setImmediate vs nextTick

Question

Node js version 0 10 was released today and introduced  setImmediate  The API changes documentation suggests using it when doing recursive nextTick calls   From what MDN says it seems very similar to process nextTick   When should I use nextTick and when should I use setImmediate

User · Answer

In simple terms  process NextTick   would executed at next tick of event loop  However  the setImmediate  basically has a separate phase which ensures that the callback registered under setImmediate   will be called only after the IO callback and polling phase   Please refer to this link for nice explanation  https   medium com the-node-js-collection what-you-should-know-to-really-understand-the-node-js-event-loop-and-its-metrics-c4907b19da4c

User · Answer

Some great answers here detailing how they both work   Just adding one that answers the specific question asked      When should I use nextTick and when should I use setImmediate      Always use setImmediate     The Node js Event Loop  Timers  and process nextTick   doc includes the following      We recommend developers use setImmediate   in all cases because it s easier to reason about  and it leads to code that s compatible with a wider variety of environments  like browser JS       Earlier in the doc it warns that process nextTick can lead to        some bad situations because it allows you to  starve  your I O by making recursive process nextTick   calls  which prevents the event loop from reaching the poll phase    As it turns out  process nextTick can even starve Promises   Promise resolve   then      gt    console log  this happens LAST         process nextTick      gt      console log  all of these         process nextTick      gt        console log     happen before           process nextTick      gt          console log     the Promise ever             process nextTick      gt            console log     has a chance to resolve                              On the other hand  setImmediate is  easier to reason about  and avoids these types of issues   Promise resolve   then      gt    console log  this happens FIRST         setImmediate      gt      console log  this happens LAST           So unless there is a specific need for the unique behavior of process nextTick  the recommended approach is to  use setImmediate   in all cases

User · Answer

I recommend you to check docs section dedicated for Loop to get better understanding  Some snippet taken from there   We have two calls that are similar as far as users are concerned  but their names are confusing    process nextTick   fires immediately on the same phase setImmediate   fires on the following iteration or  tick  of the event loop   In essence  the names should be swapped  process nextTick   fires more immediately than setImmediate    but this is an artifact of the past which is unlikely to change

User · Answer

Use setImmediate if you want to queue the function behind whatever I O event callbacks that are already in the event queue   Use process nextTick to effectively queue the function at the head of the event queue so that it executes immediately after the current function completes   So in a case where you re trying to break up a long running  CPU-bound job using recursion  you would now want to use setImmediate rather than process nextTick to queue the next iteration as otherwise any I O event callbacks wouldn t get the chance to run between iterations

User · Answer

In the comments in the answer  it does not explicitly state that nextTick shifted from Macrosemantics to Microsemantics    before node 0 9  when setImmediate was introduced   nextTick operated at the start of the next callstack   since node 0 9  nextTick operates at the end of the existing callstack  whereas setImmediate is at the start of the next callstack  check out https   github com YuzuJS setImmediate for tools and details

User · Answer

As an illustration import fs from  fs   import http from  http    const options       host   www stackoverflow com     port  80    path    index html      describe  deferredExecution        gt      it  deferredExecution    done    gt        console log  Start        setTimeout      gt  console log  TO1    0       setImmediate      gt  console log  IM1         process nextTick      gt  console log  NT1         setImmediate      gt  console log  IM2         process nextTick      gt  console log  NT2         http get options       gt  console log  IO1         fs readdir process cwd         gt  console log  IO2         setImmediate      gt  console log  IM3         process nextTick      gt  console log  NT3         setImmediate      gt  console log  IM4         fs readdir process cwd         gt  console log  IO3         console log  Done        setTimeout done  1500              will give the following output Start Done NT1 NT2 NT3 TO1 IO2 IO3 IM1 IM2 IM3 IM4 IO1  I hope this can help to understand the difference  Updated   Callbacks deferred with process nextTick   run before any other I O event is fired  while with setImmediate    the execution is queued behind any I O event that is already in the queue  Node js Design Patterns  by Mario Casciaro  probably the best book about node js js

User · Answer

I think I can illustrate this quite nicely  Since nextTick is called at the end of the current operation  calling it recursively can end up blocking the event loop from continuing  setImmediate solves this by firing in the check phase of the event loop  allowing event loop to continue normally       -----------------------   - gt           timers                 -----------------------       -----------------------             I O callbacks             -----------------------       -----------------------             idle  prepare             -----------------------        ---------------       -----------------------            incoming                      poll             lt -----    connections          -----------------------            data  etc           -----------------------        ---------------                check                  -----------------------       -----------------------   --      close callbacks           -----------------------    source  https   nodejs org en docs guides event-loop-timers-and-nexttick   Notice that the check phase is immediately after the poll phase  This is because the poll phase and I O callbacks are the most likely places your calls to setImmediate are going to run  So ideally most of those calls will actually be pretty immediate  just not as immediate as nextTick which is checked after every operation and technically exists outside of the event loop   Let s take a look at a little example of the difference between setImmediate and process nextTick   function step iteration      if  iteration     10  return    setImmediate      gt        console log  setImmediate iteration    iteration         step iteration   1      Recursive call from setImmediate handler          process nextTick      gt        console log  nextTick iteration    iteration             step 0     Let s say we just ran this program and are stepping through the first iteration of the event loop  It will call into the step function with iteration zero  It will then register two handlers  one for setImmediate and one for process nextTick  We then recursively call this function from the setImmediate handler which will run in the next check phase  The nextTick handler will run at the end of the current operation interrupting the event loop  so even though it was registered second it will actually run first   The order ends up being  nextTick fires as current operation ends  next event loop begins  normal event loop phases execute  setImmediate fires and recursively calls our step function to start the process all over again  Current operation ends  nextTick fires  etc   The output of the above code would be   nextTick iteration  0 setImmediate iteration  0 nextTick iteration  1 setImmediate iteration  1 nextTick iteration  2 setImmediate iteration  2 nextTick iteration  3 setImmediate iteration  3 nextTick iteration  4 setImmediate iteration  4 nextTick iteration  5 setImmediate iteration  5 nextTick iteration  6 setImmediate iteration  6 nextTick iteration  7 setImmediate iteration  7 nextTick iteration  8 setImmediate iteration  8 nextTick iteration  9 setImmediate iteration  9   Now let s move our recursive call to step into our nextTick handler instead of the setImmediate   function step iteration      if  iteration     10  return    setImmediate      gt        console log  setImmediate iteration    iteration             process nextTick      gt        console log  nextTick iteration    iteration         step iteration   1      Recursive call from nextTick handler          step 0     Now that we have moved the recursive call to step into the nextTick handler things will behave in a different order  Our first iteration of the event loop runs and calls step registering a setImmedaite handler as well as a nextTick handler  After the current operation ends our nextTick handler fires which recursively calls step and registers another setImmediate handler as well as another nextTick handler  Since a nextTick handler fires after the current operation  registering a nextTick handler within a nextTick handler will cause the second handler to run immediately after the current handler operation finishes  The nextTick handlers will keep firing  preventing the current event loop from ever continuing  We will get through all our nextTick handlers before we see a single setImmediate handler fire   The output of the above code ends up being   nextTick iteration  0 nextTick iteration  1 nextTick iteration  2 nextTick iteration  3 nextTick iteration  4 nextTick iteration  5 nextTick iteration  6 nextTick iteration  7 nextTick iteration  8 nextTick iteration  9 setImmediate iteration  0 setImmediate iteration  1 setImmediate iteration  2 setImmediate iteration  3 setImmediate iteration  4 setImmediate iteration  5 setImmediate iteration  6 setImmediate iteration  7 setImmediate iteration  8 setImmediate iteration  9   Note that had we not interrupted the recursive call and aborted it after 10 iterations then the nextTick calls would keep recursing and never letting the event loop continue to the next phase  This is how nextTick can become blocking when used recursively whereas setImmediate will fire in the next event loop and setting another setImmediate handler from within one won t interrupt the current event loop at all  allowing it to continue executing phases of the event loop as normal   Hope that helps   PS - I agree with other commenters that the names of the two functions could easily be swapped since nextTick sounds like it s going to fire in the next event loop rather than the end of the current one  and the end of the current loop is more  immediate  than the beginning of the next loop  Oh well  that s what we get as an API matures and people come to depend on existing interfaces

[javascript] setImmediate vs. nextTick

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