blog for the day

doc/design/assistant/blog/day_98__preferred_content.mdwn

@@ -0,0 +1,44 @@
+Started implementing [[transfer_control]]. Although I'm currently calling
+the configuration for it "preferred content expressions". (What a mouthful!)
+
+I was mostly able to reuse the Limit code (used to handle parameters like
+--not --in otherrepo), so it can already build Matchers for preferred content
+expressions in my little Domain Specific Language.
+
+Preferred content expressions can be edited with `git annex vicfg`, which
+checks that they parse properly. 
+
+The plan is that the first place to use them is not going to be inside the
+assistant, but in commands that use the `--auto` parameter, which will use
+them as an additional constraint, in addition to the numcopies setting
+already used. Once I get it working there, I'll add it to the assistant.
+
+Let's say a repo has a preferred content setting of
+"(not copies=trusted:2) and (not in=usbdrive)"
+
+* `git annex get --auto` will get files that have less than 2 trusted
+  copies, and are not in the usb drive.
+* `git annex drop --auto` will drop files that have 2 or more trusted
+  copies, and are not in the usb drive (assuming numcopies allows dropping
+  them of course).
+* `git annex copy --auto --to thatrepo` run from another repo
+  will only copy files that have less than 2 trusted copies. (And if that
+  was run on the usb drive, it'd never copy anything!)
+
+There is a complication here.. What if the repo with that preferred content
+setting is itself trusted? Then when it gets a file, its number of
+trusted copies increases, which will make it be dropped again. :-/
+
+This is a nuance that the numcopies code already deals with, but it's
+much harder to deal with it in these complicated expressions. I need to think
+about this; the three ideas I'm working on are:
+
+1. Leave it to whoever/whatever writes these expressions to write ones
+   that avoid such problems. Which is ok if I'm the only one writing
+   pre-canned ones, in practice..
+2. Transform expressions into ones that avoid such problems. (For example,
+   replace "not copies=trusted:2" with "not (copies=trusted:2 or (in=here and
+   trusted=here and copies=trusted:3))"
+3. Have some of the commands (mostly drop I think) pretend the drop
+   has already happened, and check if it'd then want to get the file back
+   again.

doc/design/assistant/transfer_control.mdwn

@@ -63,3 +63,21 @@ Some examples of using groups:
 
 The above is all well and good for those who enjoy boolean algebra, but
 how to configure these sorts of expressions in the webapp?
+
+## the state change problem
+
+Imagine that a trusted repo has setting like `not copies=trusted:2`
+This means that `git annex get --auto` should get files not in 2 trusted
+repos. But once it has, the file is in 3 trusted repos, and so `git annex
+drop --auto` should drop it again!
+
+How to fix? Can it even be fixed? Maybe care has to be taken when
+writing expressions, to avoid this problem. One that avoids it:
+`not (copies=trusted:2 or (in=here and trusted=here and copies=trusted:3))`
+
+Or, expressions could be automatically rewritten to avoid the problem.
+
+Or, perhaps simulation could be used to detect the problem. Before
+dropping, check the expression. Then simulate that the drop has happened.
+Does the expression now make it want to add it? Then don't drop it!
+How to implement this simulation?