Site owner's can create custom data serves that can be called from SearchMonkey applications to get your page data. However, this approach should only be used if the previous methods are not feasible.
The advantage of custom data services are that no changes are required on the front-end. On the back-end, a new endpoint or repurposing of an existing endpoint is required. As with OpenSearch, site owners may be able to leverage existing databases.
Some applications might have strict performance requirements, and using custom data services might slow them down considerably.