Thread: Constant "JTable" update

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Subject: Constant "JTable" update
From: Jean-Christophe ARNU <arnu@paratronic.fr>
To: pgsql-jdbc@postgresql.org
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Hello to all the pgJDBC communauty!

    My problem seems to be quite simple to solve, but I would like to get
you opinion on the different solutions available for the result to be
programmed as nicer as it can :

    My PostgreSQL database is used as a data storage for measure. On
process "feeds" the database, inserting measure in a table (measure). On
the other hand, I have a java program which is used to display such
results in tables (or charts). Up to there, nothing seems to be easier.
But I have to display data in "real time". The java program has to be
notified that a new measure (or data content in the measure table) was
inserted (repectively has changed).

    The straightforward solution seems to have database listeners on the
table that wakes-up a notifier in the java program. But such kind of
mechanism seems not to be implemented in the JDBC API (up to my small
knowledge).
    The second tortuous solutions (the one I use) is to query the database
relatively often to get the freshest results. This is quite bandwidth
consumming (assuming that some users should use a quite small bandwidth
connection). Using this kind of solution makes the Java application
slow...

    My questions are : is there any other solutions (Use of cursor maybe)?
is the first solution can be implemented while combining JDBC et
PostgreSQL functionnalities?

    Thank you for any remarks/answers/attention paid/flames/rtfm for these
problems.

--
Jean-Christophe ARNU
s/w developer
Paratronic France

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So, let me see if I understand correctly... you want your Java application
notifed when the table is updated, right?

You are correct that there isn't a JDBC api call that detects changes in the
database.  If I understand the question correctly, then your two main
solutions are :

1) Re-query the table often. (What your doing, and a drag to do. :-)

2) Write a trigger in the database to call an external application which
   notifies your Java clients that the table has been update.  This isn't a
   pure-java solution since the trigger can't be written in Java (yet) but it
   should work.  If you go this route, see
   http://www.postgresql.org/idocs/index.php?programmer-server.html
   for how to program triggers, specifically,
   http://www.postgresql.org/idocs/index.php?triggers.html will give you
   information on triggers and
   http://www.postgresql.org/idocs/index.php?programmer-pl.html
   will give you information on procedural languages that can be used. (Perl
   is a good place to start. See
   http://www.postgresql.org/idocs/index.php?plpgsql-trigger.html
   for that. You'll still have to build the communications mechanism on how
   you want to notify your client (RMI, TCP, other) so the app the trigger
   calls can communicated with your Java-clients, but this at least gets you
   started.

(Side note for others: Nothing is wrong with having a trigger call a perl
 method that does an exec call, right?)

On 08-Nov-2001 Jean-Christophe ARNU wrote:
> Hello to all the pgJDBC communauty!
>
>       My problem seems to be quite simple to solve, but I would like to get
> you opinion on the different solutions available for the result to be
> programmed as nicer as it can :
>
>       My PostgreSQL database is used as a data storage for measure. On
> process "feeds" the database, inserting measure in a table (measure). On
> the other hand, I have a java program which is used to display such
> results in tables (or charts). Up to there, nothing seems to be easier.
> But I have to display data in "real time". The java program has to be
> notified that a new measure (or data content in the measure table) was
> inserted (repectively has changed).
>
>       The straightforward solution seems to have database listeners on the
> table that wakes-up a notifier in the java program. But such kind of
> mechanism seems not to be implemented in the JDBC API (up to my small
> knowledge).
>       The second tortuous solutions (the one I use) is to query the database
> relatively often to get the freshest results. This is quite bandwidth
> consumming (assuming that some users should use a quite small bandwidth
> connection). Using this kind of solution makes the Java application
> slow...
>
>       My questions are : is there any other solutions (Use of cursor maybe)?
> is the first solution can be implemented while combining JDBC et
> PostgreSQL functionnalities?
>
>       Thank you for any remarks/answers/attention paid/flames/rtfm for these
> problems.
>


Virtually,
Ned Wolpert <ned.wolpert@knowledgenet.com>

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