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GNU cfengine

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3.23.3 Link Children

The linkchildren directive is a closely related to the cfengine model for NFS filesystems. It is a way of making links which embodies a rudimentary kind of `intelligence'.

Consider the following:

 
links:

   any::

      /usr/local/lib/emacs +> linkchildren

The word linkchildren automatically tells cfengine that it should look for an appropriate file to link to on a binary server for the current host. The exact meaning of the above statement is as follows. cfengine begins searching though the list of mountable resources, discarding any filesystems which do not belong to valid binary servers. It looks for a filesystem ending in `emacs' (the last link of the left hand side). If all is well, these file systems are already mounted and they can be searched. If no resource is found ending in `emacs', we go to the next link lib and look for a filesystem ending in `lib'. If this is not found we go to local and so on. When a match is made, cfengine then tries to locate the file by checking whether it exists relative to the matched filesystem. For example, suppose `local' matched with host:/site/host/local. It would then try to locate host:/site/host/local/lib/emacs and link all of the children therein to the local file directory /usr/local/lib/emacs.

Here is another example which makes reference to the cfengine model for mounting NFS filesystems. Suppose you have a host with some spare disk space. You want to mount /usr/local from the binary architecture server, but you also want to use the disk you have locally. The following lines

 
links:

   electron::

      /$(site)/electron/local +> linkchildren

   any::

      /usr/local              -> /$(site)/$(binserver)/local

have the effect of creating a directory /$(site)/electron/local and filling it with links to all of the files and directories on the binary server's mounted filesystem. It results in an exact copy (by linkage) on the local disk, but does not use up your local disk space. The space you have remaining could, for example, be used for software with a special license for that host. The second link links /usr/local to the `nearest' binary server. But the nearest binary server is always $(host) which means this evaluates to a file which now exists because of the first command, so on the host `electron' the directory /usr/local ends up being a link to /$(site)/electron/local which is full of links to the binary server.

If you've caught your breath after that mouthful you probably have mixed feelings about creating a bunch of links in this way. What happens if the files they point to are removed? Then you are left with a lot of useless links. Actually this is no problem for cfengine, since you can ask cfengine to simply remove links which point to non-existent files See section 3.16 files. Nevertheless, this feature clearly requires some caution and is mainly a spice for advanced users of the cfengine model.


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