In this post I describe a variation of the backend for frontend (AKA BFF) pattern that Søren Trudsø made me aware of. With this variation we create not just a backend for each frontend - one backend for the iOS app, one for the Android app and one for the web site, say - but a backend for each page in the web frontend: A single-page BFF.
The BFF patterns is a pattern for building applications on top of a system of microservices. I describe that pattern in my Microservice in .NET Core book, but you can also find good descriptions of the pattern on the web - for instance from Sam Newman. To sum it up the BFF patterns is that you create a microservice for each frontend you have. That is, if you have an iOS app there is an iOS BFF. If you have an Android app you have an Android BFF. If you have an Oculus Rift app there is an Oculus Rift BFF. Each BFF is a microservice with the sole responsibility of serving its frontend - the Android BFF is there solely to serve the Android app and does not care about the Oculus rift app whatsoever. The BFFs do nothing more than gather whatever their frontends - or apps - need and serve that in a format convenient to the frontend. The BFFs do not implement any business logic themselves, all that is delegated to other microservices. This figure illustrates the setup:
In this setup each BFF tends to grow as its frontend grows. That is, the web BFF tends to grow as the web frontend grows: When more functionality is added to existing pages, that functionality might need new endpoints to make AJAX requests to, and thus the web BFF grows a little bit. When new pages are added to the web frontend, the web BFF also grows.
Sidenote: I realize that the term "page" on a web site is somewhat fuzzy these days: Single page apps routinely swap the entire view from one thing to something completely different, giving the user the experience of going to a new "page". In this post I use the term "page" in the more traditional sense of a full page reload. You know, that thing when you follow a link and the browser loads a completely new HTML document from a new URL. I think you've encountered it before :D
The size of the web BFF might not be problem at first (or ever), but a some point enough may have been added to the web frontend to make it a problem. In this situation I have found it useful to break the web BFF down by page boundaries: In stead of having one BFF serve the entire web frontend, I will have a one BFF for each page on the web site, like so:
This way the BFFs are kept small and focused on a single task, namely serving a single page.
Notice that one or more of the pages here can be single page apps that include several views, so there need not be a direct correspondance between what the use perceives a separate views - or pages - and the single page BFFs on the backend. Rather, in such cases, there is a BFF for each single page app.