When building scalable systems with Spring Boot, running multiple instances of your application is a common practice and using @GenratedValue in Entity classes is even more common. At first glance it looks very safe but distributed environments have their own nuances. Let’s discuss, what can go wrong ?

What Is @GeneratedValue?

@GeneratedValue is a JPA annotation used to specify how the primary key of an entity should be generated. You typically use it like this:

JPA provides four main strategies for ID generation:

• AUTO
• IDENTITY
• SEQUENCE
• TABLE

Each of these behaves differently — and not all of them are safe in a multi-instance deployment.

The Core Problem: ID Collisions

In a multi-instance environment (e.g., cloud deployment with horizontal scaling), each instance may try to generate IDs independently. Without a shared mechanism, this can cause:

• Duplicate primary keys
• Failed inserts due to constraint violations
• Data corruption or overwriting
• Performance bottlenecks

Strategy Breakdown: When Collisions Happen

Here’s how each generation strategy behaves:

1. GenerationType.IDENTITY ✅ Safe

• Uses the database’s auto-increment feature.
• Each insert operation generates the ID in the DB.
• No risk of collision, even across instances.

Drawback: Slower inserts due to round-trip to the DB.

2. GenerationType.SEQUENCE ⚠️ Safe with Correct Configuration

• Uses a database sequence to generate IDs.
• Hibernate can preallocate ID blocks to reduce DB hits using allocationSize.

Risk of collision if:

• Each instance uses its own local sequence (e.g., in-memory DB).
• allocationSize is not properly configured or mismatched with the database.

✅ Best Practice:

Ensure:

• All instances use the same shared database.
• The DB sequence increments match the allocationSize.

3. GenerationType.TABLE High Collision Risk

• Stores the current ID in a table.
• Reads, increments, and writes the ID.

Risk of collision:

• Without proper locking or isolation, two instances may read the same ID before it’s updated.

Recommendation: Avoid high-concurrency, multi-instance setups.

4. GenerationType.AUTO ⚠️ Unpredictable

• JPA chooses the strategy based on the database and provider.
• Could resolve to SEQUENCE, IDENTITY, or TABLE.

Risk of collision: Depends on what strategy gets selected.

Recommendation: Avoid using AUTO in distributed systems unless you know exactly what strategy it maps to.

Alternatives for Distributed Systems

Use UUIDs

Globally unique

• Safe across all instances
• No DB coordination needed

Drawback: Larger index size; less efficient than numeric IDs

Use Custom Distributed ID Generators (e.g., Snowflake)

Use libraries or microservices that generate distributed, sortable, and unique IDs:

• Snowflake
• ULID / KSUID
• Redis/Zookeeper-based counters

These are ideal for high-scale systems needing numeric IDs without DB hits.

Summary

Conclusion

When building Spring Boot applications that run in multi-instance environments, ID generation is a critical design choice. Misconfigured ID strategies can lead to hard-to-debug issues in production.

Stick to safe strategies like UUID, properly-configured sequences, or distributed ID services to ensure your app scales safely.