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+<!-- go/cmark -->
+<!--* freshness: {owner: 'hta' reviewed: '2021-01-01'} *-->
+
+# How to write code in the `api/` directory
+
+Mostly, just follow the regular [style guide](/g3doc/style-guide.md), but:
+
+* Note that `api/` code is not exempt from the “`.h` and `.cc` files come in
+  pairs” rule, so if you declare something in `api/path/to/foo.h`, it should be
+  defined in `api/path/to/foo.cc`.
+* Headers in `api/` should, if possible, not `#include` headers outside `api/`.
+  It’s not always possible to avoid this, but be aware that it adds to a small
+  mountain of technical debt that we’re trying to shrink.
+* `.cc` files in `api/`, on the other hand, are free to `#include` headers
+  outside `api/`.
+* Avoid structs in api, prefer classes.
+
+The preferred way for `api/` code to access non-`api/` code is to call
+it from a `.cc` file, so that users of our API headers won’t transitively
+`#include` non-public headers.
+
+For headers in `api/` that need to refer to non-public types, forward
+declarations are often a lesser evil than including non-public header files. The
+usual [rules](/g3doc/style-guide.md#forward-declarations) still apply, though.
+
+`.cc` files in `api/` should preferably be kept reasonably small. If a
+substantial implementation is needed, consider putting it with our non-public
+code, and just call it from the `api/` `.cc` file.
+
+Avoid defining api with structs as it makes harder for the api to evolve.
+Your struct may gain invariant, or change how it represents data.
+Evolving struct from the api is particular challenging as it is designed to be
+used in other code bases and thus needs to be updated independetly from its usage.
+Class with accessors and setters makes such migration safer.
+See [Google C++ style guide](https://google.github.io/styleguide/cppguide.html#Structs_vs._Classes) for more.
+
+If you need to evolve existent struct in api, prefer first to convert it into a class.