Homogeneity

The concept of a `convergent script' must be refined slightly when one expects it to interoperate and cooperate with other scripts of similar intent. A script must not only be convergent within itself, but must also behave so that the set of all scripts is convergent as a suite. To do this, each script must exhibit three crucial ingredients:

• awareness: a script knows whether it succeeded or not in enforcing its requirements. It returns a nonzero exit code to indicate failure and a zero exit code to indicate success.

• convergence: applying the script twice changes nothing and has no effect if the network is already in compliance with the script's requirements.

• homogeneity (or consistency): scripts never undo the changes made by others (though scripts may enforce the same changes as others). This is a global convergence criterion over all scripts.

These conditions were of course inspired by the convergent processes of Cfengine[2,3,4] and our own Slink[7] and Distr[8], and are exactly the behaviors that we previously attempted to ensure by scripting in Prolog[9] instead of Cfengine or PIKT[24], and in Babble[10] instead of Expect[20].

Of these conditions, the first two are relatively easy to ensure for a given script. These are properties of a script in isolation from others. The third property of homogeneity is a global condition on the suite of all scripts.

These seemingly abstract ideals are easy to provide in scripts. Consider the simple task of forcing /etc/inetd.conf to have a particular form. A script that does this might contain¹:

cp /proto/inetd.conf /etc/inetd.conf

This script can be converted into a convergent one by checking that a copy is needed before making the copy:

if [ ! cmp /proto/inetd.conf \ 
           /etc/inetd.conf ] ; then 
  cp /proto/inetd.conf /etc/inetd.conf
fi

This can be made aware by arranging for it to return the proper exit codes:

if [ ! -f /proto/inetd.conf ] ; \
  exit 1
if [ ! cmp /proto/inetd.conf \ 
           /etc/inetd.conf ] ; then 
  cp /proto/inetd.conf /etc/inetd.conf
fi
exit 0

To make it homogeneous with other scripts, however, every script that modifies /etc/inetd.conf must modify it into this exact form.