Question

I’m building an application with distributed parts.
Meaning, while one part (writer) maybe inserting, updating information to a database, the other part (reader) is reading off and acting on that information.

Now, i wish to trigger an action event in the reader and reload information from the DB whenever i insert something from the writer.

Is there a simple way about this?

Would this be a good idea? :

// READER
while(true) {
    connect();

    // reload info from DB
    executeQuery("select * from foo");

    disconnect();
}

EDIT : More info

This is a restaurant Point-of-sale system.

Whenever the cashier punches an order into the db – the application in the kitchen get’s the notification. This is the kind of system you would see at McDonald’s.

The applications needn’t be connected in any way. But each part will connect to a single mySQL server.

And i believe i’d expect immediate notifications.

Answer 1

The idea you are talking about is called “polling”. As Graphain pointed out you must add a delay in the loop. The amount of delay should be decided based on factors like how quickly you want your reader to detect any changes in database and how fast the writer is expected to insert/update data.

Next improvement to your solution could be to have an change-indicator within the database. Your algo will look something like:

// READER
while(true) {
    connect();

    // reload info from DB
    change_count=executeQuery("select change_count from change_counters where counter=foo");
    if(change_count> last_change_count){
      last_change_count=change_count;
      reload();
    }

    disconnect();
}

The above change will ensure that you do not reload data unnecessarily.
You can further tune the solution to keep a row level change count so that you can reload only the updated rows.