https://catern.com/services.html

Write libraries instead of services, where possible

A service has constant administration costs which are paid by the
service provider. A library instead moves these costs to the users of
the library.

For any developer with limited resources, this means a library (where
viable) can provide the same functionality to the user, at a lower
cost to the developer than a service.

By library, I mean any software that can be run by the user: shared
objects, modules, servers, command line utilities, and others. By
service, I mean any software which the user can't run on their own;
anything which depends (usually through an API) on a service provider
for its functionality.

Usually, the centralization of these administration costs is cited as
a benefit of services. People say, "services are easy to write
because you can upgrade them centrally, so you can avoid
slow-to-upgrade users making everyone's lives worse."

But this assumes that slow-to-upgrade users can have negative effects
on everyone else. If one user can't have a negative impact on other
users, then you don't care if some users are slow to upgrade; they're
only hurting themselves.

You can prevent users from negatively impacting other users by not
sharing state or resources between users; in other words, let users
run the software themselves, by writing libraries instead of
services.

"Write a library", in this sense, can still mean writing a standalone
server reached through a network protocol. As long as the user runs
the server themselves, you're still saving the administration costs.
Both standalone servers and importable code are equally expressive,
and equally cheap for you to administrate if you're not operating a
service yourself.

By avoiding the maintenance and upgrade costs of a service, a library
can afford to contain more functionality. That's good for both the
developer and the user.