25.6. Synchronizing Source
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## 25.6. Synchronizing Source

There are various ways of using an Internet or email connection to stay up-to-date with any given area, or all areas, of the FreeBSD project sources. The primary services are Subversion and CTM.

### Warning:

While it is possible to update only parts of the source tree, the only supported update procedure is to update the entire tree and recompile all the programs that run in user space, such as those in `/bin` and `/sbin`, and kernel sources. Updating only part of the source tree, only the kernel, or only the userland programs will often result in problems ranging 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 are up-to-the-minute and the user controls when they are downloaded. It is easy to restrict updates to specific files or directories and the requested updates are generated on the fly by the server.

CTM does not interactively compare the local sources 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 are 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 more efficient than Subversion and places less strain on server resources since it is a push rather than a pull model.

There are other trade-offs. If a user inadvertently wipes out portions of the local archive, Subversion will detect and rebuild the damaged portions. CTM will not do this, and if a user deletes some portion of the source tree and does not have a backup, they will have to start from scratch from the most recent CTM “base delta” and rebuild it all with CTM.

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