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Posted to dev@thrift.apache.org by "James E. King III (JIRA)" <ji...@apache.org> on 2019/01/11 22:27:00 UTC
[jira] [Updated] (THRIFT-3593) Enable server-side to issue requests
to clients on the same connection (push notifications)
[ https://issues.apache.org/jira/browse/THRIFT-3593?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
James E. King III updated THRIFT-3593:
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> 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: Wish List
> Reporter: Sebastian Zenker
> Priority: Critical
> 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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