aphyr

13 ms·

151.
by aphyr 6y ago
Yeah, the PostgreSQL team really knocked it out of the park on this one. It was a pleasure working with them. :)
152.
by aphyr 6y ago
Fun story: after the last report which called them out for not talking about write loss by default, MongoDB updated their Jepsen page to say that the analysis didn't observe lost writes. I guess they assumed that people wouldn't..
153.
by aphyr 6y ago
Patroni's documentation also seems to suggest that even with the strongest settings, it can lose transactions; Stolon makes stronger claims.
154.
by aphyr 6y ago
I started this analysis intending to do just that--it's been difficult, however, to figure out which of the dozens of replication/HA configurations to actually test. I settled on Stolon, since it seemed to make the strongest safet
155.
by aphyr 6y ago
Yes. Jepsen and Hypothesis both descend from a long line of property-based testing systems--mostly notably, Haskell & Erlang's QuickCheck. Jepsen makes a number of unusual choices specific to testing concurrent distributed systems:
156.
Jepsen: PostgreSQL 12.3(jepsen.io)
769 points by aphyr 6y ago|140 comments
157.
by aphyr 6y ago
I've actually written a little mini-Jepsen workbench for folks who want to practice implementing a distributed key-value store. Might be worth a spin! https://github.com/jepsen-io/maelstrom
158.
by aphyr 6y ago
I've invested seven years of my life into this brand, and my choices are carefully considered.
159.
by aphyr 6y ago
I'm not convinced it is false--IIRC their claim is specifically w.r.t other Kafka side effects, and those they can control.
160.
by aphyr 6y ago
I think this is a good way to look at things, and there are vendors who do this! VoltDB, for instance, changed their defaults to be strict serializable even though it imposed a performance hit, following their Jepsen analysis. https:/
161.
by aphyr 6y ago
I'm kind of impressed any distributed system gets off the ground. These things are hard to write!
162.
by aphyr 6y ago
E.g. when each read should return the whole register's history the size of history grows O(n^2) compared to the case when the reads return just the head. If you look at Elle's transaction generators, you can cap the size of any
163.
by aphyr 6y ago
Not sure how many ways I can say this: the titles are already "Jepsen: X". HN's got a policy in place that means sometimes mods change the title to just "X". That's not something I have control over, sorry.
164.
by aphyr 6y ago
I think there are open possibilities in making leaderless single-round-trip consensus systems for log-oriented FSMs, which is what pretty much everyone WANTS. This is based on her presentation and some dinner conversation at HPTS 2019, so
165.
by aphyr 6y ago
Pretty much, yeah. There are some cases where Jepsen reaches into the guts of a database or lies to it via LD_PRELOAD shims, but generally these are Just Plain Old Binaries provided by vendors; no instrumentation required.
166.
by aphyr 6y ago
I'd like this too, but I'm not sure how to do fault injection against an Amazon-controlled service.
167.
by aphyr 6y ago
This... usually happens on Jepsen HN threads. The full title, as in the page metadata, and as originally submitted, is "Jepsen: MongoDB 4.2.6. At some point a mod drops the "Jepsen:" part, then we have this discussion, and it
168.
by aphyr 6y ago
I'd like to, but I don't have any way to do fault injection on a system someone else owns. :(
169.
by aphyr 6y ago
I like to keep in mind that MongoDB's existing feature set is maturing--occasional regressions may happen, but by and large they're making progress. The problems in this analysis were in a transaction system that's only been
170.
by aphyr 6y ago
It's been on my list for a long time, but I've also struggled to find out like... what, exactly, is the right way to do postgres replication? Every time I go into the docs I wind up with a laundry list of different mechanisms for
171.
by aphyr 6y ago
That is, a table on the jepsen.io frontpage, or at least on each product's review page, with database products and configuration on rows and consistency properties on columns, and a nice "Yay!" or "Nope!" mark in t
172.
by aphyr 6y ago
Have you ever tried distributed transactions with relational databases? I am delighted to say that yes: checking safety properties of distributed systems, including those of relational databases, is literally my job. See https:/&#x2
173.
by aphyr 6y ago
Chief, it does not have to be this hard. 3.4 clearly states: This anomaly occurred even with read concern snapshot and write concern majority 3.5: In this case, a test running with read concern snapshot and write concern majority execu
174.
by aphyr 6y ago
> I did not find any proof of this in the rest of the report. May I suggest sections 3.4, 3.5, 3.6, 3.7, 4.0, and 4.1?
175.
by aphyr 6y ago
Possibly! You're right that path dependence played a role in safety issues: the problems we found in 3.4.0-rc3 were related to grafting the new v1 replication protocol onto a system which made assumptions about how v0 behaved. That sai
176.
by aphyr 6y ago
No, I haven't! You can see a full list of analyses here: http://jepsen.io/analyses
177.
by aphyr 6y ago
I was lucky to have a good education: my B.A. involved courses in contemporary experimental physics and independent research in nonlinear quantum dynamics (esp. proofs, experimental design, writing), cognitive and social psychology (more ex
178.
by aphyr 6y ago
You just can't test lower levels of guarantees and then complain you did not get what higher levels of guarantees were designed to provide. Gently, may I suggest that you read the report, or at least the abstract? This is addressed i
179.
by aphyr 6y ago
Calvin and CRDTs aren't new, but I still think they're dramatically underappreciated! Heidi Howard's recent work on generalizing Paxos quorums is super intriguing, and from some discussion with her, I think there are open pos
180.
by aphyr 6y ago
There's a few things at play here. One is talking only about the positive results from the previous Jepsen analysis, while not discussing the negative ones. Vendors often try to represent findings in the most positive light, but this w

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