Does node.js use threads/thread pool internally?

Question

I've decided to familiarize myself with node.js and have read a several articles on the subject. What remained unclear to me is if node.js creates new threads and/or schedules tasks on threads from a thread pool when you call node.js functions.

For example if I call fs.readFile is it executed on a different thread?

If yes, [how] can I write my own function readFileCustomized or doLongOperation to run on a different thread?

Answer 1

There is no async API for file operations so node.js uses a thread pool for that. You can see it in the code of libuv.

The pool can run 4 threads:

static uv_thread_t default_threads[4];

Blocking FS tasks are posted with uv__work_submit. For example, here's how read is implemented:

int uv_fs_read(uv_loop_t* loop, uv_fs_t* req,
               uv_file file,
               void* buf,
               size_t len,
               int64_t off,
               uv_fs_cb cb) {
  INIT(READ);
  req->file = file;
  req->buf = buf;
  req->len = len;
  req->off = off;
  POST;
}

...

#define POST                                                                  \
  do {                                                                        \
    if ((cb) != NULL) {                                                       \
      uv__work_submit((loop), &(req)->work_req, uv__fs_work, uv__fs_done);    \
      return 0;                                                               \
    }                                                                         \
    else {                                                                    \
      uv__fs_work(&(req)->work_req);                                          \
      uv__fs_done(&(req)->work_req, 0);                                       \
      return (req)->result;                                                   \
    }                                                                         \
  }                                                                           \
  while (0)

If you want to implement your own threads, you can check this great introduction.