see the following for background discussion:
http://groups.google.com/group/ankisrs-users/browse_thread/thread/4db5e82f7dff74fb
- change sched index to the more efficient gid, queue, due
- drop the dynamic index support. as there's no no q/a cache anymore, it's
cheap enough to hit the cards table directly, and we can't use the index in
its new form.
- drop order by clauses (see todo)
- ensure there's always an active group. if users want to study all groups at
once, they need to create a top level group. we do this because otherwise
the 'top level group' that's active when everything is selected is not
clear.
to do:
- new cards will appear in gid order, but the gid numbers don't reflect
alphabetical sorting. we need to change the scheduling code so that it steps
through each group in turn
- likewise for the learn queue
because group deletions are likely to be a semi-common operation (esp. for new users trying out shared material), deleting groups will no longer cause a full sync. in order to avoid syncing issues, we now allow cards/facts/etc to point to an invalid group, and in that case, we just treat them like they're in the default group
Decks now have an "update sequence number". All objects also have a USN, which
is set to the deck USN each time they are modified. When syncing, each side
sends any objects with a USN >= clientUSN. When objects are copied via sync,
they have their USNs bumped to the current serverUSN. After a sync, the USN on
both sides is set to serverUSN + 1.
This solves the failing three way test, ensures we receive all changes
regardless of clock drift, and as the revlog also has a USN now, ensures that
old revlog entries are imported properly too.
Objects retain a separate modification time, which is used for conflict
resolution, deck subscriptions/importing, and info for the user.
Note that if the clock is too far off, it will still cause confusion for
users, as the due counts may be different depending on the time. For this
reason it's probably a good idea to keep a limit on how far the clock can
deviate.
We still keep track of the last sync time, but only so we can determine if the
schema has changed since the last sync.
The media code needs to be updated to use USNs too.
While thinking about media syncing I realized the current sync algorithm is
flawed in certain cases. It might be time to think about using a USN instead,
as that should also hopefully solve the skewed clock problem properly.
- if we store it inside the media folder, we inadvertently bump the folder mod
time every time sqlite creates a journal file
- close/reopen the media db as the deck is closed/opened
I removed the media database in an earlier commit, but it's now necessary
again as I decided to add native media syncing to AnkiWeb.
This time, the DB is stored in the media folder rather than with the deck.
This means we avoid sending it in a full sync, and makes deck backups faster.
The DB is a cache of file modtimes and checksums. When findChanges() is
called, the code checks to see which files were added, changed or deleted
since the last time, and updates the log of changes. Because the scanning step
and log retrieval is separate, it's possible to do the scanning in the
background if the need arises.
If the DB is deleted by the user, Anki will forget any deletions, and add all
the files back to the DB the next time it's accessed.
File changes are recorded as a delete + add.
media.addFile() could be optimized in the future to log media added manually
by the user, allowing us to skip the full directory scan in cases where the
only changes were manually added media.
Ported the sync code to the latest libanki structure. Key points:
No summary:
The old style got each side to fetch ids+mod times and required the client to
diff them and then request or bundle up the appropriate objects. Instead, we now
get each side to send all changed objects, and it's the responsibility of the
other side to decide what needs to be merged and what needs to be discarded.
This allows us to skip a separate summary step, which saves scanning tables
twice, and allows us to reduce server requests from 4 to 3.
Schema changes:
Certain operations that are difficult to merge (such as changing the number of
fields in a model, or deleting models or groups) result in a full sync. The
user is warned about it in the GUI before such schema-changing operations
execute.
Sync size:
For now, we don't try to deal with large incremental syncs. Because the cards,
facts and revlog can be large in memory (hundreds of megabytes in some cases),
they would have to be chunked for the benefit of devices with a low amount of
memory.
Currently findChanges() uses the full fact/card objects which we're planning to
send to the server. It could be rewritten to fetch a summary (just the id, mod
& rep columns) which would save some memory, and then compare against blocks
of a few hundred remote objects at a time. However, it's a bit more
complicated than that:
- If the local summary is huge it could exceed memory limits. Without a local
summary we'd have to query the db for each record, which could be a lot
slower.
- We currently accumulate a list of remote records we need to add locally.
This list also has the potential to get too big. We would need to
periodically commit the changes as we accumulate them.
- Merging a large amount of changes is also potentially slow on mobile
devices.
Given the fact that certain schema-changing operations require a full sync
anyway, I think it's probably best to concentrate on a chunked full sync for
now instead, as provided the user syncs periodically it should not be easy to
hit the full sync limits except after bulk editing operations.
Chunked partial syncing should be possible to add in the future without any
changes to the deck format.
Still to do:
- deck conf merging
- full syncing
- new http proxy
As per the forum thread, the current due counts are really demotivating when
there's a backlog of cards. In attempt to solve this, I'm trying out a new
behaviour as the default: instead of reporting all the due cards including the
backlog, the status bar will show an increasing count of cards studied that
day. Theoretically this should allow users to focus on what they've done
rather than what they have to do. The old behaviour is still there as an option.
Instead of collecting the exact number of cards, we just record whether a
group has any reviews or new cards. By not needing to calculate the exact
numbers, it runs a lot faster than before.
