Re: [Tracker] HitsAdded, HitsRemoved and HitsModified for Xesam


The diff contains a first look at the tracker-db-sqlite.c file, I added
some comments that illustrate how a journal table "Events" will be
filled up.

Note that the table will most likely become a sqlite memory table.

The reason why I don't think a GHashTable in the C code is as good is
because we want to repeat the query in the TrackerXesamLiveSearch on
this "Events" table (for example with an INNERT JOIN with Services).

If it where a GHashTable, that query would either need a lot of OR
clauses (each ServiceID in one OR) or we'd need to do a query for each
item in the table to check whether the items affect a live search.

/me is the master of pseudo code, here I go again! 

For each query in live-search-queries do

  // This one sounds like the best to me. It requires a In-Sqlite
  // In-Memory table called "Events"

  SELECT ... FROM Events, Services ... 
        WHERE   Events.ServiceID = Services.ID 
        AND     the live-search-query 
        AND     (ServiceID is in the table)

  // Pro: short arguments list, easy query
  // Con: JOIN (although the cartesian product is relatively small)


  // This one doesn't need a "Events" table in sqlite but does need a
  // In-C In-Memory GHashTable holding all the affected ServiceIDs

  SELECT ... FROM Services ... 
        WHERE   the live-search-query 
        AND     (
                           ServiceID = hashtable[0].key
                        OR ServiceID = hashtable[1].key 
                        OR ServiceID = hashtable[2].key
                        OR ServiceID = hashtable[n].key

  // Pro: no JOIN
  // Con: long arguments list


On Tue, 2008-04-29 at 17:56 +0200, Philip Van Hoof wrote:
Pre note: 

This is about the Xesam support being done (since this week) in the


Xesam requires notifying live searches about changes that affect them.
We plan to implement this with a "events" table that journals all
creates, deletes and updates that the indexer causes.

Periodically we will handle and then flush the items in that events

I made a cracktasty diagram that contains the from-a-high-distance
abstract proposal that we have in mind for this.

This is pseudo code that illustrates the periodic handler:

bool periodic_handler (...) 


  lock indexer
  update eventstable set beinghandled=1 where 1=1 (all items)
  unlock indexer

  foreach query in all livequeries
     added, modified, removed = query.execute-on (eventstable)
     query.emit_added (added)
     query.emit_removed (removed)
     query.emit_modified (modified)

  lock indexer
  delete from eventstable where beinghandled = 1
  unlock indexer

  return (!stopping)


Here's a piece of IRC log between me and jamiecc about the proposal:

pvanhoof ping jamiemcc 
pvanhoof same thing
pvanhoof I'll make a pdf
jamiemcc oh ok
pvanhoof Sending
pvanhoof ok
pvanhoof so
pvanhoof it's about the hitsadded, hitsremoved and hitsmodified signals for xesam
pvanhoof What we have in mind is using a "events" table that is a journal for all creates, deletes and 
pvanhoof Periodically we will flush that table, each create (insert), update and each delete we add a 
record in that table
pvanhoof We'll make sure the table is queryable in a similar fashion as how the Xesam query will execute
pvanhoof In the periodical handler we'll for each live search check whether it got affected by the items in 
the events table
pvanhoof In pseudo, the handler:
jamiemcc sounds feasible
pvanhoof gboolean periodic_handler (void data) {
pvanhoof   lock indexer
pvanhoof   update eventstable set beinghandled=1 where 1=1 (all items)
pvanhoof   unlock indexer
pvanhoof   foreach query in all live queries
pvanhoof      added, modified, removed = query.execute-on (eventstable)
pvanhoof      query.emit_added (added)
pvanhoof      query.emit_removed (removed)
pvanhoof      query.emit_modified (modified)
pvanhoof   done
pvanhoof   lock indexer
pvanhoof   delete from eventstable where beinghandled = 1
pvanhoof   unlock indexer
pvanhoof }
pvanhoof I've send you a diagram that you can look at as if it's a state-activity one, a ERD and a class 
diagram :) now how cool is that?? :)
pvanhoof it's just three columns, although the ERD is quite simplistic of course
jamiemcc yeah just go tit
* fritschy (~fritschy 84 19 173 195) has left #tracker
pvanhoof so, the current idea is to adapt those stored procedures into transactions that will also add this 
record to the "events" table
* fritschy (~fritschy 84 19 173 195) has joined #tracker
pvanhoof Which might not be sufficient, and we kinda lack the in-depth know-how of all the db handling of 
pvanhoof So that's a first issue we want to discuss with you
pvanhoof The other is stopping the indexing, restarting it (locking it, in the pseudo code): what you think 
about that
jamiemcc ok I will need to think about it - I iwll probably reply later tonight and we can discuss tomorrow
pvanhoof I adapted my initial proposal to have two short critical sections rather than letting the entire 
periodic handler be one critical section
pvanhoof that way the lock is smaller
jamiemcc the indexer will be seaparte process so will need to be locked via dbus signals
pvanhoof by just adding a column to the events table
pvanhoof yes but I guess we want any such locking to be short
jamiemcc well yes 
pvanhoof then once the items that are to be handled are identified, we for each live-search check whether 
the live-search is affected
pvanhoof and we perform the necessary hitsadded, hitsremoved and hitsmodified signals if needed
pvanhoof if all is done, we simply purge the handled items from the events table
jamiemcc the query results will be store din temp tables
pvanhoof which is the second location where we want the indexer to be locked-out
jamiemcc remember a query may be a cursor so wont include entire result set
pvanhoof No okay, but that's something the check needs to worry about 
pvanhoof so ottela is working on a query for the live-search
jamiemcc ok cool
pvanhoof and if we only want to update if the client has the affected item visible, due to cursor-usage
pvanhoof then i guess we'll somehow need to get that info into trackerd
jamiemcc any reason we dont store whats change din memory rather than sqlite table?
pvanhoof oh, that's abstract right now
jamiemcc o
jamiemcc ok
pvanhoof "tracker's event table" can also be a hashtable for me ..
jamiemcc yeah fine
pvanhoof implementation detail
pvanhoof since it doesn't need to be persistent ...
pvanhoof difference is that either we use a memory table and still a transaction for the three stored 
pvanhoof or we adapt code
jamiemcc prefer hashtable as amount of data will be small
jamiemcc can even be a list
pvanhoof ok, your comments/ideas on this would of course be very useful btw
jamiemcc yeah I will think about it more tonight and get back to you
pvanhoof sounds great
pvanhoof I'll make a mail about this to the mailing list? or I await your ideas tomorrow?
pvanhoof I'll just wait for now
jamiemcc you cna mail if you like
jamiemcc I will reply to it

tracker-list mailing list
tracker-list gnome org
Philip Van Hoof, freelance software developer
home: me at pvanhoof dot be 
gnome: pvanhoof at gnome dot org

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