When calling service::run() on a service instance, a loop starts (which mainly consists in polling the open ZMQ sockets as we saw earlier). The loop keeps running until a call to service::stop() is made for the instanciated service.
As you probably have figured out, service::run() is blocking. Therefore n3rv::service has another running method called run_async(), which starts the running loop inside a dedicated thread.
There will be many situations where you will want to add extra code in a service’s running loop, and for that n3rv provides 2 distinct mechanisms:
Probably the simplest method, n3rv::service defines the hkloop() virtual method, that is called for every iteration of the main loop. All you have to do then is to override hkloop() in your service-inherited class definition and put inside the code you want to be executed in the main loop.
class example: public n3rv::service {
using n3rv::service::service;
public:
/* We override the hkloop() virtual method.*/
void hkloop() {
/* Put HERE the code you want in
running loop*/
}
}
A more elaborate way to insert code in the main loop consists in using the service::register_main() method.
class example: public n3rv::service {
using n3rv::service::service;
public:
/* We declare a new loop callback */
static void* loop_callback(void* objref) {
example* self = static_cast<example*>(objref);
/* Put HERE the code you want in
running loop*/
}
int initialize() {
//We register the callback to be run inside main loop.
this->register_main("00_loop_callback",example::loop_callback);
}
}
With this method, all the registered callbacks will be executed sequencially at every iteration of the main loop. The execution order is determined by the callbabck identifier (cbid), so if this matters to you i suggest that you use callback identifiers like 00_name, 01_name,..
The main advantage of this method is that you can dynamically unregister a callback at any time with the help of service::unregister_main(const char* cbid), or register new ones.
Both machanisms are usable together. hkloop() gets executed first, then the registered loop callbacks in the order you chose. Nevertheless to avoid confusion and errors, i suggest that you stick to only one of these, following your needs.
You may discover that your main loop is not running as fast as you’d want: it may be because of the the value of poll_timeout. if this is the case you might consider lowering it.
Now a final warning: If you want to save yourself a lot of troubles and repeated headaches, don’t put blocking instructions in your main loop’s code. That would potentially result in horrible perf loss and unexpected behaviours.
class example: public n3rv::service {
using n3rv::service::service;
public:
/* We override the hkloop() virtual method.*/
void hkloop() {
//You're doing it wrong !
while(1) {}
}
}