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From "Jens Alfke (JIRA)" <>
Subject [jira] [Commented] (COUCHDB-2052) Add API for discovering feature availability
Date Fri, 07 Feb 2014 16:20:20 GMT


Jens Alfke commented on COUCHDB-2052:

> The replicator attempts to use optimized endpoints of source and target nodes and, on
a 404, falls back to less optimal endpoints

This is definitely elegant and Web-like, but not optimal over slow mobile connections with
high latency. If multiple resources need to be preflighted this way, it can start to add noticeable
delay to the replication. I'm not saying that this approach is unacceptable; just be aware
that it has a cost (one that CouchDB developers may not be keeping in mind since they're used
to thinking of replication as being between two _servers_ with fast Ethernet connections :)

> 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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