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From "Jens Alfke (JIRA)" <>
Subject [jira] [Commented] (COUCHDB-2052) Add API for discovering feature availability
Date Fri, 07 Feb 2014 16:36:21 GMT


Jens Alfke commented on COUCHDB-2052:

OK, here are examples from the Couchbase Sync Gateway:

(1) WebSocket transport for the changes feed. The query param "?feed=websocket" is recognized
and triggers a change-over to the WebSocket protocol, with the server sending a message for
every change. If you send such a request to CouchDB it will ignore the unrecognized feed type
and send back a 'normal' changes feed instead; this can trigger a lot of wasted data transfer.

(2) The body of a PUT/POST request sent to the server may be gzip-encoded ("Content-Encoding:
gzip"). As far as I can tell the HTTP spec doesn't provide any way for the client to discover
whether the server supports this. If the server doesn't, and also doesn't check the Content-Encoding
request header, it'll end up trying to read the raw data and either barfing on it or (worse)
storing it without any indication that it's zipped.

(3) When the Gateway parses an incoming document in multipart/related format, it looks at
the attachment bodies' "Content-Disposition" headers to discover which named attachments they
correspond to, instead of (as CouchDB does or used to) assuming that they appear in the same
order as the objects in the JSON body's `_attachments` dictionary. A client might not be able
to generate the MIME bodies in exactly the same order as in the dictionary, so it might need
to check for this capability and abort the replication if it's not there, since the alternative
is getting the attachments mixed up.

> 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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