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?
