Multithreading and parallel processes with c++ - c++

Question

I would like to create a c++ webserver that will perform a task for each user that lands on my website. Since the task might be computationally heavy (for now just a long sleep), I'd like to handle each user on a different thread. I'm using mongoose to set up a webserver.
The different processes (in my code below just one, aka server1) are set up correctly and seem to function correctly. However, the threads seem to be queuing one after the other so if 2 users hit the end point, the second user must wait until the first user finishes. What am I missing? Do the threads run out of scope? Is there a "thread-manager" that I should be using?
#include "../../mongoose.h"
#include <unistd.h>
#include <iostream>
#include <stdlib.h>
#include <thread>
//what happens whenever someone lands on an endpoint
void myEvent(struct mg_connection *conn){
//long delay...
std::thread mythread(usleep, 2*5000000);
mythread.join();
mg_send_header(conn, "Content-Type", "text/plain");
mg_printf_data(conn, "This is a reply from server instance # %s",
(char *) conn->server_param);
}
static int ev_handler(struct mg_connection *conn, enum mg_event ev) {
if (ev == MG_REQUEST) {
myEvent(conn);
return MG_TRUE;
} else if (ev == MG_AUTH) {
return MG_TRUE;
} else {
return MG_FALSE;
}
}
static void *serve(void *server) {
for (;;) mg_poll_server((struct mg_server *) server, 1000);
return NULL;
}
int main(void) {
struct mg_server *server1;
server1 = mg_create_server((void *) "1", ev_handler);
mg_set_option(server1, "listening_port", "8080");
mg_start_thread(serve, server1);
getchar();
return 0;
}

Answer 1

Long running requests should be handled like this:
static void thread_func(struct mg_connection *conn) {
sleep(60); // simulate long processing
conn->user_data = "done"; // Production code must not do that.
// Other thread must never access connection
// structure directly. This example is just
// for demonstration.
}
static int ev_handler(struct mg_connection *conn, enum mg_event ev) {
switch (ev) {
case MG_REQUEST:
conn->user_data = "doing...";
spawn_thread(thread_func, conn);
return MG_MORE; // Important! Signal Mongoose we are not done yet
case MG_POLL:
if (conn->user_data != NULL && !strcmp(conn->user_data, "done")) {
mg_printf(conn, "HTTP/1.0 200 OK\n\n Done !");
return MG_TRUE; // Signal we're finished. Mongoose can close this connection
}
return MG_FALSE; // Still not done

Answer 2

Caveat: I'm not familiar with mongoose
My assumptions:
The serve function is polling for incoming connections
If the thread executing mg_poll_server is the same thread that triggers the call to ev_handler then your problem is the fact that ev_handler calls myEvent which starts a long running operation and blocks the thread (i.e., by calling join). In this case you're also blocking the thread which is handling the incoming connections (i.e., A subsequent client must wait for the first client to finish their work), which seems is the behavior you describe seeing.
I'm not sure what the real task is supposed to do so I can't say for sure how you should fix this. Perhaps in your use-case it may be possible to call detach otherwise you might keep track of executing threads and defer calling join on them until the server is shutdown.

Answer 3

James Adkison is absolutely right. So, if instead the beginning of the code looks like this:
void someFunc(struct mg_connection *conn){
usleep(2*5000000);
std::cout << "hello!" << std::endl;
std::cout<< "This finished from server instance #"<<conn<<std::endl;
mg_send_header(conn, "Content-Type", "application/json");
mg_printf_data(conn, "{\"message\": \"This is a reply from server instance # %s\"}",
// (char *) conn->server_param);
}
void myEvent(struct mg_connection *conn){
std::thread mythread(someFunc,conn);
mythread.detach();
std::cout<< "This is a reply from server instance #"<<(char *) conn->server_param<<std::endl;
}
static int ev_handler(struct mg_connection *conn, enum mg_event ev) {
if (ev == MG_REQUEST) {
myEvent(conn);
return MG_TRUE;
} else if (ev == MG_AUTH) {
//.... exactly as before
//....
then the program works. Basically the difference is replacing .join() with .detach(). someFunc is running now in parallel for 2 users -- so that's great!. Thanks!