The DocumentDB API supports optimistic CONCURRENCY control (OCC) through HTTP entity tags, or ETags. Every DocumentDB API resource has an ETag, and the ETag is set on the server every time a document is updated. The ETag header and the current VALUE are included in all response messages. ETags can be used with the If-Match header to allow the server to decide whether a resource should be updated. The If-Match value is the ETag value to be checked against. If the ETag value matches the server ETag value, the resource is updated. If the ETag is no longer current, the server rejects the OPERATION with an "HTTP 412 Precondition failure" response CODE. The client then re-fetches the resource to ACQUIRE the current ETag value for the resource. In addition, ETags can be used with the If-None-Match header to determine whether a re-fetch of a resource is needed.

The DocumentDB API supports optimistic concurrency control (OCC) through HTTP entity tags, or ETags. Every DocumentDB API resource has an ETag, and the ETag is set on the server every time a document is updated. The ETag header and the current value are included in all response messages. ETags can be used with the If-Match header to allow the server to decide whether a resource should be updated. The If-Match value is the ETag value to be checked against. If the ETag value matches the server ETag value, the resource is updated. If the ETag is no longer current, the server rejects the operation with an "HTTP 412 Precondition failure" response code. The client then re-fetches the resource to acquire the current ETag value for the resource. In addition, ETags can be used with the If-None-Match header to determine whether a re-fetch of a resource is needed.