Partitioned consensus and its impact on Spanner’s latency

Re:Partitioned consensus and its impact on Spanner’s latency by jamesblonde on Friday December 14, 2018 @02:41PM (#18683901)

I really like Daniel's writing and exposition of the area. There are, however, a number of assumptions here which don't hold for what he calls 'partitioned consensus' systems. The whole article is written with Calvin and Spanner in mind. We, however, build our platform on a 'partitioned consensus' system that is built on a fully 2-phase commit DB, NDB, with a transaction coordinator at every node. It doesn't fit in his model - cross-partition transactions across availability zones on google cloud take just a couple of milliseconds - nowhere near the "10ms for single-region deployments" that he claims. He assumes partitions have a leader, which again is not true for NDB. But, he knows this - as Spanner is the competition, not NDB.

NDB's concurrency model is "lock-aware programming". You, as a programmer, decide whether you need to lock a row for writing or reading or whether you don't need a lock at all. Calvin serializes transactions for you, which is great, but you pay the price in terms of scalability (nowhere near NDB) and latency (nowhere near NDB). Spanner is a global OLTP DB, which is not comparable to NDB or Calvin.

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Re:Partitioned consensus and its impact on Spanner’s latency by abadid on Friday December 14, 2018 @02:57PM (#18684070)

I have not worked with NDB, nor read any research papers or documentation about it. So that's why I didn't have it in mind when I wrote that post.

But I'm a little confused by your comment: How is it possible to partition consensus without having more than leader? To me, the definition of "partitioned consensus" is that there is more than one consensus group, which means more than one leader.

Also, FYI, Calvin does not serialize transactions. It processes transactions in parallel. But it guarantees equivalence to a predetermined serial order. That distinction is important. As far as scalability, I discussed that in my previous post. Calvin doesn't have any scalability constraints that can be reached by known real-world workloads.

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Re:Partitioned consensus and its impact on Spanner’s latency by jamesblonde on Friday December 14, 2018 @03:07PM (#18684180)

I should have been more clear - Calvin serializes cross-partition transactions. NDB does not. There is a Transaction Coordinator (TC) on every node. TCs can execute cross-partition transactions in parallel, but programmers need to write "lock-aware" programs (more late). TCs can fail-over if one fails - so, it blocks for just a few seconds (Transaction inactive timeouts are typically just a couple of seconds). There are no leaders in each partition, because every node is a potential TC. There are primary TCs for each partition, but failure does not require a leader election.

NDB has "lock-aware" programming - you don't get "global consensus". You decide, as a programmer, that this row could be accessed concurrently by another process, so you lock it, with either a read of write lock. Linearizability is easily implemented by acquiring a lock on a well-known row, but, of course, kills scalability.

In our Usenix FAST paper on HopsFS on Spotify's Hadoop workload, we had 1m ops/sec on HDFS, which was about 10m ops/sec on NDB. We ran out of hardware. There are workloads that big. [edited for clarity]

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Re:Partitioned consensus and its impact on Spanner’s latency by abadid on Friday December 14, 2018 @03:12PM (#18684230)

Please, please read the Calvin paper http://www.cs.umd.edu/~abadi/papers/calvin-sigmod12.pdf. The assumption that Calvin serializes cross-partition transactions is a common misunderstanding is 100% inaccurate. The paper shows how Calvin gets better parallelism on cross-partition transactions than traditional systems.

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Re:Partitioned consensus and its impact on Spanner’s latency by jamesblonde on Friday December 14, 2018 @03:16PM (#18684275)

Ok, sorry about that if it wasn't correct. But are you still not globally ordering every cross-partition transaction - "every scheduler to piece together its own view of a global transaction order by interleaving (in a deterministic, round-robin manner) all sequencers’ batches for that epoch". Even if the sequencers are distributed and execute the transactions in parallel, they need to agree on a total order. This contrast with the lock-aware programming model in NDB, where programmers can allow cross-partition transactions to proceed immediately in parallel if they are sure that they don't conflict.

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Re:Partitioned consensus and its impact on Spanner’s latency by abadid on Friday December 14, 2018 @03:46PM (#18684551)

Yes, that's what I refer to as "unified consensus". Batching helps to overcome the scalability challenges (i.e. order batches rather than each individual xact). I talk about this briefly at: http://dbmsmusings.blogspot.com/2018/09/newsql-database-syst...

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