possible EventEmitter memory leak detected

Question

I am getting following warning    node  warning  possible EventEmitter memory leak detected  11 listeners added  Use emitter setMaxListeners   to increase limit  Trace       at EventEmitter  lt anonymous gt   events js 139 15      at EventEmitter  lt anonymous gt   node js 385 29      at Server  lt anonymous gt   server js 20 17      at Server emit  events js 70 17      at HTTPParser onIncoming  http js 1514 12      at HTTPParser onHeadersComplete  http js 102 31      at Socket ondata  http js 1410 22      at TCP onread  net js 354 27    I wrote code like this in server js   http createServer      function  req  res           listen 3013     How to fix this

User · Answer

I was facing the same issue  but i have successfully handled with async await  Please check if it helps  let dataLength   25  Before   nbsp  nbsp for  let i   0  i  lt  dataLength  i       nbsp  nbsp  nbsp  nbsp  nbsp  nbsp sftp get remotePath  fs createWriteStream xyzProject   data i  name      nbsp  nbsp   After   nbsp  nbsp for  let i   0  i  lt  dataLength  i       nbsp  nbsp  nbsp  nbsp  nbsp  nbsp await sftp get remotePath  fs createWriteStream xyzProject   data i  name      nbsp  nbsp

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The accepted answer provides the semantics on how to increase the limit  but as  voltrevo pointed out that warning is there for a reason and your code probably has a bug   Consider the following buggy code     Assume Logger is a module that emits errors var Logger   require    Logger js     for  var i   0  i  lt  11  i            BUG  This will cause the warning       As the event listener is added in a loop     Logger on  error   function  err            console log  error writing log      err               Logger writeLog  Hello        Now observe the correct way of adding the listener     Good  event listener is not in a loop Logger on  error   function  err        console log  error writing log      err       for  var i   0  i  lt  11  i          Logger writeLog  Hello        Search for similar issues in your code before changing the maxListeners  which is explained in other answers

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Replace  on   with once     Using once   removes event listeners when the event is handled by the same function   If this doesn t fix it  then reinstall restler with this in your package json  restler    git   github com danwrong restler git 9d455ff14c57ddbe263dbbcd0289d76413bfe07d   This has to do with restler 0 10 misbehaving with node   you can see the issue closed on git here  https   github com danwrong restler issues 112 However  npm has yet to update this  so that is why you have to refer to the git head

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Node Version   v11 10 1  Warning  message from stack trace     process on  warning   e   gt  console warn e stack       node 17905  MaxListenersExceededWarning  Possible EventEmitter memory leak detected  11 wakeup listeners added  Use emitter setMaxListeners   to increase limit MaxListenersExceededWarning  Possible EventEmitter memory leak detected  11 wakeup listeners added  Use emitter setMaxListeners   to increase limit     at  addListener  events js 255 17      at Connection addListener  events js 271 10      at Connection Readable on   stream readable js 826 35      at Connection once  events js 300 8      at Connection  send   var www html fleet-node-api node modules http2 lib protocol connection js 355 10      at processImmediate  timers js 637 19      at process topLevelDomainCallback  domain js 126 23     After searching for github issues  documentation and creating similar event emitter memory leaks  this issue was observed due to node-apn module used for iOS push notification   This resolved it       You should only create one Provider per-process for each   certificate key pair you have  You do not need to create a new   Provider for each notification  If you are only sending notifications   to one app then there is no need for more than one Provider       If you are constantly creating Provider instances in your app  make   sure to call Provider shutdown   when you are done with each provider   to release its resources and memory    I was creating provider object each time the notification was sent and expected the gc to clear it

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Put this in the first line of your server js  or whatever contains your main Node js app    require  events   EventEmitter prototype  maxListeners   0   and the error goes away

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I was having this till today when I start grunt watch  Finally solved by  watch      options        maxListeners  99      livereload  true          The annoying message is gone

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You said you are using process on  uncaughtException   callback   Where are you executing this statement  Is it within the callback passed to http createServer If yes  different copy of the same callback will get attached to the uncaughtException event upon each new request  because the function  req  res          gets executed everytime a new request comes in and so will the statement process on  uncaughtException   callback  Note that the process object is global to all your requests and adding listeners to its event everytime a new request comes in will not make any sense  You might not want such kind of behaviour  In case you want to attach a new listener for each new request  you should remove all previous listeners attached to the event as they no longer would be required using   process removeAllListeners  uncaughtException

