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From "Robert Newson (JIRA)" <>
Subject [jira] [Commented] (COUCHDB-2052) Add API for discovering feature availability
Date Fri, 07 Feb 2014 15:56:29 GMT


Robert Newson commented on COUCHDB-2052:

Let's not forget that "discoverability" is already part of CouchDB. The replicator attempts
to use optimized endpoints of source and target nodes and, on a 404, falls back to less optimal
endpoints, thus gracefully degrading for older versions.

There's a lot of abstract talk here and I'm not sure it's helping. Could we take three different
examples and then ponder how they would be made discoverable? Preferably ones not trivially
discoverable by whether they 404.

Some of the ideas above sound extremely cumbersome and more than we need.

> Add API for discovering feature availability
> --------------------------------------------
>                 Key: COUCHDB-2052
>                 URL:
>             Project: CouchDB
>          Issue Type: Improvement
>      Security Level: public(Regular issues) 
>          Components: HTTP Interface
>            Reporter: Jens Alfke
> I propose adding to the response of "GET /" a property called "features" or "extensions"
whose value is an array of strings, each string being an agreed-upon identifier of a specific
optional feature. For example:
> 	{"couchdb": "welcome", "features": ["_bulk_get", "persona"]}, "vendor": …
> Rationale:
> Features are being added to CouchDB over time, plug-ins may add features, and there are
compatible servers that may have nonstandard features (like _bulk_get). But there isn't a
clear way for a client (which might be another server's replicator) to determine what features
a server has. Currently a client looking at the response of a GET / has to figure out what
server and version thereof it's talking to, and then has to consult hardcoded knowledge that
version X of server Y supports feature Z.
> (True, you can often get away without needing to check, by assuming a feature exists
but falling back to standard behavior if you get an error. But not all features may be so
easy to detect — the behavior of an unaware server might be to ignore the feature and do
the wrong thing, rather than returning an error — and anyway this adds extra round-trips
that slow down the operation.)

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