如何检查是否已在数据库中插入具有某些值的特定行

Question

I am using Informix DB. This question may not be tied to one specific database. But I want to know how I can in Java, continuously probe into a Database and check if a certain row has been added to a table in the DB. Basically, the flow is:

1. My Java application should use JDBC to check if a certain table is populated.
2. If no, it should wait until a row has been inserted.

My question how can I have Java be aware of a row insertion. I am not expecting to add any triggers or anything, but in pure Java be able to check that the row is added.

Some thoughts that come to my mind are continuously call DB for the row, or periodically (every half-hour or so) call DB and check if the row is available. But what I am looking for is something like a Listener which can do this.

Answer 1

There is no facility in the Informix DBMS to signal when a particular row arrives in a table.

Well, I say that, but there is the DB-Cron facility which can periodically execute tasks (inside the server), and you could conceivably schedule a task to poll for the data to see if it has arrived and to send a message (somehow) to indicate that it has. It would be non-trivial, especially the part the indicates that it has arrived.

The JDBC protocol (and SQL protocols generally) are essentially synchronous; the client sends a request and waits for an answer from the DBMS.

So, pragmatically, if your delay period is half an hour, you can either create an admin task to handle the processing (you could write a Java UDR to be executed in the server by the server if that's crucial to you), or you can arrange for the Java (client-side) program to poll periodically to find out whether the information you need is there. A half-hour delay is not going to stress anything, even with a moderate number of processes polling for separate values (or even the same value). On the other hand, you normally try to avoid polling when you can. You'll need to strike a balance between responsiveness to the special data arriving and general system responsiveness. On the whole, general system responsiveness is more important, so keep the polling interval as large as you can.

If your polling interval needed to be sub-second, then the balance would be different - the job would be a lot harder.