Sebastian Zenker created THRIFT-3593:
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Summary: 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
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Reporter: Sebastian Zenker
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.
service FanController
{
void setTargetTemperature(int t);
int getTargetTemperature();
int getActualTemperature();
int getActualRPM();
}
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.
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);
}
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:
service FanController
{
void setTargetTemperature(int t);
signal void targetTemperatureChanged(int t);
signal void actualTemperatureChanged(int t);
signal actualRPMChanged(int t);
}
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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