Exercise: Switch releases with a link

Many applications keep each release in its own directory and point a link called current at the one in use, so an upgrade or a rollback is one quick command. Build that on servera, run into the classic mistake when you repoint the link, and save a configuration file with a hard link before deleting a release.

Switch releases with a link

Create and repoint a symbolic link to a directory.
Keep a file's data alive with a hard link.
Recognise a dangling link.

Start the exercise

On workstation, start the exercise. It removes any release directories and links from an earlier run from student's home on servera.

[student@workstation ~]$ lab start sa-links

Later tasks assume that you are logged in to the server they name: ssh student@servera from workstation, then sudo -i for a root shell.

Create two releases

On servera, as student, make two release directories, each with its own app.conf. echo "text" > file writes a line into a file (chapter 5 explains it):

[student@workstation ~]$ ssh student@servera
[student@servera ~]$ mkdir -p release/v1 release/v2
[student@servera ~]$ echo "version 1" > release/v1/app.conf
[student@servera ~]$ echo "version 2" > release/v2/app.conf

Point current at version 1

Create a symbolic link current to release/v1, and read the configuration through it.

[student@servera ~]$ ln -s release/v1 current
[student@servera ~]$ ls -l current
lrwxrwxrwx. 1 student student 10 Oct  3 09:59 current -> release/v1
[student@servera ~]$ cat current/app.conf
version 1

The target is a relative path. It is resolved from the directory the link is in, so the link keeps working if you move or copy the whole tree together.

Try to repoint it the obvious way

Upgrade to version 2 by running the same command with v2, and look carefully at what happened:

[student@servera ~]$ ln -s release/v2 current
[student@servera ~]$ ls -l current release/v1
lrwxrwxrwx. 1 student student   10 Oct  3 09:59 current -> release/v1

release/v1:
total 4
-rw-r--r--. 1 student student 10 Oct  3 09:59 app.conf
lrwxrwxrwx. 1 student student 10 Oct  3 09:59 v2 -> release/v2

current still points to v1. Because current leads to a directory, ln treated it as the destination directory, exactly as cp and mv would, and created a new link called v2 inside release/v1. That stray link is even broken: its relative target, release/v2, is looked up from inside release/v1.

Remove the stray link:

[student@servera ~]$ rm release/v1/v2

Repoint the link properly

-n tells ln to treat an existing link to a directory as a plain name, and -f to replace it:

[student@servera ~]$ ln -sfn release/v2 current
[student@servera ~]$ ls -l current
lrwxrwxrwx. 1 student student 10 Oct  3 09:59 current -> release/v2
[student@servera ~]$ cat current/app.conf
version 2

A rollback is the same command with v1.

Save v2's configuration with a hard link

Before deleting the v2 directory, keep its configuration by giving the file a second name in your home directory. Compare the inode numbers.

[student@servera ~]$ ln release/v2/app.conf app.conf.saved
[student@servera ~]$ ls -li release/v2/app.conf app.conf.saved
261898 -rw-r--r--. 2 student student 10 Oct  3 09:59 app.conf.saved
261898 -rw-r--r--. 2 student student 10 Oct  3 09:59 release/v2/app.conf

One inode, two names, link count 2. No data was copied.

Delete release v2 and compare the two links

[student@servera ~]$ rm -r release/v2
[student@servera ~]$ cat current/app.conf
cat: current/app.conf: No such file or directory
[student@servera ~]$ cat app.conf.saved
version 2
[student@servera ~]$ ls -l app.conf.saved
-rw-r--r--. 1 student student 10 Oct  3 09:59 app.conf.saved

current now dangles: it points to a name that is gone. The hard link kept the data, and its link count is back to 1.

Grade and finish

Check your work with lab grade sa-links, then put the exercise away with lab finish sa-links.