Unique identifiers for entries can now be used more efficiently.
Internally, a hash array has replaced the alist used so far to
keep track of the files in which an ID is defined. This makes it
quite fast to find an entry by ID.
There is now a new link type which looks like this:
id:GLOBALLY-UNIQUE-IDENTIFIER
This link points to a specific entry. When you move the entry to
a different file, for example if you move it to an archive
file, this link will continue to work.
The file /org-id.el/ contains an API that can be used to write
code using these identifiers, including creating IDs and finding
them wherever they are.
Org has its own method to create unique identifiers, but if the
system has /uuidgen/ command installed (Mac's and Linux systems
generally do), it will be used by default. You an also select
the method by hand, using the variable =org-id-method=.
If the ID system ever gets confused about where a certain ID is,
it initiates a global scan of all agenda files with associated
archives, all files previously known containing any IDs, and all
currently visited Org-mode files to rebuild the hash. You can
also initiate this by hand: =M-x org-id-update-id-locations=.
Running this command will also dump into the =*Messages*= buffer
information about any duplicate IDs. These should not exist, but
if you /copy/ an entry with its properties, duplicate IDs will
inevitably be produced. This is unavoidable in a plain text
system that allows you to edit the text in arbitrary ways, and a
portion of care on your side is needed to keep this system clean.
The hash is stored in the file =~/.emacs.d/.org-id-locations=.
This is also a change from previous versions where the file was
=~/.org=id-locations=. Therefore, you can remove this old file
if you have it. I am not sure what will happen if the =.emacs.d=
directory does not exists in your setup, but in modern Emacsen, I
believe it should exist. If you do not want to use IDs across
files, you can avoid the overhead with tracking IDs by
customizing the variable =org-id-track-globally=. IDs can then
still be used for links inside a single file.
IDs will also be used when you create a new link to an Org-mode
buffer. If you use =org-store-link= (normally at =C-c l=) inside
en entry in an Org-mode buffer, and ID property will be created
if it does not exist, and the stored link will be an =id:= link.
If you prefer the much less secure linking to headline text, you
can configure the variable =org-link-to-org-use-id=. The default
setting for this variable is =create-if-interactive=, meaning
that an ID will be created when you store a link interactively,
but not if you happen to be in an Org-mode file while you create
a remember note (which usually has a link to the place where you
were when starting remember).
When ID's are created, Org tries to remember in which file the id is
located. However, if an ID is created in a remember buffer, no file
is known. This patch makes sure that this condition does not lead to
an error.
Carsten Dominik