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From Ilya Khlopotov <iil...@apache.org>
Subject API versioning
Date Mon, 27 Apr 2020 14:55:10 GMT
Hello,

The topic of API versioning was brought in the [Streaming API in CouchDB 4.0](https://lists.apache.org/thread.html/ra8d16937cca332207d772844d2789f932fbc4572443a354391663b9c%40%3Cdev.couchdb.apache.org%3E)
thread. The tread proposes to add new API endpoints to introduce a response structure change.
The alternative approach could be to implement proper support for different API versions.


It would benefit CouchDB project if we would have support for API versioning. Adding new endpoint
is easy but it is very hard to deprecate or change the old ones. With proper API versioning
we can avoid the need to rewrite all client applications at the same time. 

rnewson mentioned a good blog post about API versioning (https://www.troyhunt.com/your-api-versioning-is-wrong-which-is/).
The main idea of the blog post is. There is no perfect solution it would be the best to support
all options so the user can choose which one to use.

In that spirit I propose to implement four different ways of specifying the API version (per
endpoint):

- Path based -  `/_v{version_number}/{db}/_all_docs`
- Query parameter based  - `/{db}/_all_docs?_v={version_number}`
- Accept / Content-Type headers in the form of `application/couchdb; _v={version_number},application/json`
- Custom header - X-Couch-API: v2

The server would include response version in two places:
- Custom header - `X-Couch-API: v2`
- `Content-type: application/couchdb; _v={version_number},application/json`

Implementation wise it would go as follows:
1) we teach chttpd how to extract version (we set version to `1` if it is not specified)
2) we change arity of chttpd_handlers:url_handler/2 to pass API version
3) we would update functions in chttpd_httpd_handlers.erl to match on API version
  ```
  url_handler(<<"_all_dbs">>, 1)        -> fun chttpd_misc:handle_all_dbs_req/1;
  url_handler(<<"_all_dbs">>, 2)        -> fun chttpd_misc_v2:handle_all_dbs_req/1;
  ...
  db_handler(<<"_design">>, 1)       -> fun chttpd_db:handle_design_req/2;
  db_handler(<<"_design">>, 2)       -> fun chttpd_db_v2:handle_design_req/2;
  ...
  design_handler(<<"_view">>, 1)    -> fun chttpd_view:handle_view_req/3;
  design_handler(<<"_view">>, 2)    -> fun chttpd_view_v2:handle_view_req/3;
  ```
4) Modify chttpd:send_response to set response version (pass additional argument)

I don't expect the implementation to exceed 20 lines of code (not counting changes in arity
of functions in chttpd_httpd_handlers).

Best regards,
iilyak 

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