[Twisted-Python] Question about using the adbapi

[Pandelis Theodosiou]

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.brandl at 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
>
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