What are Cosmos DB triggers and user-defined functions (UDFs)?

Cosmos DB supports two additional server-side JavaScript constructs beyond stored procedures: triggers and user-defined functions (UDFs). Both run on the Cosmos DB compute engine but serve different purposes.

Triggers are JavaScript functions attached to a container that fire automatically before or after a write operation (create, replace, delete). There are two kinds:

• Pre-triggers — Execute before the write is committed. Can modify the request (e.g., auto-populate a createdAt timestamp, validate field formats, generate a composite field). If the pre-trigger throws, the write is rejected.
• Post-triggers — Execute after the write succeeds. Useful for updating aggregate documents, sending notifications, or maintaining denormalized data. Post-triggers run within the same transaction scope as the write.

Triggers must be explicitly listed in the SDK request options — they do not fire automatically unless you opt in per request. This is a common gotcha: unlike SQL database triggers, Cosmos DB triggers require the client to declare PreTriggerInclude or PostTriggerInclude on each write request:

await container.CreateItemAsync(item, partitionKey, new ItemRequestOptions {
    PreTriggers = new List<string> { "validateTimestamp" },
    PostTriggers = new List<string> { "updateUserAggregate" }
});

User-Defined Functions (UDFs) are JavaScript helper functions callable from within a Cosmos DB query using the udf. prefix. They let you extend the query language with custom computation:

-- Query using a UDF to categorize price ranges
SELECT c.id, udf.getPriceCategory(c.price) AS category
FROM c

UDFs cannot modify data — they are read-only transformations used in SELECT and WHERE clauses. They also have no access to external systems or the Cosmos DB context object (unlike stored procedures).

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