Getting starting with a Legacy-Code-Project


Day zero

Imagine the following situation: you are starting a new job, you are looking forward to your bright future. Of course you are planning to use the newest technologies and frameworks. And then you are allowed to take a first look into the source you have to work with. No tests, no spec, which fit to the source, of course no doc but a lot of  ( angry ) customers, which are strongly coupled to this mess. And now you are allowed to work with this kind of … whatever.

We call this Legacy-Code and I guess this situation is a common one, every developer or most of them will face a situation like this during his/her career. So what can we do to get out of this? I want to show you some base-techniques which will help you.

Accept the fact: this is the code to work on and it currently solves real problems!

No developer is planning to create legacy code. There is always a reason like: we needed to get in buisness or we had failed. Or the old developers had not enough resources to solve all upcoming issues or develop automatic tests. 10 years ago I faced this situation again and again: nobody want to write automatic tests because it costs a lot of time and you need some experience how to design your architecture in a way that its testable. And there were not so much tools out in those days.

The code is there for a reason and you need to accept this: this working legacy code ensures that you got the job. So even when its hard try to be polite when reading the code. Someone invested a lot of lifetime to keeps it up and running. And hopefully this guy is still in the company and you can ask him some questions.

You can kill him later ;-).

Check, if there is any source-control-management

The first thing you should check is the existence of an Source-Control-Management-Tool like Subversion, Git or Perforce. If not: get one, learn how to use it and put all your legacy code into source control! Do it now, do not discuss. If any of the other developers are concerned about using one install a SCM-tool on you own developer-pc and use it there. I promise: it will save your life some day. One college accidentally killed  his project-files after 6 weeks of work. He forgot the right name of his backup-folder and removed the false on, the one containing the current source. He tried to save disk-space, even in those old day disk-space was much cheaper than manpower.

To avoid errors like this: use a SCM-tool.

Check-in all your files!

Now you have a working SCM-tool check if all source-files, scripts and Makefiles are checked-in. If not: start doing this. The target of this task is just to get a reproducible build for you. Work on this until you are able to build from scratch after checking out your product. And when this works write a small KickStarter-Doc how to build everything from scratch after a clean checkout. Of course this will not work in the beginning. Of course you will face a lot of issues like a broken build, wrong paths or a different environment. But this is also a sign of legacy-code: not reproducible builds. Normally not all related files like special Makefiles are checked in. Or sometimes the environment differs between the different developer-PCs. And this causes a lot of hard to reproduce issues.

Do you know the phrase: “It worked on my machine?” after facing a new bug. Sometimes the developer was right. The issue was caused by different environments between the developer machines ( for instance different compiler version, different IDE, different kernel, different whatever … ).

When you have checkin all you files try to ensure that everyone is using the same tools: same compiler version, same libs, same IDE and document this in your KickStarter-Doc. Let’s try other guy’s to work with this and fix all upcoming issues.

This can slow down the ongoing development tasks. To avoid this you can learn how to work with branches with your SCM-tool ( for instance this doc shows how to do branches in git: https://git-scm.com/book/en/v2/Git-Branching-Basic-Branching-and-Merging ).

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