So, a common technique to deploy a new release of your app might use a directory structure something like this:

 - app/
  - current/ -> releases/2015-01-01-1346/
  - releases/
    - 2015-01-01-1023/
    - 2015-04-10-1346/

Then, when you want to deploy a new version of your app or site you clone the source into app/releases/ and run whatever build and configuration processes you need (composer install for example) and then ln -snf current releases/2015-04-10-1346 and your new release is deployed:

 - app/
  - current/ -> releases/2015-04-10-1346/
  - releases/
  …

However, did you know that ln is not an atomic operation (it doesn’t occur instantaneously). What actually happens is current/ is unlinked then a new symlink is created in it’s place—this leaves a small window where there is no current/ directory.

An atomic solution

Instead use mv which is atomic. Create a new symlink ln -s releases/2015-04-10-1346/ latest then move it over the top mv -Tf latest current and your done—absolute zero downtime.