Currently coprocessors can't do any streaming operations. I think that would be a necessary feature to perform long running operations on the server (like scans) that in turn could produce a lot of data. GroupBy type aggregates come to mind, but there are many more cases.
Somewhere I read about some approach for server side cursors (can't find that discussion now). I think a simpler approach would be allowing a coprocessor to register new InternalScanners that it could implement, and then have some way of accessing the scanner via the normal ClientScanner mechanism. Maybe by just exposing long HRegionServer.addScanner(InternalScanner) through RegionServerServices. and adding public ResultScanner getScanner(long scannerId) ... on HTable, and similar on all other clients (I don't know anything about the client beside the HTable Java client). Or something similar (just making this up here). That way all major parts are already in place (Client Scanners are good in performing caching, the coprocessor could just wrap "real" internal scanners, etc). The problem is just about how to wire up the parts. Thoughts? Are questions like this better asked on the dev list? Thanks. -- Lars
