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60 lines
2.8 KiB
Markdown
60 lines
2.8 KiB
Markdown
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# Mongoose Internals
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Mongoose is multithreaded web server. `mg_start()` function allocates
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web server context (`struct mg_context`), which holds all information
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about web server instance:
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- configuration options. Note that mongoose makes internal copies of
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passed options.
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- SSL context, if any
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- user-defined callbacks
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- opened listening sockets
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- a queue for accepted sockets
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- mutexes and condition variables for inter-thread synchronization
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When `mg_start()` returns, all initialization is quaranteed to be complete
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(e.g. listening ports are opened, SSL is initialized, etc). `mg_start()` starts
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two threads: a master thread, that accepts new connections, and several
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worker threads, that process accepted connections. The number of worker threads
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is configurable via `num_threads` configuration option. That number puts a
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limit on number of simultaneous requests that can be handled by mongoose.
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When master thread accepts new connection, a new accepted socket (described by
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`struct socket`) it placed into the accepted sockets queue,
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which has size of 20 (see [code](https://github.com/cesanta/mongoose/blob/3892e0199e6ca9613b160535d9d107ede09daa43/mongoose.c#L486)). Any idle worker thread
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can grab accepted sockets from that queue. If all worker threads are busy,
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master thread can accept and queue up to 20 more TCP connections,
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filling up the queue.
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In the attempt to queue next accepted connection, master thread blocks
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until there is space in a queue. When master thread is blocked on a
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full queue, TCP layer in OS can also queue incoming connection.
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The number is limited by the `listen()` call parameter on listening socket,
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which is `SOMAXCONN` in case of Mongoose, and depends on a platform.
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Worker threads are running in an infinite loop, which in simplified form
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looks something like this:
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static void *worker_thread() {
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while (consume_socket()) {
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process_new_connection();
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}
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}
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Function `consume_socket()` gets new accepted socket from the mongoose socket
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queue, atomically removing it from the queue. If the queue is empty,
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`consume_socket()` blocks and waits until new sockets are placed in a queue
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by the master thread. `process_new_connection()` actually processes the
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connection, i.e. reads the request, parses it, and performs appropriate action
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depending on a parsed request.
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Master thread uses `poll()` and `accept()` to accept new connections on
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listening sockets. `poll()` is used to avoid `FD_SETSIZE` limitation of
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`select()`. Since there are only a few listening sockets, there is no reason
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to use hi-performance alternatives like `epoll()` or `kqueue()`. Worker
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threads use blocking IO on accepted sockets for reading and writing data.
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All accepted sockets have `SO_RCVTIMEO` and `SO_SNDTIMEO` socket options set
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(controlled by `request_timeout_ms` mongoose option, 30 seconds default) which
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specify read/write timeout on client connection.
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