How in general does Node js handle 10 000 concurrent requests

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I understand that Node js uses a single-thread and an event loop to process requests only processing one at a time  which is non-blocking   But still  how does that work  lets say 10 000 concurrent requests  The event loop will process all the requests  Would not that take too long   I can not understand  yet  how it can be faster than a multi-threaded web server  I understand that multi-threaded web server will be more expensive in resources  memory  CPU   but would not it still be faster  I am probably wrong  please explain how this single-thread is faster in lots of requests  and what it typically does  in high level  when servicing lots of requests like 10 000   And also  will that single-thread scale well with that large amount  Please bear in mind that I am just starting to learn Node js

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Adding to slebetman s answer for more clarity on what happens while executing the code   The internal thread pool in nodeJs just has  4 threads by default  and  its not like the whole request is attached to a new thread from the thread pool the whole execution of request happens just like any normal request  without any blocking task    just that whenever a request has any long running or a heavy operation like db call  a file operation or a http request  the task is queued to the internal thread pool which is provided by libuv  And as nodeJs provides 4 threads in internal thread pool by default every 5th or next concurrent request waits until a thread is free and once these operations are over the callback is pushed to the callback queue  and is picked up by event loop and sends back the response    Now here comes another information that its not once single callback queue  there are many queues     NextTick queue Micro task queue Timers Queue IO callback queue  Requests  File ops  db ops  IO Poll queue  Check Phase queue or SetImmediate close handlers queue   Whenever a request comes the code gets executing in this order of callbacks queued    It is not like when there is a blocking request it is attached to a new thread   There are only 4 threads by default  So there is another queueing happening there    Whenever in a code a blocking process like file read occurs   then calls a function which utilises thread from thread pool and then once the operation is done   the callback is passed to the respective queue and then executed in the order   Everything gets queued based on the the type of callback and processed in the order mentioned above

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I understand that Node js uses a single-thread and an event loop to   process requests only processing one at a time  which is non-blocking     I could be misunderstanding what you ve said here  but  one at a time  sounds like you may not be fully understanding the event-based architecture   In a  conventional   non event-driven  application architecture  the process spends a lot of time sitting around waiting for something to happen  In an event-based architecture such as Node js the process doesn t just wait  it can get on with other work   For example  you get a connection from a client  you accept it  you read the request headers  in the case of http   then you start to act on the request  You might read the request body  you will generally end up sending some data back to the client  this is a deliberate simplification of the procedure  just to demonstrate the point    At each of these stages  most of the time is spent waiting for some data to arrive from the other end - the actual time spent processing in the main JS thread is usually fairly minimal   When the state of an I O object  such as a network connection  changes such that it needs processing  e g  data is received on a socket  a socket becomes writable  etc  the main Node js JS thread is woken with a list of items needing to be processed   It finds the relevant data structure and emits some event on that structure which causes callbacks to be run  which process the incoming data  or write more data to a socket  etc  Once all of the I O objects in need of processing have been processed  the main Node js JS thread will wait again until it s told that more data is available  or some other operation has completed or timed out    The next time that it is woken  it could well be due to a different I O object needing to be processed - for example a different network connection  Each time  the relevant callbacks are run and then it goes back to sleep waiting for something else to happen   The important point is that the processing of different requests is interleaved  it doesn t process one request from start to end and then move onto the next   To my mind  the main advantage of this is that a slow request  e g  you re trying to send 1MB of response data to a mobile phone device over a 2G data connection  or you re doing a really slow database query  won t block faster ones   In a conventional multi-threaded web server  you will typically have a thread for each request being handled  and it will process ONLY that request until it s finished  What happens if you have a lot of slow requests  You end up with a lot of your threads hanging around processing these requests  and other requests  which might be very simple requests that could be handled very quickly  get queued behind them   There are plenty of others event-based systems apart from Node js  and they tend to have similar advantages and disadvantages compared with the conventional model   I wouldn t claim that event-based systems are faster in every situation or with every workload - they tend to work well for I O-bound workloads  not so well for CPU-bound ones

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Single Threaded Event Loop Model Processing Steps    Clients Send request to Web Server  Node JS Web Server internally maintains a Limited Thread pool to provide services to     the Client Requests  Node JS Web Server receives those requests and places them into a Queue  It is known as    Event Queue     Node JS Web Server internally has a Component  known as    Event Loop     Why it got this name is that it uses indefinite loop to receive requests and process them  Event Loop uses Single Thread only  It is main heart of Node JS Platform    Processing Model  Event Loop checks any Client Request is placed in    Event Queue  If not then wait for incoming requests for indefinitely  If yes  then pick up one Client Request from Event Queue   Starts process that Client Request If that Client Request Does Not requires any Blocking IO Operations  then process everything  prepare response and send it back to client  If that Client Request requires some Blocking IO Operations like interacting with Database  File System  External Services then it will follow different approach  Checks    Threads availability from Internal Thread Pool Picks up one Thread    and assign this Client Request to that thread  That Thread is    responsible for taking that request  process it  perform Blocking IO    operations  prepare response and send it back to the Event Loop  very nicely explained by  Rambabu Posa for more explanation go throw this Link

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What you seem to be thinking is that most of the processing is handled in the node event loop. Node actually farms off the I/O work to threads. I/O operations typically take orders of magnitude longer than CPU operations so why have the CPU wait for that? Besides, the OS can handle I/O tasks very well already. In fact, because Node does not wait around it achieves much higher CPU utilisation.

