How the single threaded non blocking IO model works in Node js

Question

I m not a Node programmer  but I m interested in how the single threaded non blocking IO model works  After I read the article understanding-the-node-js-event-loop  I m really confused about it  It gave an example for the model   c query      SELECT SLEEP 20        function  err  results  fields         if  err           throw err              res writeHead 200    Content-Type    text html          res end   lt html gt  lt head gt  lt title gt Hello lt  title gt  lt  head gt  lt body gt  lt h1 gt Return from async DB query lt  h1 gt  lt  body gt  lt  html gt          c end               Que  When there are two request A comes first  and B since there is only a single thread  the server-side program will handle the request A firstly  doing SQL querying is asleep statement standing for I O wait  And The program is stucked at the I O waiting  and cannot execute the code which renders the web page behind  Will the program switch to request B during the waiting  In my opinion  because of the single thread model  there is no way to switch one request from another  But the title of the example code says that everything runs in parallel except your code        P S I m not sure if I misunderstand the code or not since I have   never used Node  How Node switch A to B during the waiting  And can   you explain the single threaded non blocking IO model of Node in a   simple way  I would appreciate it if you could help me

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if you read a bit further -  Of course  on the backend  there are threads and processes for DB access and process execution  However  these are not explicitly exposed to your code  so you can   t worry about them other than by knowing that I O interactions e g  with the database  or with other processes will be asynchronous from the perspective of each request since the results from those threads are returned via the event loop to your code    about -  everything runs in parallel except your code  - your code is executed synchronously  whenever you invoke an asynchronous operation such as waiting for IO  the event loop handles everything and invokes the callback  it just not something you have to think about   in your example  there are two requests A  comes first  and B  you execute request A  your code continue to run synchronously and execute request B  the event loop handles request A  when it finishes it invokes the callback of request A with the result  same goes to request B

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Node js is built upon libuv  a cross-platform library that abstracts apis syscalls for asynchronous  non-blocking  input output provided by the supported OSes  Unix  OS X and Windows at least    Asynchronous IO  In this programming model open read write operation on devices and resources  sockets  filesystem  etc   managed by the file-system don t block the calling thread  as in the typical synchronous c-like model  and just mark the process  in kernel OS level data structure  to be notified when new data or events are available  In case of a web-server-like app  the process is then responsible to figure out which request context the notified event belongs to and proceed processing the request from there  Note that this will necessarily mean you ll be on a different stack frame from the one that originated the request to the OS as the latter had to yield to a process  dispatcher in order for a single threaded process to handle new events   The problem with the model I described is that it s not familiar and hard to reason about for the programmer as it s non-sequential in nature   You need to make request in function A and handle the result in a different function where your locals from A are usually not available    Node s model  Continuation Passing Style and Event Loop   Node tackles the problem leveraging javascript s language features to make this model a little more synchronous-looking by inducing the programmer to employ a certain programming style  Every function that requests IO has a signature like function      parameters      callback  and needs to be given a callback that will be invoked when the requested operation is completed  keep in mind that most of the time is spent waiting for the OS to signal the completion - time that can be spent doing other work   Javascript s support for closures allows you to use variables you ve defined in the outer  calling  function inside the body of the callback - this allows to keep state between different functions that will be invoked by the node runtime independently  See also Continuation Passing Style   Moreover  after invoking a function spawning an IO operation the calling function will usually return control to node s event loop  This loop will invoke the next callback or function that was scheduled for execution  most likely because the corresponding event was notified by the OS  - this allows the concurrent processing of multiple requests   You can think of node s event loop as somewhat similar to the kernel s dispatcher  the kernel would schedule for execution a blocked thread once its pending IO is completed while node will schedule a callback when the corresponding event has occured   Highly concurrent  no parallelism  As a final remark  the phrase  everything runs in parallel except your code  does a decent job of capturing the point that node allows your code to handle requests from hundreds of thousands open socket with a single thread concurrently by multiplexing and sequencing all your js logic in a single stream of execution  even though saying  everything runs in parallel  is probably not correct here - see Concurrency vs Parallelism - What is the difference    This works pretty well for webapp servers as most of the time is actually spent on waiting for network or disk  database   sockets  and the logic is not really CPU intensive - that is to say  this works well for IO-bound workloads

