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HBase, mail # dev - memstore timestamp and visible timestamp


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Re: memstore timestamp and visible timestamp
Wei Tan 2012-08-03, 21:58
Hi Lars,

   I agree with you on that, this comparison only makes sense when we have
two concurrent put to the same key, i.e., (k, v1) and (k, v2), AND we ask
the region server to assign their timestamp. The relations of ts(kv1) and
ts (kv2) can be non-deterministic, which is fine. A desired feature, which
seems to be already satisfied in the current implementation, is that if
ts(kv1) < ts(kv2), it probably should be visible earlier as the latest
value (i.e., memts(kv1)<memts(kv2)), otherwise it will never be!

   Thank you Lars!

Best Regards,
Wei

Wei Tan
Research Staff Member
IBM T. J. Watson Research Center
19 Skyline Dr, Hawthorne, NY  10532
[EMAIL PROTECTED]; 914-784-6752

From:   lars hofhansl <[EMAIL PROTECTED]>
To:     "[EMAIL PROTECTED]" <[EMAIL PROTECTED]>,
Cc:     "[EMAIL PROTECTED]" <[EMAIL PROTECTED]>
Date:   08/03/2012 05:45 PM
Subject:        Re: memstore timestamp and visible timestamp

We are also mixing concepts here.
memTS is a regionserver local concept, there is no distributed aspect to
this.
The whole memTS vs TS discussion is somewhat pointless as memTS is an
internal concept and TS is part of the client visible data.
There can be a single Put adding many columns with the same rowkey, all
with different TSs and all these changes are still only visible
atomically, which is handled by the memTS.

-- Lars
----- Original Message -----
From: Vladimir Rodionov <[EMAIL PROTECTED]>
To: "[EMAIL PROTECTED]" <[EMAIL PROTECTED]>; lars hofhansl
<[EMAIL PROTECTED]>
Cc: "[EMAIL PROTECTED]" <[EMAIL PROTECTED]>
Sent: Friday, August 3, 2012 2:27 PM
Subject: RE: memstore timestamp and visible timestamp

Time can go backwards. One time a year. By one hour.
I may be wrong but it seems that the situation described by TS (memTS1 >
memTS2 and ts1 < ts2) is possible but under concurrent
updates in a distributed environment the only way to guarantee "fairness"
of operations is to put all of them into one global queue.
I really doubt that this is what people need (and want).
Upd: It is possible to keep a queue per server-row inside RS. This is the
question of how do we define order of requests in
concurrent environment. We can have one global queue, one queue per RS or
(at the lowest granularity) one queue per key-row but
the most efficient way (and of course not the most fair) - add the element
of randomness - let OS decide which thread it will give time slot to
first.

Best regards,
Vladimir Rodionov
Principal Platform Engineer
Carrier IQ, www.carrieriq.com
________________________________________
From: lars hofhansl [[EMAIL PROTECTED]]
Sent: Friday, August 03, 2012 12:11 PM
To: [EMAIL PROTECTED]
Cc: [EMAIL PROTECTED]
Subject: Re: memstore timestamp and visible timestamp

I see. This is not as much a stated guarantee but a fact following from
the implementation.
The memTS is handed out per region server - which is fine, because the
only consistency guarantee HBase makes is for KVs of the same row,
and these are always colocated in the same region (and hence the same
region server).
Since the region server also hands out the TSs based on wall clock time
(and assuming time does not go backwards) it follows that a KV assigned a
later memTS cannot have an earlier TS.

Of course that is not the case if you use client assigned TSs.

Maybe I should write a followup blog post that more clearly describes the
relationship (or rather the absence thereof) between the memTS and the TS.
The gist is that the memTS is strictly internal to guarantee ACID
properties (and HBase could have used readlocks for this as well, and if
it did that would be transparent to the outside),
whereas the TS is an application level concept, it is part of the data (so
to speak).
-- Lars
________________________________
From: Wei Tan <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED]
Cc: "[EMAIL PROTECTED]" <[EMAIL PROTECTED]>
Sent: Friday, August 3, 2012 7:21 AM
Subject: Re: memstore timestamp and visible timestamp

Hi Lars,

   Appreciate your reply. Actually I read your blog posting and then had
that question. I am very interested in how you guarantee this:

   Also note that if you use the Region Server assigned TSs then mTS1<mTS2
implies TS1<=TS2 (the update might happen with the same ms).

  In case you have a pointer explaining this, I would like to read.
Otherwise I will dig into the code later today. I remember reading 0.92.0
code and do not find much clue. But I will try again.

Best Regards,
Wei

Wei Tan
Research Staff Member
IBM T. J. Watson Research Center
19 Skyline Dr, Hawthorne, NY  10532
[EMAIL PROTECTED]; 914-784-6752

From:   lars hofhansl <[EMAIL PROTECTED]>
To:     "[EMAIL PROTECTED]" <[EMAIL PROTECTED]>,
"[EMAIL PROTECTED]" <[EMAIL PROTECTED]>,
Date:   08/02/2012 07:35 PM
Subject:        Re: memstore timestamp and visible timestamp

Hi Wei,

you have to distinguish between "visible to other concurrent scanners" and
"visible to a client".
What's visible to a client is determined by what the a client wants to see
based on the application visible timestamp (TS).

The visibility to concurrent scanners is controlled by the memstoreTS
(mTS) to avoid "strange" states sue to parallel updates.
HBase here guards against partially visible "transactions" (i.e. a Put of
many columns that fails after it applied the changes to some of the
columns).

The scenario you describe below is indeed desired. Note that a client can
request seeing the older versions too so the older edit (in terms of TS is
not lost).
Also note that if you use the Region Server assigned TSs then mTS1<mTS2
implies TS1<=TS2 (the update might happen with the same ms).

If you do not mind a longer read, I have written about this here:
http://hadoop-hbase.blogspot.com/2012/03/acid-in-hbase.html

Let me know if that makes any sense.

From: Wei Tan <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED]