not supported on BB-10?
Doest anyone got the same error?
In essence you should only be using a single instance of the NetworkAccessManager but passing multiple requests through it. The documentation (http://developer.blackberry.com/cascades/reference/qnetworkaccessmanager.html) specifies the following:
One QNetworkAccessManager should be enough for the whole Qt
application.
...
QNetworkAccessManager has an asynchronous API. When the replyFinished slot above is called, the parameter it takes is the QNetworkReply object containing the downloaded data as well as meta-data (headers, etc.).
...
Note: QNetworkAccessManager queues the requests it receives. The number of requests executed in parallel is dependent on the protocol. Currently, for the HTTP protocol on desktop platforms, 6 requests are executed in parallel for one host/port combination.
So basically what you should be doing is sending multiple requests through the same NetworkAccessManager and then handling the response based on the meta-data. The NetworkAccessManager will handle the async processing for you.
I don't get one thing in RMI. It's a bit confusing actually.
On client side, we have the business interface (Hello.class), the client code (HelloClient.class) and the remote stub (probably Hello_stub.class) and on server side we have the server code (HelloImpl.class), the business interface (Hello.class) and the skeleton .
For Java 5 onwards, we don't create stubs but still they are c=in picture i believe.
So, how does the communication happen ?
The client calls method on Hello.class which then calls Hello_stub.class for all n/w operations. The Hello_stub.class calls the skeleton which then calls Hello.class and then calls methods on HelloImpl.class ?
I am a bit confused after reading Head first EJB :) .It would be glad if someone clarified it.
When the stub's method is called:
It gets a TCP connection to s target out of the client connection pool, or creates one if there isn't a pooled connection
Bundles up the call and the arguments into a serializable object.
Writes the object to the connection along with some other stuff like a JRMP protocol header and a remote objectID.
Reads the reply object from the connection.
Returns the connection to the pool, where it gets closed after a certain idle time.
If the reply object is an exception, throws it.
Otherwise returns the reply object as the method result.
At the server, a thread sits on the listening socket, accepting connections, creating threads, and dispatching incoming remote calls to the correct remote object via the specified object ID.
This is done via reflection. RMI skeletons haven't been used since 1998, except in the case of stubs you deliberately generate with rmic -v1.1, but the principle is the same either way.