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authorMatthias Beyer <>2016-08-24 14:12:05 +0200
committerMatthias Beyer <>2016-08-24 14:22:20 +0200
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parenteafd8c1ae593dba31f5bfbd6618b89918f6f68a4 (diff)
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+# libimagref
+This library crate contains functionality to generate _references_ within the
+imag store.
+It can be used to create references to other files on the filesystem (reachable
+via a filesystem path). It differs from `libimagentrylink`/external linking as
+it is designed exclusively for filesystem references, not for URLs.
+A reference can have several properties, for example can a reference track the
+content of a filesystem path by hashing the content with a hashsum (SHA1) and
+one can check whether a file was changed by that.
+As files can get big (think of `debian.iso`) _partial hashing_ is supported
+(think of "hash the first 2048 bytes of a file).
+The library contains functionality to re-find a moved file automatically by
+checking the content hash which was stored before.
+Permission changes can be tracked as well.
+So this library helps to resemble something like a _symlink_.
+## Limits
+Please understand that this is _not_ intended to be a version control system or
+something like that.
+We also can not use _real symlinks_ as we need imag-store-objects to be able to
+link stuff.
+## Usecase
+This library offers functionality to refer to content outside of the store.
+It can be used to refer to _nearly static stuff_ pretty easily - think of a
+Maildir - you add new mails by fetching them, but you mostly do not remove mails
+and if you do you end up with a "null pointer" in the store, which can then be
+handled properly.
+As this library supports custom hashes (you don't have to hash the full file,
+you can also parse the file and hash only _some_ content) this is pretty
+For example if you want to implement a imag module which tracks a certain kind
+of files which constantly change... but the first 5 lines do never change
+after the file is created - you can write a custom hasher that only uses the
+first 5 lines for the hash.
+## Internals
+Internally, in the store, the file gets created under
+`/ref/<hash of the path to the file to refer to>`.
+If the content of the file is hashed, we can still re-find the file via the
+content hash (which is stored in the header of the store entry).
+The reference object can, after the path was re-found, be updated.