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https://issues.apache.org/jira/browse/THRIFT-3593?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15367605#comment-15367605
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Sebastian Zenker commented on THRIFT-3593:
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In my opinion, "push notifications" should become part of the IDL. Most modern
programming languages support some kind of subject-observer pattern. Either it
is directly build-in into the language itself (e.g. C#'s event keyword) or can
be added by using a library (e.g. boost::signal2 or Qt for C++). Conceptually,
what a user wants is:
* Notify ALL clients about some changes and not just one client. E.g. if
something happens asynchronously at the server (e.g. out of disk space), the
server wants to inform all clients.
* All notifications are usually "oneway" methods which return nothing. If such
a notification would return something back to the server, the server needs some
strategy, on how all received return values (which may differ) would be merged
into one return value. But in 99% of all cases the server isn't interested in
such a return value. Supporting blocking calls in both directions may also lead
to dead-locks in case both parties (client and server) call a blocking method
at the same time.
* For those notifications the user probably wants to define them in one service
definition instead of creating two services (the "real" service + a second for
just the notifications) which are logically one service
I also have concerns with using threads to handle multiplexing the
communication in both directions, as it may can lead to race conditions in case
a "push notification" surpasses a response message. Using an addition thread
per connection probably also causes troubles when we have a server with more
than 10k clients.
> Enable server-side to issue requests to clients on the same connection (push
> notifications)
> -------------------------------------------------------------------------------------------
>
> Key: THRIFT-3593
> URL: https://issues.apache.org/jira/browse/THRIFT-3593
> Project: Thrift
> Issue Type: Wish
> Components: AS3 - Compiler, AS3 - Library, C glib - Compiler, C glib
> - Library, C# - Compiler, C# - Library, C++ - Compiler, C++ - Library, Cocoa
> - Compiler, Cocoa - Library, Compiler (General), D - Compiler, D - Library,
> Dart - Compiler, Dart - Library, Delphi - Compiler, Delphi - Library,
> Documentation, Erlang - Compiler, Erlang - Library, Go - Compiler, Go -
> Library, Haskell - Compiler, Haskell - Library, Haxe - Compiler, Haxe -
> Library, Java - Compiler, Java - Library, JavaScript - Compiler, JavaScript -
> Library, Lua - Compiler, Lua - Library, Node.js - Compiler, Node.js -
> Library, Perl - Compiler, Perl - Library, PHP - Compiler, PHP - Library,
> Python - Compiler, Python - Library, Ruby - Compiler, Ruby - Library,
> Smalltalk - Compiler, Smalltalk - Library, Swift - Compiler, Test Suite,
> Tutorial
> Reporter: Sebastian Zenker
> Labels: push
>
> In our applications, we have very often the use case, that we actively want
> to inform all connected Thrift clients about state changes on the server
> side. Let me use a stupid example to explain what I whish. Let's assume we
> have service which represents a fan controller. This service allows to
> configure a target temperature and can be requested for the actual
> temperature and actual RPM.
> {code}
> service FanController
> {
> void setTargetTemperature(int t);
> int getTargetTemperature();
> int getActualTemperature();
> int getActualRPM();
> }
> {code}
> Our client application allows the user to set the target temperature and
> display the actual temperature and RPM.
> To implement such an application, we currently have two options when using
> the Thrift framework:
> 1.) Every client requests the actual temperature and RPM once per second.
> With other words: every client implements polling.
> 2.) We split service FanController into two different Thrift services. One
> which allows to configure the fan controller and a second one which is used
> by the server to notify all its clients about state changes. The first one is
> implemented by the "real" server and the second one is implemented by all
> clients and consists of some oneway methods only. So from a Thrift point of
> view, both sides are server & client. E.g.
> {code}
> service FanController
> {
> void setTargetTemperature(int t);
> int getTargetTemperature();
> void RegisterEvents(string hostname, int port); //use to tell the server,
> that it should establish a connection to hostname+port which implements
> FanControllerEvents
> void UnregisterEvents(string hostname, int port);
> }
> service FanControllerEvents
> {
> oneway void targetTemperatureChanged(int t);
> oneway void actualTemperatureChanged(int t);
> oneway void actualRPMChanged(int rpm);
> }
> {code}
> Both approaches have massive drawbacks. I think it is not worth the effort to
> explain why solution #1 (polling) sucks. But also solution #2 doesn't work
> well, because:
> * It requires every client to register its FanControllerEvents service at
> the server side by using FanController::RegisterEvents(). This doesn't work,
> in case the client resides behind a NAT-router, because so the "real" server
> cannot establish a TCP connection to the client.
> * It always requires at least two TCP connections which makes firewall
> configurations more complex.
> * The "real" server needs to maintain a list with all connected clients in
> the application logic. In case the actual RPM or temperature changes, the
> server needs to iterate over the list of all connected clients and call the
> corresponding function. Maintaining the list in the application logic adds
> extra complexity at the server side, which can be avoided and may be better
> part of the Thrift framework.
> * How to handle the case, if only 1 of the 2 TCP connections gets
> interrupted?
> * The fan controller service - which is logically one thing - gets splitted
> into two Thrift services: FanController + FanControllerEvents which decreases
> readability of the IDL file.
> To solve such a use case, my recommendation is the following: Add a new
> keyword like "signal" to the IDL language. Wouldn't it be cool to be able to
> define something like:
> {code}
> service FanController
> {
> void setTargetTemperature(int t);
> signal void targetTemperatureChanged(int t);
> signal void actualTemperatureChanged(int t);
> signal actualRPMChanged(int t);
> }
> {code}
> E.g. DBus (a IPC framework very often used in Linux environments) allows to
> specify signals in their interfaces. See also:
> http://dbus.freedesktop.org/doc/dbus-tutorial.html#signalprocedure
> It's a very intrusive wish, as it will effect all code generators and runtime
> libraries. What do you think?
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