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https://issues.apache.org/jira/browse/HBASE-15576?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15222033#comment-15222033
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Phil Yang commented on HBASE-15576:
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{quote}
Even if the client asks for partial result?
{quote}
If client setAllowPartialResults, next() will still be blocked if we has no
cell returned in heartbeat message, and next() will return if client get some
cells.
I think we should do this based on chunking protocol because we can get break
position easily by time limit logic at server side to prevent timeout at client
side. So we can work on master first.
{quote}
The new Scan would be made with the client volunteered mvcc, something we do
not support currently..
{quote}
If I am not wrong, in HFile we don't save mvcc for each Cells. Coincidently I
had a discussion with [~Apache9] and other workmates, we think we should save
mvcc in HFile to solve some issues on replication. So maybe we can do it first
as an independent task? :)
> Support stateless scanning and scanning cursor
> ----------------------------------------------
>
> Key: HBASE-15576
> URL: https://issues.apache.org/jira/browse/HBASE-15576
> Project: HBase
> Issue Type: New Feature
> Reporter: Phil Yang
> Assignee: Phil Yang
>
> After 1.1.0 released, we have partial and heartbeat protocol in scanning to
> prevent responding large data or timeout. Now for ResultScanner.next(), we
> may block for longer time larger than timeout settings to get a Result if the
> row is very large, or filter is sparse, or there are too many delete markers
> in files.
> However, in some scenes, we don't want it to be blocked for too long. For
> example, a web service which handles requests from mobile devices whose
> network is not stable and we can not set timeout too long(eg. only 5 seconds)
> between mobile and web service. This service will scan rows from HBase and
> return it to mobile devices. In this scene, the simplest way is to make the
> web service stateless. Apps in mobile devices will send several requests one
> by one to get the data until enough just like paging a list. In each request
> it will carry a start position which depends on the last result from web
> service. Different requests can be sent to different web service server
> because it is stateless.
> Therefore, the stateless web service need a cursor from HBase telling where
> we have scanned in RegionScanner when HBase client receives an empty
> heartbeat. And the service will return the cursor to mobile device although
> the response has no data. In next request we can start at the position of
> cursor, without the cursor we have to scan from last returned result and we
> may timeout forever. And of course even if the heartbeat message is not empty
> we can still use cursor to prevent re-scan the same rows/cells which has beed
> skipped.
> Obviously, we will give up consistency for scanning because even HBase client
> is also stateless, but it is acceptable in this scene. And maybe we can keep
> mvcc in cursor so we can get a consistent view?
> HBASE-13099 had some discussion, but it has no further progress by now.
> API:
> In Scan we need a new method setStateless to make the scanning stateless and
> need another timeout setting for stateless scanning. In this mode we will not
> block ResultScanner.next() longer than this timeout setting. And we will
> return Results in next() as usual but the last Result (or only Result if we
> receive empty heartbeat) has a special flag to mark it a cursor. The cursor
> Result has only one Cell. Users can scan like this:
> {code}
> while( r = scanner.next() && r != null && !r.isCursor()){
> //just like before
> }
> if(r != null){
> // scanning is not end, it is a cursor
> } else {
> // scanning is end
> }
> scanner.close()
> {code}
> Implementation:
> We will have two options to support stateless scanning:
> Only one rpc like small scanning, not supporting batch/partials and cursor is
> row level. It is simple to implementation.
> Support big scanning with several rpc requests, supporting batch/partials and
> cursor is cell level. It is a little complex because we need seek at server
> side and we should make sure total time of rpc requests not exceed timeout
> setting.
> Or we can make it by two phases, support one-shot first?
> Any thoughts? Thanks.
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