How to Prefetch Oracle Sequence IDs in a Distributed Environment

Running a distributed application is both an exciting and challenging endeavor. One common issue developers encounter is how to effectively manage database sequences across multiple application servers to avoid conflicts and ensure data integrity. This post explores a viable solution for prefetching Oracle sequence IDs in a distributed environment using Java and Oracle database.

The Challenge: Managing Sequence IDs in a Distributed Application

Imagine you’re working on a Java application distributed across five servers, all connected to a single Oracle 9i database. You need to prefetch a batch of 100 IDs from a sequence to maintain performance and efficiency. In a single-threaded environment, this task is straightforward—simply fetch the IDs using a couple of SQL queries.

Here’s how it might look:

SELECT seq.nextval FROM dual;
ALTER SEQUENCE seq INCREMENT BY 100;
SELECT seq.nextval FROM dual;

While the first query retrieves the current sequence ID for immediate use, the second query fetches the next available ID after incrementing the sequence. Easy, right?

However, things become complex in a distributed environment where multiple threads across different servers are trying to access the same sequence concurrently. Synchronizing access on the Java side isn’t feasible since parts of your application may not even run on the same JVM or physical machine. What to do in this case?

A Robust Solution: Always Increment by 100

One effective approach to managing this issue is to modify the sequence to always increment by 100. By setting the increment value statically, you ensure that each nextval call retrieves a batch of 100 sequence numbers. Here’s how you can set this up:

Step 1: Create the Sequence

You can create an Oracle sequence that starts at 100 and increments by 100 as follows:

CREATE SEQUENCE so_test START WITH 100 INCREMENT BY 100 NOCACHE;

Step 2: Retrieve Sequence IDs

Once the sequence is set up, you can retrieve both the first and last sequence IDs using the following query:

SELECT so_test.nextval - 99 AS first_seq, so_test.currval AS last_seq FROM dual;

This will give you a range of sequence IDs for your application to use. For example:

 FIRST_SEQ   LAST_SEQ
---------- ----------
         1        100
       101        200
       201        300

Step 3: Be Aware of DDL Operations

While handling sequences, keep in mind that certain Data Definition Language (DDL) operations can produce implicit commits. Take caution while executing DDL commands, as they can affect your transaction’s integrity. For instance:

SELECT * FROM xx;
-- no rows selected

INSERT INTO xx VALUES ('x');
-- 1 row created

ALTER SEQUENCE so_test INCREMENT BY 100;
-- Sequence altered

ROLLBACK;
-- Rollback complete

SELECT * FROM xx;
-- should show 'x' still there

A Note on Dynamic Increments

If your scenario requires dynamically adjusted increments instead of fixed values, this solution may need further refinement. While this approach works well when you can define a clear increment, dynamic conditions will require additional logic to handle concurrency and state management across distributed nodes.

Conclusion

Implementing a simple solution by adjusting the sequence to an increment of 100 can significantly alleviate the challenges faced when prefetching Oracle sequence IDs in a distributed Java application. While this approach provides a foundation, keep in mind the need for careful transaction management, especially when working with DDL operations.

By proactively managing your sequences and considering how they operate in a concurrent environment, you can develop robust applications that maintain integrity and efficiency across distributed systems.