I see 2 other options: a) keep the last (longitude, latitude) pair for every receiver stored in your script, too, besides the db. When a new pair of values arrives, compare it with the current and only if it is different, send the "INSERT" query to the database.
b) When a new pair of values arrives, send a query (or stored procedure) that tells the database to check and "INSERT" only if the pair is different that the latest. Pandelis Theodosiou On Mon, May 9, 2011 at 1:50 PM, Albert Brandl <albert.bra...@weiermayer.com>wrote: > Hi! > > In the application I'm currently working on, we gather data from > different sources and store them in a database, but only if the > values have changed. > > One example is GPS information. We don't want to store hundreds of > identical values that are sent by the receiver, so we have to compare the > last written row with the current latitude / longitude pair. > > I use a subclass of NMEARreceiver class to determine the current latitude > and longitude. Then I query the database using a ConnectionPool instance > to get the previous values for latitude and longitude. If the result > differs from the current values, a new entry is stored, again using the > ConnectionPool. > > The problem is: How do I "connect" the current latitude and longitude to > the database query? The query returns a Deferred object, but this expects > a callback with only one argument, the result of the query. > > Storing them as instance variables of NMEAReceiver does not seem to be > the right way, since I don't know how long it takes to query the database > - the relevant GPS information might be overwritten by a new NMEA > telegram when the connection pool finally triggers the callback > > One workaround I've found is to add this information to the SQL query, > but this seems quite weird to me (and doesn't work in the case that the > query does not return a result). > > A different approach would be to implement a callback with three > arguments (result, latitude and longitude), and wrap it in a lambda > function that fills latitude and longitude with the current values. > Would this would be a better way to handle this requirement? > > How would you implement this? > > Thanks & best regards, > > Albert > -- > Albert Brandl > Weiermayer Solutions GmbH | Abteistraße 12, A-4813 Altmünster > phone: +43 (0) 720 70 30 14 | fax: +43 (0) 7612 20 3 56 > web: http://www.weiermayer.com > > _______________________________________________ > Twisted-Python mailing list > Twisted-Python@twistedmatrix.com > http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python > <http://int.ask.com/web?siteid=10000861&webqsrc=999&l=dis&q=UPDATE> <http://int.ask.com/web?siteid=10000861&webqsrc=999&l=dis&q=triple%20>
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