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.
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
- 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
the field cache (fsums table) also needs to store the model id to preserve the
old behaviour of limiting duplicate checks to a given model, and to ensure
we're actually comparing against the same fields
removed the dingsbums and wcu importers; will accept them back if the authors
port them to the new codebase.
Anki used random 64bit IDs for cards, facts and fields. This had some nice
properties:
- merging data in syncs and imports was simply a matter of copying each way,
as conflicts were astronomically unlikely
- it made it easy to identify identical cards and prevent them from being
reimported
But there were some negatives too:
- they're more expensive to store
- javascript can't handle numbers > 2**53, which means AnkiMobile, iAnki and
so on have to treat the ids as strings, which is slow
- simply copying data in a sync or import can lead to corruption, as while a
duplicate id indicates the data was originally the same, it may have
diverged. A more intelligent approach is necessary.
- sqlite was sorting the fields table based on the id, which meant the fields
were spread across the table, and costly to fetch
So instead, we'll move to incremental ids. In the case of model changes we'll
declare that a schema change and force a full sync to avoid having to deal
with conflicts, and in the case of cards and facts, we'll need to update the
ids on one end to merge. Identical cards can be detected by checking to see if
their id is the same and their creation time is the same.
Creation time has been added back to cards and facts because it's necessary
for sync conflict merging. That means facts.pos is not required.
The graves table has been removed. It's not necessary for schema related
changes, and dead cards/facts can be represented as a card with queue=-4 and
created=0. Because we will record schema modification time and can ensure a
full sync propagates to all endpoints, it means we can remove the dead
cards/facts on schema change.
Tags have been removed from the facts table and are represented as a field
with ord=-1 and fmid=0. Combined with the locality improvement for fields, it
means that fetching fields is not much more expensive than using the q/a
cache.
Because of the above, removing the q/a cache is a possibility now. The q and a
columns on cards has been dropped. It will still be necessary to render the
q/a on fact add/edit, since we need to record media references. It would be
nice to avoid this in the future. Perhaps one way would be the ability to
assign a type to fields, like "image", "audio", or "latex". LaTeX needs
special consider anyway, as it was being rendered into the q/a cache.
SQLAlchemy is a great tool, but it wasn't a great fit for Anki:
- We often had to drop down to raw SQL for performance reasons.
- The DB cursors and results were wrapped, which incurred a
sizable performance hit due to introspection. Operations like fetching 50k
records from a hot cache were taking more than twice as long to complete.
- We take advantage of sqlite-specific features, so SQL language abstraction
is useless to us.
- The anki schema is quite small, so manually saving and loading objects is
not a big burden.
In the process of porting to DBAPI, I've refactored the database schema:
- App configuration data that we don't need in joins or bulk updates has been
moved into JSON objects. This simplifies serializing, and means we won't
need DB schema changes to store extra options in the future. This change
obsoletes the deckVars table.
- Renamed tables:
-- fieldModels -> fields
-- cardModels -> templates
-- fields -> fdata
- a number of attribute names have been shortened
Classes like Card, Fact & Model remain. They maintain a reference to the deck.
To write their state to the DB, call .flush().
Objects no longer have their modification time manually updated. Instead, the
modification time is updated when they are flushed. This also applies to the
deck.
Decks will now save on close, because various operations that were done at
deck load will be moved into deck close instead. Operations like undoing
buried card are cheap on a hot cache, but expensive on startup.
Programmatically you can call .close(save=False) to avoid a save and a
modification bump. This will be useful for generating due counts.
Because of the new saving behaviour, the save and save as options will be
removed from the GUI in the future.
The q/a cache and field cache generating has been centralized. Facts will
automatically rebuild the cache on flush; models can do so with
model.updateCache().
Media handling has also been reworked. It has moved into a MediaRegistry
object, which the deck holds. Refcounting has been dropped - it meant we had
to compare old and new value every time facts or models were changed, and
existed for the sole purpose of not showing errors on a missing media
download. Instead we just media.registerText(q+a) when it's updated. The
download function will be expanded to ask the user if they want to continue
after a certain number of files have failed to download, which should be an
adequate alternative. And we now add the file into the media DB when it's
copied to th emedia directory, not when the card is commited. This fixes
duplicates a user would get if they added the same media to a card twice
without adding the card.
