How to correctly make "latest" symlinks

"Latest symlink"

A "latest" symlink, is a symbolic link (on Linux, Unix etc) which links to the "latest" version of a file.

Suppose we have a file which takes some effort to create, which is generated periodically or in response to some stimulus (e.g. user activity). Then we want to create a "latest version" symlink.

Ideally the properties should be

• latest symlink always points at the latest version (duuh!)
• latest symlink always exists
• latest symlink never points at a partially completed, broken, missing or otherwise bad file

Sometimes people do this in a way which won't work.

How to create a symlink

Dead easy, right? Just call the "symlink" function. 

 int symlink(const char *oldpath, const char *newpath);

 DESCRIPTION
       symlink()  creates  a  symbolic  link  named newpath which contains the
       string oldpath.



But if we call "symlink" and the newpath param points to an already existing file (including a symlink) then it will return EEXIST.

The wrong (obvious) way

try:
  unlink(dest)
except OSError:
  pass
symlink(src, dest)

The correct (not so obvious) way

templink = dest + '.temp'
symlink(src, templink)
rename(templink, dest) 

Why?

Because we want to avoid a race condition where the destination symbolic link does not exist. Renaming files is atomic and will instantly replace the existing link with a new one; no other program can possibly see a non-existing file.

Other wrong ways

Some possibly common, but wrong (or even wronger) ways to do this

• Just create the "latest" file using our "do lots of work" process directly. This is really bad, as during file creation, another process can see a partially completed file. If your code looks like this: write file header; do lots of work to create file body; write file footer, then there is a really good chance that another process sees an incomplete file.
• Create a different file, then copy the file (file copying isn't atomic)

Why I use Python

There are a lot of holy wars between programming language advocates in the industry.

I use Python.

Why?

Because I have found that programs written in Python are significantly more likely to Work Correctly. They more often run correctly the first time, therefore I spend less time debugging.

There are lots of other reasons, but it's mainly the "It just works".

A few years ago, I had 10+ years of industry experience of Perl, but only 1 year of Python. I already found that my Python programs initially worked correctly FAR MORE OFTEN.

This is comparing a language that I'd been using commercially, most days, for 10 years, with something that I'd only just picked up.

That's why I use Python.

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Other stuff?

DEBUGGING: Stack traces. In Perl, it's possible to get a stack trace from an exception, but you sometimes have to jump through hoops to do it. Likewise, in PHP, stacktraces are available in principle, but don't always appear when you need them. Java? Stack traces usually appear, but are often huge and difficult to read (even better: in some versions of Java, the compiler "optimises" out line numbers from stack traces) In Python, stack traces Just Work. Like everything else.