rystsov

Yep, I still consider them to be cutting edge. Paxos was written in 1990 but the industry adopted it only in 2010s. For example I've looked through pBFT and it doesn't mention reconfiguration protocol which is essential for industry use. I've found one from 2012 so it should be getting ripe by now.

I was following Jepsen results since Kyle's first post and it's amazing that the blog post series became a well respected company

The report revealed the following unknown consistency issues which we had to fix:

  - duplicated writes by default
  - aborted read with InvalidTxnState
  - lost transactional writes

The first issue was caused by a client: starting with recent versions the client has idempotency on by default but when the server-side doesn’t support it (we had idempotency behind a feature flag) the client doesn’t complain. We will enable idempotency by default in 21.1.1 so it shouldn't be an issue. Also it's possible to turn the feature flag on for the older versions.

The other two issues were related to the transactions; we haven’t chaos tested the abort operation since it’s very similar to commit but even the tiny difference in logic was enough to hide the bug. It’s fixed now too.

Hey folks, I wrote this post and I'm happy to answer questions

Cool! What did you test? I've played with Jepsen and Cosmos DB when I was at Microsoft but we had to ditch ssh, write custom agent and inject faults with PowerShell command lets.

It wasn't a big surprise for us. Redpanda is a complex distributed system with multiple components even at the core level: consensus, idmepotency, transactions so we were ready that something might be off (but we were pleased to find that all the safety issues were with the things which were behind the feature flags at the time).

Also we have internal chaos test and by the time partnership with Kyle started we already identified half of the consistency issues and sent PRs with fixes. The issues got in the report because by the time we started the changes weren't released yet. But it is acknowledged in the report

> The Redpanda team already had an extensive test suite— including fault injection—prior to our collaboration. Their work found several serious issues including duplicate writes (#3039), inconsistent offsets (#3003), and aborted reads/circular information flow (#3036) before Jepsen encountered them

We missed other issues because haven't exercised some scenario. As soon as Kyle found the issues we were able to reproduce them with the in-house chaos tests and fix. This dual testing (jepsen + existing chaos harness) approach was very beneficial. We were able to check the results and give feedback to Kyle if he found a real thing or if it looks more like an expected behavior.

We fixed all the consistency (safety) issues, but there are several unresolved availability dips. We'll stick with Jepsen (the framework) until we're sure we fixed then too. But then we probably rely just on the in house tests.

Clojure is very powerful language and I was truly amazed how fast Kyle for able to adjust his tests to new information but we don't have clojure expertise and even simple tasks take time. So it's probably wiser to use what we already know even it it a bit more verbose.

The mess is mostly the result of the mismatch between the classic database transactional model and kafka transactional model (G0 anomaly). If you read the documentation without the database background it seems ok, but when you notice the differences between the models it becomes hard to understand if it's a bug or property of the Kafka protocol.

There is a lot of research happening around this area even in the database world. The list of the isolation levels isn't final and some of the recent developments include PC-PSI and NMSI which also seem to "violate" the order. I hope one day we get the formal academic description of the Kafka model. It looks very promising.

Different systems solve different problems and have different functional characteristics. Actually one of the thing which Kyle highlighted in his report is write cycles (G0 anomaly), it isn't a problem of the Redpanda implementation but a fundamental property of the Kafka protocol. Records in Kafka protocol don't have preconditions and they don't overwrite each other (unlike the database operations) so it doesn't make sense to enforce order on the transactions and it's possible to run them in parallel. It gives enormous performance benefits and doesn't compromise safety.

Hey folks, I was working with Kyle Kingsbury on this report from the Redpanda side and I'm happy to help if you have questions

Yes and no, we do both. For ack=-1 we wait for fsync & replication confirmation but for other modes we relaxed the behavior.