The old DeckStorage object had its upgrade code split in a previous commit;
the opening and upgrading code has been merged back together, and put in a
separate storage.py file. The correct way to open a deck now is import anki; d
= anki.Deck(path).
deck.getCard() -> deck.sched.getCard()
same with answerCard
deck.getCard(id) returns a Card object now.
And the DB wrapper has had a few changes:
- sql statements are a more standard DBAPI:
- statement() -> execute()
- statements() -> executemany()
- called like execute(sql, 1, 2, 3) or execute(sql, a=1, b=2, c=3)
- column0 -> list
Cards had developed quite a lot of cruft from incremental changes, and a
number of important attributes were stored in names that had no bearing to
their actual use.
Added:
- position, which new cards will be sorted on in the future
- flags, which is reserved for future use
Renamed:
- type to queue
- relativeDelay to type
- noCount to lapses
Removed:
- all new/young/matureEase counts; the information is in the revlog
- firstAnswered, lastDue, lastFactor, averageTime and totalTime for the same
reason
- isDue, spaceUntil and combinedDue, because they are no longer used. Spaced
cards will be implemented differently in a coming commit.
- priority
- yesCount, because it can be inferred from reps & lapses
- tags; they've been stored in facts for a long time now
Also compatibility with deck versions less than 65 has been dropped, so decks
will need to be upgraded to 1.2 before they can be upgraded by the dev code.
All shared decks are on 1.2, so this should hopefully not be a problem.
Previously we had an index on the value field, which was very expensive for
long fields. Instead we use a separate column and take the first 8 characters
of the field value's md5sum, and index that. In decks with lots of text in
fields, it can cut the deck size by 30% or more, and many decks improve by
10-20%. Decks with only a few characters in fields may increase in size
slightly, but this is offset by the fact that we only generate a checksum for
fields that have uniqueness checking on.
Also, fixed import->update reporting the total # of available facts instead of
the number of facts that were imported.
When you call operations like deleteCards(), suspendCards() and so on, it is
now necessary to call deck.reset() afterwards. This allows the calling code to
delay a reset if necessary. If the calling code calls a function that says the
caller must reset, the caller should be sure to call .reset() and fetch the
current card again. Failure to do the latter will result in answerCard()
attempting to remove the card from the queue, when the queue has been cleared.
In various parts of the code we need to get all cards of a given category
(new, failed, etc) regardless of whether they're suspended, buried, etc. So we
store the true type in the obsolete relativeDelay column and add in index for
it, because it's cheaper than putting indices on reps & successive.
- reimplement reviewEarly and newEarly by replacing parts of the scheduler,
instead of adding special conditions
- remove references to isDue and priority (1,2,3,4) which is not necessary
anymore
- add option to switch between per-day scheduling and due now scheduling
- newCardsToday() -> newCardsDoneToday()
- don't decrement counts for suspended cards
- make sure to update type when suspending/unsuspending
- fix findCards()
- set hardInterval = 1-1.1 on upgrade, or the default per day scheduling doesn't
make sense
Previously we used getCard() to fetch a card at the time. This required a
number of indices to perform efficiently, and the indices were expensive in
terms of disk space and time required to keep them up to date. Instead we now
gather a bunch of cards at once.
- Drop checkDue()/isDue so writes are not necessary to the DB when checking
for due cards
- Due counts checked on deck load, and only updated once a day or at the end
of a session. This prevents cards from expiring during reviews, leading to
confusing undo behaviour and due counts that go up instead of down as you
review. The default will be to only expire cards once a day, which represents
a change from the way things were done previously.
- Set deck var defaults on deck load/create instead of upgrade, which should
fix upgrade issues
- The scheduling code can now have bits and pieces switched out, which should
make review early / cram etc easier to integrate
- Cards with priority <= 0 now have their type incremented by three, so we can
get access to schedulable cards with a single column.
- rebuildQueue() -> reset()
- refresh() -> refreshSession()
- Views and many of the indices on the cards table are now obsolete and will
be removed in the future. I won't remove them straight away, so as to not
break backward compatibility.
- Use bigger intervals between successive card templates, as the previous
intervals were too small to represent in doubles in some circumstances
Still to do:
- review early
- learn more
- failing mature cards where delay1 > delay0
