How to use Pub/sub with hiredis in C++? - redis

Question

I am trying to test this pub/sub function of redis with hiredis client via c++.
I can see that subscribing to certain channel seems to be easy enough to do through redisCommand Api.
However I am wondering how the reply is coming back when somebody publish to the certain server.
Thank you

Answer 1

https://github.com/redis/hiredis/issues/55
aluiken commented on Mar 2, 2012
void onMessage(redisAsyncContext *c, void *reply, void *privdata) {
redisReply *r = reply;
if (reply == NULL) return;
if (r->type == REDIS_REPLY_ARRAY) {
for (int j = 0; j < r->elements; j++) {
printf("%u) %s\n", j, r->element[j]->str);
}
}
}
int main (int argc, char **argv) {
signal(SIGPIPE, SIG_IGN);
struct event_base *base = event_base_new();
redisAsyncContext *c = redisAsyncConnect("127.0.0.1", 6379);
if (c->err) {
printf("error: %s\n", c->errstr);
return 1;
}
redisLibeventAttach(c, base);
redisAsyncCommand(c, onMessage, NULL, "SUBSCRIBE testtopic");
event_base_dispatch(base);
return 0;
}

Answer 2

This is a late answer, but you can try redis-plus-plus, which is based on hiredis, and written in C++ 11.
Disclaimer: I'm the author of this library. If you have any problem with this client, feel free to let me know. If you like it, also feel free to star it :)
Sample code:
Redis redis("tcp://127.0.0.1:6379");
// Create a Subscriber.
auto sub = redis.subscriber();
// Set callback functions.
sub.on_message([](std::string channel, std::string msg) {
// Process message of MESSAGE type.
});
sub.on_pmessage([](std::string pattern, std::string channel, std::string msg) {
// Process message of PMESSAGE type.
});
sub.on_meta([](Subscriber::MsgType type, OptionalString channel, long long num) {
// Process message of META type.
});
// Subscribe to channels and patterns.
sub.subscribe("channel1");
sub.subscribe({"channel2", "channel3"});
sub.psubscribe("pattern1*");
// Consume messages in a loop.
while (true) {
try {
sub.consume();
} catch (...) {
// Handle exceptions.
}
}
Check the doc for detail.

Answer 3

Observer pattern is what we see in pub/sub feature of Redis.
All subscribers are observers and subject is the channel which is being modified by publishers.
When a publisher modifies a channel i.e. executes command like redis-cli> publish foo value
This change is communicated by Redis server to all observers (i.e. subscribers)
So Redis server has list of all observers for a particular channel.