Posted by 0x54MUR41 10 hours ago
I think it’s more tractable to define this problem space starting from the concept of (strict) serializability, which is really a generalization of the concept of thread safety. Every software engineer has an intuitive understanding of it. Lack of serializability can lead to execution-dependent behavior, which usually results in hard-to-diagnose bugs. Thus, all systems should strive towards serializability, and the database can be a tool in achieving it.
Various non-serializable levels of database transaction isolation are relaxations of the serializability guarantee, where the database no longer enforces the guarantee and it’s up to the database user to ensure it through other means.
The isolation phenomena are a useful tool for visualizing various corner cases of non-serializability, but they are not inherently tied to it. It's possible to achieve serializability while observing all of the SQL phenomena. For example, a Kubernetes cluster with carefully-written controllers can be serializable.
The combination of transactions, isolation levels, and MVCC is such a huge undertaking to cover all at once, specially when comparing how it's done across multiple DBs which I attempted here. Always a balance between technical depth, accessibility to people with less experience, and not letting it turn into an hour-long read.
If anything, I’d say it might be better to start with the lower isolation levels first, highlight the concurrency problems that can arise with them, and gradually introduce higher isolation levels until you get to serializability. That feels a bit more intuitive rather than downward progression from serializability to read uncommitted as presented here.
It also might be nice to see a quick discussion of why people choose particular isolation levels in practice, e.g. why you might make a tradeoff under high concurrency and give up serializability to avoid waits and deadlocks.
But excellent article overall, and great visualizations.
More notation, more citations, more better.
Unsure why "strict" (L + S) is in braces: Linearizability ("L") is what resembles safety in SMP systems the most?
Using read-committed ofc means having to keep locking details in mind. Like, UNIQUE doesn't just guard against bad data entry, it can also be necessary for avoiding race conditions. But now that I know, I'd rather do that than take the serializable performance hit, and also have to retry xacts and deal with the other caveats at the bottom of https://www.postgresql.org/docs/current/transaction-iso.html
https://dev.mysql.com/doc/refman/8.4/en/set-transaction.html...
https://mariadb.com/docs/server/reference/sql-statements/adm...
I don't know about MyISAM though (who uses it anyway ;-) ).
I'm using the past tense here, but VoltDB is still going. I don't think it's as well-known as it deserves to be.
Oracle and SQL Server also default to read committed, not serializable. Serializable looks good in text books but is rarely used in practice.
If the data is fairly straightforward like just one-to-many CRUD with no circular references, you would be able to do it without transactions, just table relationships would be enough to ensure consistency.
And no I'd never expect people to know the isolation levels by heart, but if you know there are different ones and they behave differntly that's pretty good and tells me you are curious about how things work under the hood.
It goes into not only different isolation levels, but also some ambiguity in the traditional ACID definition.
I believe a 2nd edition is imminent.
Am I missing something or this statement is incomplete? Also I find the visualization of commit weird, it “points to” the header of the table, but then xmax gets updated “behind the scenes”? Isnt xmax/xmin “the mechanism behind how the database knows what is committed/not committed”? Also, there could be subtransactions, which make this statement even more contradictory?
I enjoyed the visualizations and explanations otherwise, thanks!