[jira] [Created] (COUCHDB-1682) Allow filtered _changes to time out, returning last_seq processed

["Nathan Vander Wilt (JIRA)" <ji...@apache.org>]

Nathan Vander Wilt created COUCHDB-1682:
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             Summary: Allow filtered _changes to time out, returning last_seq processed
                 Key: COUCHDB-1682
                 URL: https://issues.apache.org/jira/browse/COUCHDB-1682
             Project: CouchDB
          Issue Type: Improvement
            Reporter: Nathan Vander Wilt


Right now a filtered _changes query ?since=0 on a database with a high update_seq can take a very long time to return. If this request is performed through a proxy or through a browser with a timeout, it may never complete as far as the client is concerned.

Right now CouchDB itself ignores any polling timeout for such a request — i.e. it does not time out while the _changes results are still processing. This is okay, as it at least lets patient clients get a result.

I propose, though, that the timeout value be respected during the "initial" (e.g. in the context of a fresh replication) request. When the timeout is reached, the client should get back a valid response, with incomplete (even empty!) results and a last_seq corresponding to how far it had processed changes in the background. Then the client/replicator could record a checkpoint and request processing of the next batch.

The net result would be that the initial replication request would not be unbounded in time. Even if a response is "timed out" by a proxy/browser within 30 seconds or 5 minutes, assuming the client is aware of this limit they could set a bit lower timeout and get back a last_seq that keeps them from having to (futile-ly) try again from since=0.

Unfortunately, this does slightly change the semantics of the query: it is as if limit=0 when the client provided no (or a different) limit and may be expecting last_seq to ± match current_seq for such a request. So perhaps this behaviour would need to be enabled by its own query parameter, ?batch=please or something.

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