By way of analogy, think of NodeJS as a waiter taking the customer orders while the I/O chefs prepare them in the kitchen. Other systems have multiple chefs, who take a customers order, prepare the meal, clear the table and only then attend to the next customer.

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If you have to ask this question then you re probably unfamiliar with what most web applications services do  You re probably thinking that all software do this   user do an action                  v  application start processing action     -- gt  loop                -- gt  busy processing  end loop     -- gt  send result to user   However  this is not how web applications  or indeed any application with a database as the back-end  work  Web apps do this   user do an action                  v  application start processing action     -- gt  make database request            -- gt  do nothing until request completes  request complete     -- gt  send result to user   In this scenario  the software spend most of its running time using 0  CPU time waiting for the database to return   Multithreaded network app   Multithreaded network apps handle the above workload like this   request -- gt  spawn thread                -- gt  wait for database request                       -- gt  answer request request -- gt  spawn thread                -- gt  wait for database request                       -- gt  answer request request -- gt  spawn thread                -- gt  wait for database request                       -- gt  answer request   So the thread spend most of their time using 0  CPU waiting for the database to return data  While doing so they have had to allocate the memory required for a thread which includes a completely separate program stack for each thread etc  Also  they would have to start a thread which while is not as expensive as starting a full process is still not exactly cheap   Singlethreaded event loop  Since we spend most of our time using 0  CPU  why not run some code when we re not using CPU  That way  each request will still get the same amount of CPU time as multithreaded applications but we don t need to start a thread  So we do this   request -- gt  make database request request -- gt  make database request request -- gt  make database request database request complete -- gt  send response database request complete -- gt  send response database request complete -- gt  send response   In practice both approaches return data with roughly the same latency since it s the database response time that dominates the processing   The main advantage here is that we don t need to spawn a new thread so we don t need to do lots and lots of malloc which would slow us down   Magic  invisible threading  The seemingly mysterious thing is how both the approaches above manage to run workload in  parallel   The answer is that the database is threaded  So our single-threaded app is actually leveraging the multi-threaded behaviour of another process  the database   Where singlethreaded approach fails  A singlethreaded app fails big if you need to do lots of CPU calculations before returning the data  Now  I don t mean a for loop processing the database result  That s still mostly O n   What I mean is things like doing Fourier transform  mp3 encoding for example   ray tracing  3D rendering  etc   Another pitfall of singlethreaded apps is that it will only utilise a single CPU core  So if you have a quad-core server  not uncommon nowdays  you re not using the other 3 cores   Where multithreaded approach fails  A multithreaded app fails big if you need to allocate lots of RAM per thread  First  the RAM usage itself means you can t handle as many requests as a singlethreaded app  Worse  malloc is slow  Allocating lots and lots of objects  which is common for modern web frameworks  means we can potentially end up being slower than singlethreaded apps  This is where node js usually win   One use-case that end up making multithreaded worse is when you need to run another scripting language in your thread  First you usually need to malloc the entire runtime for that language  then you need to malloc the variables used by your script   So if you re writing network apps in C or go or java then the overhead of threading will usually not be too bad  If you re writing a C web server to serve PHP or Ruby then it s very easy to write a faster server in javascript or Ruby or Python   Hybrid approach  Some web servers use a hybrid approach  Nginx and Apache2 for example implement their network processing code as a thread pool of event loops  Each thread runs an event loop simultaneously processing requests single-threaded but requests are load-balanced among multiple threads   Some single-threaded architectures also use a hybrid approach  Instead of launching multiple threads from a single process you can launch multiple applications - for example  4 node js servers on a quad-core machine  Then you use a load balancer to spread the workload amongst the processes   In effect the two approaches are technically identical mirror-images of each other

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Adding to slebetman answer    When you say Node JS can handle 10 000 concurrent requests they are essentially non-blocking requests i e  these requests are majorly pertaining to database query   Internally  event loop of Node JS is handling a thread pool  where each thread handles a non-blocking request and event loop continues to listen to more request after delegating work to one of the thread of the thread pool  When one of the thread completes the work  it send a signal to the event loop that it has finished aka callback  Event loop then process this callback and send the response back   As you are new to NodeJS  do read more about nextTick to understand how event loop works internally   Read blogs on http   javascriptissexy com  they were really helpful for me when I started with JavaScript NodeJS

[node.js] How, in general, does Node.js handle 10,000 concurrent requests?

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