Slog: Cheating the low-latency vs. strict serializability tradeoff | Hacker News Reader

Slog: Cheating the low-latency vs. strict serializability tradeoff

dbmsmusings.blogspot.com

62 pointsevanweaver10 comments

How is Slog different (or better) than multiple asynchronously-replicated regional deployments of Spanner? Or, even simpler, a single Spanner deployment where different sets of groups have different replication configurations. That is, in fact, a fairly common deployment of Spanner internally at Google, with different sets of voting replicas for data that's most frequently accessed from different places. As an example: Paxos groups that host European users' data have a quorum of voting replicas in the EU, while groups that host US users' data have a quorum of voting replicas in the US. Data can be explicitly moved across these different sets of groups, e.g., in response to a user changing the location of their accesses. All the normal location resolution methods still work as in the simple case of uniform replication configurations (see Directories and Placement section in the original OSDI 2012 paper).

Let's say you have a transaction that writes data located in different quorums atomically. This is called a "multi-home" transaction in SLOG. Most of the machinery in the paper is dealing with that case. SLOG can do that with half-round trip latency. No idea how long it would take the version of Spanner you described to do that, but presumably at least two full round trips across quorums for two-phase commit.

The paper [1] compares Slog against Spanner, both in theoretical terms and using benchmarks. If I'm understanding the paper correctly, I think it makes sense why Slog is better -- or at least, can do better in theory.

Both systems operate very similarly for local transactions that only touch data "owned" by a single master region; they just relay the transaction to be executed by the master. For multi-region transactions, Spanner uses a coordinator to perform two-phase commit, which acquires locks on all regions before allowing the transaction to proceed.

Slog does something similar, but effectively pipelines the locking to achieve higher throughput. First there's a global coordination step that globally-orders the transaction, without any locking (which means this step can use batching for high throughput). Then, each region's master independently acquires local locks in that global order, and replicates those locks as transactions so that replicas can deterministically apply them in the same order. Finally, each replica independently executes the transaction once it sees that all of the locks have been acquired. So a lock blocks the execution of conflicting transactions, but it doesn't block their replication. Once the replication is done, the locking overhead of actually executing the transactions should be comparable to a non-distributed DB.

All of this communication has a latency penalty, of course; there's no avoiding that for a consistent distributed DB. But the point is that it provides better throughput for transactions with conflicts. For transactions that only touch one region, the latency is still just a single round-trip to the master region, and that can be very fast if your client locality is high.

The benchmark results are heavily normalized, since it wasn't possible to do an apples-to-apples comparison on the same replication topology. So they don't demonstrate convincingly that Slog is faster than Spanner, in numerical terms. However, they do show that Spanner's throughput drops off much more quickly with increasing contention, compared to Slog.

[1]: http://www.vldb.org/pvldb/vol12/p1747-ren.pdf

"Cheating" is doing all of the work here. They are sending transactions to their "home location" which is worst case all the way on the other side of the planet and then they are only counting the time to apply the transaction at that location, instead of including the latency between the client and the homed frontend. So it's low latency if you draw the system boundaries in such a way but from the perspective of the client it's still high latency.

... unless you preload the cheat setup, right?

IOW you start to suspect that the client may execute a multi-region transaction. Then you can prepare by syncing data across regions.

Latency is measured from the client. In the example in the post (and more details in the paper), you see the latency tail from when clients access data that is far from them. The challenge is to make multi-home transactions no worse than regular Paxos latency. In previous systems, this required multiple rounds of communication across the homes that are involved in that transaction. In systems like PNUTS, they would disable such transactions altogether. SLOG's ability to handle such transactions with latency no worse than Paxos is a big step forward.

It feels like they've snuck in an element of deterministic databases. This would explain why they don't take the penalty of the 2-phase commit round-trips. What's more interesting is how they've implemented this by just waiting for sufficient replica updates to appear in the database until they're confident in being strictly serializable. Of course, if those updates don't make it because of a network partition then the transaction will hang until it's over. Hopefully they really can count on there never being a network partition. Then again, a network partition would halt all related multi-homed transactions anyway so I guess it's a moot point.

Yes --- the post explicitly states that deterministic execution is a prerequisite.

SLOG is CP from CAP, so indeed suffers from unavailability in the event of a network partition.

CockroachDB has a different cheat where the voting nodes would "follow the sun" as different regions would be the most highly active at different times of a 24h period. This has then been generalized to "follow the work" where each voting range keeps its voting nodes near the location of high activity. I'd like to see how this benchmarks to Slog.

See what I wrote below regarding Spanner. The same thing applies to the CockroachDB solution. If you run 2PC for multi-region transactions that is very slow (increases latency), and prevents conflicting transactions from running for longer periods of time (decreases throughput).

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