25.6 Synchronizing Your Source

There are various ways of using an Internet (or email) connection to stay up-to-date with any given area of the FreeBSD project sources, or all areas, depending on what interests you. The primary services we offer are Subversion and CTM.

Warning: While it is possible to update only parts of your source tree, the only supported update procedure is to update the entire tree and recompile both userland (i.e., all the programs that run in user space, such as those in /bin and /sbin) and kernel sources. Updating only part of your source tree, only the kernel, or only userland will often result in problems. These problems may range from compile errors to kernel panics or data corruption.

Subversion uses the pull model of updating sources. The user (or a cron script) invokes the svn program, and it brings files up-to-date. Subversion is the preferred means of updating local source trees. The updates you receive are up-to-the-minute and you get them when, and only when, you want them. You can easily restrict your updates to the specific files or directories that are of interest to you. Updates are generated on the fly by the server, according to what you have and what you want to have.

CTM, on the other hand, does not interactively compare the sources you have with those on the master archive or otherwise pull them across. Instead, a script which identifies changes in files since its previous run is executed several times a day on the master CTM machine, any detected changes being compressed, stamped with a sequence-number and encoded for transmission over email (in printable ASCII only). Once received, these “CTM deltas” can then be handed to the ctm_rmail(1) utility which will automatically decode, verify and apply the changes to the user's copy of the sources. This process is far more efficient than Subversion, and places less strain on our server resources since it is a push rather than a pull model.

There are other trade-offs, of course. If you inadvertently wipe out portions of your archive, Subversion will detect and rebuild the damaged portions for you. CTM will not do this, and if you wipe some portion of your source tree out (and do not have it backed up) then you will have to start from scratch (from the most recent CTM “base delta”) and rebuild it all with CTM.