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Well  to give some perspective  let me compare node js with apache  Apache is a multi-threaded HTTP server  for each and every request that the server receives  it creates a separate thread which handles that request  Node js on the other hand is event driven  handling all requests asynchronously from single thread  When A and B are received on apache  two threads are created which handle requests  Each handling the query separately  each waiting for the query results before serving the page  The page is only served until the query is finished  The query fetch is blocking because the server cannot execute the rest of thread until it receives the result  In node  c query is handled asynchronously  which means while c query fetches the results for A  it jumps to handle c query for B  and when the results arrive for A arrive it sends back the results to callback which sends the response  Node js knows to execute callback when fetch finishes   In my opinion  because it s a single thread model  there is no way to switch from one request to another   Actually the node server does exactly that for you all the time  To make switches   the asynchronous behavior  most functions that you would use will have callbacks  Edit The SQL query is taken from mysql library  It implements callback style as well as event emitter to queue SQL requests  It does not execute them asynchronously  that is done by the internal libuv threads that provide the abstraction of non-blocking I O  The following steps happen for making a query    Open a connection to db  connection itself can be made asynchronously  Once db is connected  query is passed on to the server  Queries can be queued  The main event loop gets notified of the completion with callback or event  Main loop executes your callback eventhandler   The incoming requests to http server are handled in the similar fashion  The internal thread architecture is something like this   The C   threads are the libuv ones which do the asynchronous I O  disk or network   The main event loop continues to execute after the dispatching the request to thread pool  It can accept more requests as it does not wait or sleep  SQL queries HTTP requests file system reads all happen this way

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Node js is based on the event loop programming model  The event loop runs in single thread and repeatedly waits for events and then runs any event handlers subscribed to those events  Events can be for example   timer wait is complete next chunk of data is ready to be written to this file theres a fresh new HTTP request coming our way   All of this runs in single thread and no JavaScript code is ever executed in parallel  As long as these event handlers are small and wait for yet more events themselves everything works out nicely  This allows multiple request to be handled concurrently by a single Node js process    There s a little bit magic under the hood as where the events originate  Some of it involve low level worker threads running in parallel    In this SQL case  there s a lot of things  events  happening between making the database query and getting its results in the callback  During that time the event loop keeps pumping life into the application and advancing other requests one tiny event at a time  Therefore multiple requests are being served concurrently         According to   Event loop from 10 000ft - core concept behind Node js

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Okay  most things should be clear so far    the tricky part is the SQL  if it is not in reality running in another thread or process in it   s entirety  the SQL-execution has to be broken down into individual steps  by an SQL processor made for asynchronous execution    where the non-blocking ones are executed  and the blocking ones  e g  the sleep  actually can be transferred to the kernel  as an alarm interrupt event  and put on the event list for the main loop   That means  e g  the interpretation of the SQL  etc  is done immediately  but during the wait  stored as an event to come in the future by the kernel in some kqueue  epoll      structure  together with the other IO operations  the main loop can do other things and eventually check if something happened of those IOs and waits   So  to rephrase it again  the program is never  allowed to get  stuck  sleeping calls are never executed  Their duty is done by the kernel  write something  wait for something to come over the network  waiting for time to elapse  or another thread or process      The Node process checks if at least one of those duties is finished by the kernel in the only blocking call to the OS once in each event-loop-cycle  That point is reached  when everything non-blocking is done   Clear   -   I don   t know Node  But where does the c query come from

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The function c query   has two argument  c query  Fetch Data    Post-Processing of Data     The operation  Fetch Data  in this case is a DB-Query  now this may be handled by Node js by spawning off a worker thread and giving it this task of performing the DB-Query   Remember Node js can create thread internally   This enables the function to return instantaneously without any delay  The second argument  Post-Processing of Data  is a callback function  the node framework registers this callback and is called by the event loop   Thus the statement c query  paramenter1  parameter2  will return instantaneously  enabling node to cater for another request   P S  I have just started to understand node  actually I wanted to write this as comment to  Philip  but since didn t have enough reputation points so wrote it as an answer

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Node js uses libuv behind the scenes  libuv has a thread pool  of size 4 by default   Therefore Node js does use threads to achieve concurrency   However  your code runs on a single thread  i e   all of the callbacks of Node js functions will be called on the same thread  the so called loop-thread or event-loop   When people say  Node js runs on a single thread  they are really saying  the callbacks of Node js run on a single thread

[node.js] How the single threaded non blocking IO model works in Node.js

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