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https://issues.apache.org/jira/browse/THRIFT-3593?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15364803#comment-15364803
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Kyle Johnson commented on THRIFT-3593:
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In principle this seems like a good idea, but all sorts of issues creep in when
attempting to codify bidirectional communication. If the server is sending an
event back to a client, does the server still respond to function calls? What
about in a threaded server? How does the threaded server know which client to
send an event to?
In my mind, the primary concern of defining two thrift services as a workaround
to the lack of bidirectional communication is the fact that it's not firewall
friendly. But why not do something akin to passive FTP, namely where the new
server (the original client and event recipient) actually connects with the
client (the original server and event generator)? Essentially this would just
be a new transport over which client and server would communicate.
This gets around the firewall issue *and* it keeps things simple so that we can
define the desired behavior in the bidirectional communication. Furthermore,
no servers, handlers or processors need to be changed as this could all be
handled at the transport level.
Thoughts?
> Add new IDL keyword 'signal' to be able that a server can actively send
> messages to all its connected clients to prevent polling
> --------------------------------------------------------------------------------------------------------------------------------
>
> 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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