I am working on a regular iPhone app which pulls data from a server (XML, JSON, etc...), and I'm wondering what is
the best way to implement synching data. Criteria are speed (less network data exchange), robustness (data recovery in case update fails), offline access and flexibility (adaptable when
the structure of
the database changes slightly, like a new column). I know it varies from app to app, but can you guys share some of your strategy/experience?
For me, I'm thinking of something like this:
1) Store Last Modified Date in iPhone
2) Upon launching, send a message like getNewData.php?lastModifiedDate=...
3) Server will process and send back only modified data from last time.
4) This data is formatted as so:
<+><data id="..."></data></+> // add this to SQLite/CoreData
<-><data id="..."></data></-> // remove this
<%><data id="..."><attribute>newValue</attribute></data></%> // new modified value
I don't want to make <+, <-, <%... for each attribute as well, because it would be too complicated, so probably when receive a <% field, I would just remove
the data with
the specified id and then add it again (assuming id here is not some automatically auto-incremented field).
5) Once everything is downloaded and updated, I will update
the Last Modified Date field.
The main problem with this strategy is: If
the network goes down when I am updating something =
the Last Modified Date is not yet updated = next time I relaunch
the app, I will have to go through
the same
thing again. Not to mention potential inconsistent data. If I use a temporary table for update and make
the whole
thing atomic, it would work, but then again, if
the update is too long (lots of data change),
the user has to wait a long time until new data is available. Should I use Last-Modified-Date for each of
the data field and update data gradually?
