When is it essential to use namespaces in Apex REST class URLs?

Using namespaces in Apex REST class URLs is essential when MyDomain is enabled because it helps to uniquely identify the components and avoid conflicts with other packages or Salesforce features. When MyDomain is active, the URL structure includes the namespace of the managed package, which is vital for distinguishing REST service endpoints. This ensures that requests are correctly routed to the appropriate resources and methods defined in your Apex classes.

In environments where MyDomain is not enabled, the use of namespaces is less critical since the public endpoint structure is more straightforward and does not require the same level of distinction among components. However, enabling MyDomain changes the context in which Apex can be executed, particularly in multi-tenant architectures like Salesforce, making namespaces necessary for correct routing and execution of the endpoint. This avoids accidental overlap between different applications or integrations that might exist in the same Salesforce organization.