| > We didn't get into the semantics of Redis' MULTI...EXEC, which the documentation mischaracterizes as "Transactions". I'm happy that at least you didn't use BEGIN...COMMIT. Hmmm, this is a subtler issue than you make it out to be, I think, though I generally agree with you. The quality issues with Redis's technical design here interrelate substantially with user expectations/perceptions/squishier stuff. The term "transaction" is anchored in most users' minds to a typical RDBMS transactional model assuming: a) some amount of state capture (e.g. snapshot) at the beginning of the transaction and b) "atomicity" of commit being interpreted as "all requested changes are atomically performed (or none are)" rather than "all requested changes are atomically attempted". Redis has issues with both of those, so I'm sympathetic to your statement that what they call "transactions" is mis-characterized and would be better described as "best-effort command batching". It's poor naming/branding to call it "transactions", and I don't think it had to be this way: MULTI/EXEC "transactions" should have been deprecated long ago--in favor of Redis scripts and other changes that should have been made in the Redis engine. First, a defense of scripts: Redis scripts are, to a certain variety of user who wants transaction-esque functionality, not ideal. Those users may be reluctant to engage with a full procedural programming language rather than the database's query language. However, there's substantial overlap between those users and the ones who will be extremely confused by and unhappy with the existing MULTI/EXEC model--they're the folks with the most specific (wrong, in Redis) assumptions of how transactions should work, and suffer the most from them not working that way. Lua scripts, unfamiliar or not, are likely less troublesome in the long run for this cohort. Specifically, requiring users to be explicit about failure behavior of specific commands via call() vs. pcall() would remove one of the worst sharp edges of the MULTI/EXEC world. Scripts can't answer other transaction-related needs, though. Ideally, I would have preferred that Redis go in the direction of a uniform set of conditions that can be applied to most write commands. There already are conditions in Redis, but they're not uniformly available: SET + NX/XX conditions single-key writes; WATCH semantically/implicitly conditions later EXEC commands with "if version of $key matches the version retrieved by the WATCH statement", etc. If that type of functionality were made explicit and uniformly available to all or most write operations, a further chunk of transaction-related needs could be addressed. When making single commands conditional isn't enough, scripts used to atomically batch-attempt commands could be invoked with parameters used to conditionalize those scripts' internal commands, and so on. A final simple affordance in support of transaction-ish behavior would be a connection-scoped value type: either a modifier for arbitrary commands to have them operate on an empty database scoped to the connection, or a simple list-like value for connections to "stash" arbitrary data. This wouldn't fundamentally change any semantics, but would, at the cost of some indirection, marginally reduce the need for clients complexity when buffering conditions/commands for later flush via a pseudo-"commit"-script. This is somewhat hair-splitting, though: MULTI/EXEC is already such a connection-scoped buffer, just one that stages commands and not data. My hunch is that a data-only buffer to be consumed by scripts instead of "EXEC" would be an improvement here, but I may well be wrong. Now, the system that results from these changes is still not as ergonomic/low friction as traditional transactions, and is especially unergonomic when users have to manually capture undo state and decide on rollback semantics during the failure of script execution. As Antirez mentioned in an adjacent comment, AOF can help ensure appropriate conistency in the face of database crashes during script execution, but database level reconciliation--aka "what is the equivalent of 'rollback' for a given script"--is still on the user to work out. But that's what we're really talking about here, isn't it? That lack of undo (that is: the ability to capture and discard transactional state a la MVCC) is at the root of most of the weird and not-quite-transactional capabilities of Redis in this era. Antirez is totally right that adding those capabilities would have substantially complicated the Redis engine, and I believe him when he says that made it not worth it to do so. Given that, I'd have vastly preferred a Redis which embraced providing tools that work in spite of/with full acknowledgement of that lack, rather than concealing it/confusing users by mis-branding MULTI/EXEC as "transactions". |