Alle lunedì 07 gennaio 2008, fde ha scritto:
2) The application insert data from more than one workstation. So there
is a while when the data the user see are not the real data inserted in
the db, so I have to refresh the grid. For this purpose do I have to
refresh the full table with a "select * from table" or there is an other
way ?
I'm developing an application for internal use at our company, based on a
Postgres database and wxPython interface.
There will be about 30 to 40 clients at the end of the story, all accessing
data with different functions but definetly everyone needs to know if some
data has changed.
The task of syncronizing the different clients is done by a mix of SQLAlchemy
(www.sqlalchemy.org) and Pyro (pyro.sourceforge.net) and works more or less
like this:
- a client reads its data (with a specific interface or a grid depending on
what the client does) through SQLAlchemy and subscribes to a Pyro server for
news
- whenever a client changes some data, the database "intercepts" changes on
COMMIT and "informs" other clients through the Pyro server: basically the
database works out the "type" of the data being changed/inserted and
broadcasts the IDs with Pyro.
The only drawback I can see is that at COMMIT point many clients are likely to
access the database and refresh informations: so far this is a minor problem,
since data do not change that much and most clients are for production
tracking so they read few database rows at one time.
Answering your question, with a similar method any client would easily know if
rows were inserted or deleted with count(), as well as retrieve and refresh
changed rows.
Personal note: I would really recommend to study and use SQLAlchemy, because
it is very flexible: in the end the db abstraction and access looks very
pythonic, and you can mostly forget to write and maintain SQL code.
Here's a small example of how to browse a table with changes and deletions:
from myDatabase import database, mappers
db = database.myDb(engine="postgres://dbuser@localhost:5432/database")
db.session.begin()
try:
for c in mappers.Customer.query.all():
if c.name == '':
db.session.delete(c)
else:
c.name = c.name.upper()
db.session.commit()
except:
db.session.rollback()
raise
finally:
db.session.close()
···
--
Cordialmente
Stefano Bartaletti
Responsabile Software
G.Tosi Spa Tintoria
Skype account: stefano.bartaletti
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21052 Busto Arsizio (VA)
Tel. +39 0331 34 48 11
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