Handling Sequence Overflows

In this lesson, you will learn how to proactively manage database sequence limits to prevent application crashes caused by primary key exhaustion in high-volume systems.

Understanding Sequence Exhaustion

In EF Core, we often rely on HiLo algorithms or database-native sequences (like IDENTITY or SEQUENCE objects) to generate primary keys. While these are convenient, they are backed by fixed-size numeric types. If you define a sequence with a BIGINT (which is large) or, more dangerously, an INT (which caps at roughly 2.1 billion), the system will eventually run out of available values. When a sequence hits its MAXVALUE, the database will throw an exception, effectively halting all inserts.

Monitoring and Management

In long-running production systems, you cannot wait for the overflow to occur. Instead, you must monitor the sequence’s current value relative to its defined limit.

You can query your database's system views (such as sys.sequences in SQL Server or pg_sequences in PostgreSQL) to track usage. A common strategy involves implementing a background service or an EF Core interceptor that alerts your team when a sequence reaches a predefined percentage of its capacity (e.g., 80%).

Illustration: Checking Sequence Status

If you are using a sequence for your OrderId in SQL Server, you can run a check to see how close you are to the limit:

// Example SQL query to check sequence exhaustion
var currentUsage = await context.Database.SqlQueryRaw<long>(@"
    SELECT CAST(current_value AS BIGINT) FROM sys.sequences 
    WHERE name = 'OrderSequence'").SingleAsync();

const long MaxInt = 2147483647; // If using INT
if (currentUsage > (MaxInt * 0.8)) 
{
    _logger.LogWarning("Sequence 'OrderSequence' is at 80% capacity!");
}

Remediation Strategies

When a sequence is nearing overflow, you have two primary options:

1. Cycle the Sequence: Configure the sequence to CYCLE (start over), though this requires ensuring your business logic handles duplicate IDs or that you have a secondary unique key.
2. Alter/Migrate to BIGINT: If you are currently using INT, you must perform a migration to convert the column and the underlying sequence to BIGINT. This is a significant schema change that requires careful planning to avoid locking large tables.

Key Takeaway: Never assume a sequence will last forever; monitor your sequence values programmatically and plan for schema migration to larger data types before you hit the integer ceiling.