[
https://issues.apache.org/jira/browse/HBASE-15576?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15221841#comment-15221841
]
stack commented on HBASE-15576:
-------------------------------
Hello [~yangzhe1991]
bq. 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.
Why you say the above? With Scanner chunking, don't we return early if we hit
thresholds (size, time?). Re:
https://blogs.apache.org/hbase/entry/scan_improvements_in_hbase_1 But perhaps
you are referring to the case where even with this in place, we can go on
longer than a timeout?
What will the Cursor return? The top of the merge sort heap and the mvcc? Yeah,
HBASE-13099, doing like dynamodb relates here. The Cursor would be maintained
internally? You see any need of our exposing it to the client?
bq. Only one rpc like small scanning, not supporting batch/partials and cursor
is row level. It is simple to implementation.
How would this work against a big row?
On your 'another question', if the Put/Delete are after your Scan start, then
you will not see them because of mvcc. If move/split, the mvcc should still
apply; if it doesn't, it is a bug. The issue as to what happens when a delete
comes in after the Scan starts and then a compaction runs clearing your Cell --
and the Scan is totally stateless -- is a problem. I think that even for
stateful scanners, if a move and then a compaction, it could fail to read Cells
that were present when the Scan started (Having Scanner 'degrade to row level'
-- i.e. change how its working mid-scan -- would be erratic behavior and not
sure how you'd throw an exception for a Cell you don't is missing).
Resetting the seeks on the server-side is expensive but you know this already
I'd say.
> 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.
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
