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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, 19:43
Hi Lars,

   "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."

   I assume that this applies ONLY when we talk about two KVs in the SAME
row? I read the code of put() finding that a row is locked entering a put,
and then TS assigned, and later memTS assigned. This makes sense since
only after this put is done can another put obtain the row lock, and
therefore a larger TS and memTS will be obtained.

  However, this does NOT hold for two KVs who belong to different rows,
right? Say we have two KVs,  KV1 can enter the put earlier and get a
smaller TS1, but it can be delayed a little bit in the code path, and
possibly get a memTS after KV2, correct?

  Again, thanks :-)

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 03:14 PM
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]
Cc:
Sent: Thursday, August 2, 2012 3:35 PM
Subject: memstore timestamp and visible timestamp

Hi,

  I have a question regarding the correlation between the visible
timestamp of a KV (denoted as ts) and its memstore timestamp (aka, the
write number, denoted as memts). Reading the HRegion.java code it seems
that these two are independently assigned. Let's assume two concurrent
put: (k, v1) and (k, v2)
  Suppose somehow memts(k,v1) < memts(k, v2) then (k,v1) will be committed
and visible before (k,v2).
If ts(k,v1) < ts(k, v2), then after both KVs commits, (k,v2) becomes the
latest version.
else, if ts(k,v1) > ts(k, v2), then after a "later"(w.r.t. MVCC) KV
commits, it immediately become stale and still not visible. --- Is it a
desirable feature?
  Am I understanding it correctly, that memts(k,v1) < memts(k, v2) does
not indicate that ts(k,v1) < ts(k, v2), and vice versa?
PS: let's talk about the hbase region server assigned, not user assigned,
visible timestamp.

  Thanks,

Wei

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