User · Answer

You need to clear all listeners before creating new ones using   Client   Server  socket removeAllListeners       Assuming socket is your client socket   or created server socket   You can also subscribe from specific event listeners like for example removing the connect listener like this   this socket removeAllListeners  connect

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I prefer to hunt down and fix problems instead of suppressing logs whenever possible  After a couple days of observing this issue in my app  I realized I was setting listeners on the req socket in an Express middleware to catch socket io errors that kept popping up  At some point  I learned that that was not necessary  but I kept the listeners around anyway  I just removed them and the error you are experiencing went away  I verified it was the cause by running requests to my server with and without the following middleware   socketEventsHandler req  res  next            req socket on  error   function err                console error  ------REQ ERROR               console error err stack                      res socket on  error   function err                console error  ------RES ERROR               console error err stack                      next            Removing that middleware stopped the warning you are seeing  I would look around your code and try to find anywhere you may be setting up listeners that you don t need

User · Answer

Thanks to RLaaa for giving me an idea how to solve the real problem root cause of the warning  Well in my case it was MySQL buggy code    Providing you wrote a Promise with code inside like this   pool getConnection  err  conn    gt       if err  reject err     const q    SELECT   from  a table      conn query q       err  rows    gt         conn release        if err  reject err          do something         conn on  error    err    gt          reject err            Notice there is a conn on  error   listener in the code  That code literally adding listener over and over again depends on how many times you call the query  Meanwhile if err  reject err  does the same thing   So I removed the conn on  error   listener and voila    solved  Hope this helps you

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In my case  it was child stderr pipe process stderr  which was being called when I was initiating 10  or so  instances of the child  So anything  that leads to attach an event handler to the same EventEmitter Object in a LOOP  causes nodejs to throw this error

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I am getting this warning too when install aglio on my mac osx   I use cmd fix it   sudo npm install -g npm next   https   github com npm npm issues 13806

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I d like to point out here that that warning is there for a reason and there s a good chance the right fix is not increasing the limit but figuring out why you re adding so many listeners to the same event  Only increase the limit if you know why so many listeners are being added and are confident it s what you really want   I found this page because I got this warning and in my case there was a bug in some code I was using that was turning the global object into an EventEmitter  I d certainly advise against increasing the limit globally because you don t want these things to go unnoticed

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Our team s fix for this was removing a registry path from our  npmrc   We had two path aliases in the rc file  and one was pointing to an Artifactory instance that had been deprecated   The error had nothing to do with our App s actual code but everything to do with our development environment

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By default  a maximum of 10 listeners can be registered for any single event   If it s your code  you can specify maxListeners via   const emitter   new EventEmitter   emitter setMaxListeners 100     or 0 to turn off the limit emitter setMaxListeners 0    But if it s not your code you can use the trick to increase the default limit globally   require  events   EventEmitter prototype  maxListeners   100    Of course you can turn off the limits but be careful      turn off limits by default  BE CAREFUL  require  events   EventEmitter prototype  maxListeners   0    BTW  The code should be at the very beginning of the app   ADD  Since node 0 11 this code also works to change the default limit   require  events   EventEmitter defaultMaxListeners   0

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Sometimes these warnings occur when it isn t something we ve done  but something we ve forgotten to do   I encountered this warning when I installed the dotenv package with npm  but was interrupted before I got around to adding the require  dotenv   load   statement at the beginning of my app  When I returned to the project  I started getting the  Possible EventEmitter memory leak detected  warnings   I assumed the problem was from something I had done  not something I had not done   Once I discovered my oversight and added the require statement  the memory leak warning cleared

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i was having the same problem  and the problem was caused because i was listening to port 8080  on 2 listeners   setMaxListeners   works fine  but i would not recommend it   the correct way is to  check your code for extra listeners  remove the listener or change the port number on which you are listening  this fixed my problem

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This is explained in the node eventEmitter documentation  What version of Node is this  What other code do you have  That isn t normal behavior   In short  its  process setMaxListeners 0    Also see  node js - request - How to    emitter setMaxListeners

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As pointed out by others  increasing the limit is not the best answer  I was facing the same issue  but in my code I was nowhere using any event listener  When I closely looked into the code  I was creating a lot of promises at times  Each promise had some code of scraping the provided URL  using some third-party library   If you are doing something like that  then it may be the cause  Refer this thread on how to prevent that  What is the best way to limit concurrency when using ES6 s Promise all

[node.js] possible EventEmitter memory leak detected

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