Memory Leak in node.js


Never, ever forget that node.js is single threaded and expects every callback to return to the event loop. Otherwise it will never be garbage collected.


This behavior was experienced in the Softwerkskammer platform. In order to show small avatars next to member’s names we load them from gravatar or take an uploaded pic. These are then binhexed and inlined in the page’s content. We started doing this every time the memberlist is requested. This is very slow and additionally causes unnecessary load to gravatar’s site. We then introduced a local cache using node-cache. I wanted to get rid of this caching and persist the gravatar along with other profile data in our DB.

Of course, we do not know, if a gravatar has been updated. Therefor we need to regularly check gravatar’s services.


We had code like that (no caching anymore):

function imageDataFromGravatar (url, callback) {
  request.get(url, function (error, response, body) {
  if (error) { return callback({image: null}); }

  var image = 'data:' + response.headers['content-type'] + ';base64,' + new Buffer(body).toString('base64');

  callback({image: image});

module.exports = {
  getImage: function (member, callback) {
    var url = this.avatarUrl(, 16);
    imageDataFromGravatar(url, function (data) {

The function “getImage” was called like that (for each member in a loop):

if (!member.getPersistedAvatarData()) {
  avatarProvider.getImage(member, function (imageData) {
    store.saveMember(member, callback);
} else {
  if (/*actuality check*/) {
    avatarProvider.getImage(member, function (imageData) {
      var oldAvatar = member.getPersistedAvatarData();
      if (member.getPersistedAvatarData() !== oldAvatar) {
        store.saveMember(member, function () { /* background op */ });

Do you spot the horror?

        store.saveMember(member, function () { /* background op */ });

I really thought, if I don’t give the “saveMember” function a callback method (which is expected by that method, it eventually calls a mongodb function that expects a callback), it just performs its stuff in the background. – *haha* background, *haha* background, *haha* background – today I am laughing at that. A few weeks ago I have been completely naiv.

So what did I do?

quite simple, changing the last few lines to:

if (/*actuality check*/) {
  avatarProvider.getImage(member, function (imageData) {
    return store.saveMember(member, callback); // never, ever "fork" stuff in node by not having return values *I AM IDIOT*
} else {

(please note the comment after the important line)

That’s it. I hope it helps you to not do the same mistakes.

P.S.: Finding this took me multiple days, spending lots of hours of effort.

Do you take it? – i.T.A.K.E.

I haven’t written an article for a long time. It’s time to come back.

During the last two days the i.T.A.K.E. unconference took place in Bucharest. I had the honor to be one of the presenters there. But I don’t want to speak about what I presented. Instead I want to thank the organizers and the visiting crowd that they created such an awesome event.


I felt really welcome and taken care of. I want to explicitly mention the organizers of Mozaic Works. They are a family that is open minded and open hearted and they let us be part of their family. The location was pleasant and – apart from minor problems (audio and video) – it went very smooth.


You people have been a great mix of experienced and fresh faces. Everyone has been very open and curious. I had many interesting and inspiring conversations about coding and team dynamics. It especially is worth going out with you in the evening.


In talks, I learned about a frontend developer’s perspective of the code (thanks @gion_13), found an easy way to explain “ports and adapters” (thanks @johan_alps and @sanlaville) and much more. But what really made my week was the keynote of @felienne. I usually do not like to listen to academics talking about software development. She changed my opinion.

Bottom Line

I am sure the event will happen again next year. It is definitely a “Not-To-Miss-Conference” if you are passionate about software craftsmanship and giving and taking.

Currently I am still sitting in my room in the conference hotel. A warm feeling is creeping up my belly when I think back of the last hours. Thank you Bucharest!


If it’s Crap, Call it Crap – not Legacy

Tonight we were discussing code quality issues. During this discussion one thing came up to my mind:

Everybody is calling old code “legacy”, and now legacy is used as a synonym for code of poor quality.

Why that?

