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+# About APIs
+
+The [Jeff Bezos' API memo](https://gist.github.com/kislayverma/d48b84db1ac5d737715e8319bd4dd368) is one of the most talked stories about API programming.
+
+It is, in my opinion, also one of those things which are successful for some environments, but not for all.
+
+## The levels of API accessibility
+
+An "operation" in your application can be in one of the following levels of "API accessibility":
+
+* -oo The operation cannot be invoked in isolation easily.
+For instance, it is embedded in an MVC controller, mixed with form handling and HTML generation, and thus the best approach to invoke it programatically is to simulate a browser
+* 0 The operation can be invoked, in-process, by calling a function or method, but requiring complex setup or using complex types (e.g. others than lists, maps, numbers and strings)
+* 1 The operation can be invoked, in-process, by calling a function without complex setup and using plain types
+* 2 The operation can be invoked, off-process, by calling a function without complex setup and using plain types
+* 3 The operation can be invoked via a command line tool
+* 4 The operation can be invoked via a network call
+
+Many proponents of APIs propose level 4 as the target.
+This obviously allows your operations to be integrated in separate processes via network calls, which is the most powerful way of API access.
+They will also reason that this will force your application to have a clean architecture with separation of concerns.
+
+Note also that doing proper testing will probably force your operations to be tested to be in levels 0-2, as otherwise it will be annoyingly complex to test them.
+
+We propose that the architecture benefits of level 4 are also present in levels 0-3, but achieving these levels requires much less effort than achieving level 4 (where you need to add a network protocol, handle aspects such as authentication/authorization, marshalling/unmarshalling, etc.), so unless you require level 4, you can stay in levels 0-3.
+Going to level 3 instead of 0 should be easy when creating new operations, so that's the level of API accessibility we recommend new code to adhere to by default.
+
+Note also that level 3 can provide many benefits of level 4, but with less development overhead, so it's a level we recommend considering explicitly, as it is often overlooked.
+
+Level -oo is typical of legacy applications.
+Note that we consider the distance between level -oo and the rest of levels much bigger than the distance between the rest of levels.