On Wed, Aug 28, 2013 at 3:51 PM, Andy Bradford <amb-fos...@bradfords.org>wrote:
> 2.1 Identification Of Artifacts > > A particular version of a particular file is called an > "artifact". Each artifact has a universally unique name which is > the SHA1 hash of the content of that file expressed as 40 > characters of lower-case hexadecimal. ... > Changing (or adding or removing) a single byte in a file results > in a completely different artifact ID. And since the artifact ID > is the name of the artifact, making any change to a file results > in a new artifact. In this way, artifacts are immutable. > A slight addition, which i only recently learned but might be of interest to someone else out there: the content is immutable in the logical sense, but not a physical sense. Fossil will occasionally (or more often, or less often) replace the content of a blob with a delta generated from a newer version (potentially long after the original blob was saved). When fetching the blob, all such deltas (there may be a series of them) are expanded/applied, and the SHA1 is checked against the expanded content. Whenever Fossil saves anything of note (e.g. any user content, but not a config entry), it queues up that content for a "before-commit check", to ensure (before committing the SQL transaction) that it can actually read back/process what it wrote and that everything is legal for extraction purposes. Only if that check passes does the transaction get committed, otherwise it gets rolled back and the content is not saved. i've never actually seen that part fail, but know from having gone through the code that Fossil makes sure that if it's going to fail, it will fail before Fossil can commit it (in the SQL sense, not 'checkin' sense). Put briefly: when you tell fossil to give you the contents of file abcdef...., it may internally go through several versions of that file on its way to generating the one you requested, applying deltas as it goes. The end result is that the content is logically immutable, and always convertible to its original form (as determined by SHA1 comparison), but Fossil doesn't actually store it immutably. -- ----- stephan beal http://wanderinghorse.net/home/stephan/ http://gplus.to/sgbeal
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