My first guess is: If it was me who wrote the code, “legacy” sounds much nicer than saying “this code is crap”. “Crap” sounds harsh, brutal, nasty.

But there’s more to it. Many developers have to deal with existing code on a daily basis. Much code. Code that is running at customers’ sites. This code cannot be all crap. People use it, so it must be somehow valuable to them. We even earn money with selling it to new customers. – But it slowly kills us. It kills our ability to change it to modernize it.

Why bother?

Most of us are aware of the fact that today’s produced code will be tomorrow’s legacy. If we allow the use of the word “legacy” as a synonym for “crap” then we have a perfect excuse for producing crappy code. – We should care more of tomorrow’s legacy. We should be proud of yesterday’s work. Today, tomorrow, next year, in five years!

For example: We live in a legacy house, more than a hundred years old. And we are proud of it. We are proud of the people who have built it. Of course there are necessary refurbishments and modernizations, but performing them is usually possible with quite low effort. – I drive a car that is 20 years old. And I love it. – Both the house and the car are legacy, but not crap.

I have a Russian friend that possessed a 7 year old Lada. It was slightly legacy but complete crap. We once bought a brand new replacement part (a rear axle). It was cheap, but crap from the start. Of course the car was running most of the time, but we often had the feeling that we were lucky to reach our destination.


Let’s imagine a huge class hierarchy. Nowadays we know that in most cases huge hierarchies are not a good idea if you want to write maintainable code. Some years ago this knowledge was not so wide-spread. But the now so big hierarchy started small, believe it or not. So if you start building a hierarchy today you are potentially laying the foundation for tomorrow’s hell. And you should consider yourself a crap-maker, because you should know better than – let’s say – 10 years ago. The knowledge that deep hierarchies are problematic and should be avoided is now a “best practice” and has been around for at least five years. This means that if you are now dealing with a deep hierarchy that is less than five years old, you can frankly call that crap.

If you even think of extending this hierarchy with more depth, that is also crap. No excuses. If a structure you have to deal with already is too complicated, adding more is not a way of producing legacy, it is just a horrible practice and it always has been. As soon as you sense the code you are working on is getting too complex, stop. Refactor or refurbish it. It will never get easier in the future. The more you add, the more difficult it will be.


Think – don’t just code!

Legacy Software – Why I love working with it!

For years I worked as a consultant and coach. Both on agile methods and coding. I guess I was of some help on a few occasions. At least that is what some customers stated. But in the end I always left them alone. I often felt relief when I left them. But on the other hand it was also like surrender. The software was not finished. Most software is never finished; development goes on and on and on and …

This is probably nothing new to you. Maybe you have similar experiences. But probably you belong to the majority of people in software development: employed developers.

Buried Under Tons of Code

Old code. Untested. Unreadable. Unloved.

Is this what you dreamed of when you started your career? – Probably not. You may have dreamt of creating new shiny software from scratch using great technologies and fancy tools.

But you don’t quit your job. Perhaps you are a masochist? – I don’t think so. There are many reasons to face the challenge working in such an awkward place:

  • Responsibility: You feel responsible for the product. Although it is not the “Big Fun”, it is your job to keep the thing alive and even add more features to it.
  • Challenge: You think it is much harder to work in a situation like yours than to do small projects from scratch. You know that every software will become legacy after a certain age or size.
  • Fear: You are afraid of being the one to produce the nightmare from scratch that you are now working on. For you it is easier to say: “I just clean up other people’s mess!”
  • Hope: You are just staying because you have hope that you will be amongst the chosen few to start the next product from scratch.

In my case, it is a mixture of the first two aspects. In addition I have a strong belief that every legacy product can be changed to the better so that further development will be possible for a long period of time. This work is not funny but deeply satisfying. In our current product everyone in the team is trying his best while working on the huge codebase. And you know what: The software and the code become better each day.

We are all learning. We are all improving ourselves. Maybe some of us will become masters in the end. Who knows…