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. -- Lars ----- Original Message ----- 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