Also, changed the group code to ensure parents are automatically created when
a group is added.
As discussed on the forums, moving to a single collection requires moving some
deck-level configuration into groups so users can have different settings like
new cards/day for each top level item.
Also:
- store id in groups
- add mod time to gconf updates
- move the limiting code that's not specific to scheduling into groups.py
- store the current model id per top level group
- moved tags into json like previous changes, and dropped the unnecessary id
- added tags.py for a tag manager
- moved the tag utilities from utils into tags.py
Like the previous change, models have been moved from a separate DB table to
an entry in the deck. We need them for many operations including reviewing,
and it's easier to keep them in memory than half on disk with a cache that
gets cleared every time we .reset(). This means they are easily serialized as
well - previously they were part Python and part JSON, which made access
confusing.
Because the data is all pulled from JSON now, the instance methods have been
moved to the model registry. Eg:
model.addField(...) -> deck.models.addField(model, ...).
- IDs are now timestamped as with groups et al.
- The data field for plugins was also removed. Config info can be added to
deck.conf; larger data should be stored externally.
- Upgrading needs to be updated for the new model structure.
- HexifyID() now accepts strings as well, as our IDs get converted to strings
in the serialization process.
Rather than use a combination of id lookups on the groups table and a group
configuration cache in the scheduler, I've moved the groups and group config
into json objects on the deck table. This results in a net saving of code and
saves one or more DB lookups on each card answer, in exchange for a small
increase in deck load/save work.
I did a quick survey of AnkiWeb, and the vast majority of decks use less than
100 tags, and it's safe to assume groups will follow a similar pattern.
All groups and group configs except the default one will use integer
timestamps now, to simplify merging when syncing and importing.
defaultGroup() has been removed in favour of keeping the models up to date
(not yet done).
- should never skip recording graves, for the sake of merging
- 1.0 upgrade will fail on decks that have the same fact creation date. need
to work around this in the future
The approach of using incrementing id numbers works for syncing if we assume
the server is canonical and all other clients rewrite their ids as necessary,
but upon reflection it is not sufficient for merging decks in general, as we
have no way of knowing whether objects with the same id are actually the same
or not. So we need some way of uniquely identifying the object.
One approach would be to go back to Anki 1.0's random 64bit numbers, but as
outlined in a previous commit such large numbers can't be handled easy in some
languages like Javascript, and they tend to be fragmented on disk which
impacts performance. It's much better if we can keep content added at the same
time in the same place on disk, so that operations like syncing which are mainly
interested in newly added content can run faster.
Another approach is to add a separate column containing the unique id, which
is what Mnemosyne 2.0 will be doing. Unfortunately it means adding an index
for that column, leading to slower inserts and larger deck files. And if the
current sequential ids are kept, a bunch of code needs to be kept to ensure ids
don't conflict when merging.
To address the above, the plan is to use a millisecond timestamp as the id.
This ensures disk order reflects creation order, allows us to merge the id and
crt columns, avoids the need for a separate index, and saves us from worrying
about rewriting ids. There is of course a small chance that the objects to be
merged were created at exactly the same time, but this is extremely unlikely.
This commit changes models. Other objects will follow.
the initial plan was to zero the creation time and leave the cards/facts there
until we have a chance to garbage collect them on a schema change, but such an
approach won't work with deck subscriptions
- cards in final review are first reset as rev cards so that type==queue and
they can be restored correctly
- new cards in learning have type set to 1 so they too can be restored
correctly
instead of completely resetting a card like we did in resetCards() in the
past, forgetCards() just puts the card back in the new queue and leaves the
factor and revlog alone. If users want to complete reset a card, they'll need to
export it.
- use negative numbers to denote second intervals
- record the rev ivl when leaving lrn queue
- improve revlog upgrade
- don't truncate precision when recording time taken
reps should now be equal to the number of entries in the revlog, and only
exists so that we can order by review count in the browser efficiently
streak is no longer necessary as we have a learn queue now
originally the plan was to get the user to "forget learning cards" or "remove
final drill" when switching between categories, but that's cumbersome and not
intuitive
The undo code was using triggers and a temporary table to write out all changed rows before making a change. This made for powerful undo/redo support, but had some problems:
- creating the tables and triggers wasn't cheap, especially on mobile devices
- likewise, every data modification required writing into two separate databases, almost doubling the amount of writes required
- it was possible to leave the DB in an inconsistent state if an undoable operation is followed by a non-undoable operation that references the undoable operation, and the user then rolls back the undoable operation.
To address these issues, we simplify undo by integrating it with the autosave changes:
- .save() can be passed a name to mark a rollback point. If the user undoes the change, any changes since the last save are lost
- autosaves happen every 5 minutes, and are pushed back on a .save(), so the maximum work a user can lose is 5 minutes.
- reviews are handled separately, so we can let the user undo multiple reviews at once
- if necessary, special cases could be added for other operations like marking
This means that if a user does two damaging operations in a row they won't be able to restore the first one, but such an event is both unlikely, and is also covered by the backups made each time a deck is opened.
