Comment by TheMiller | Hacker News Reader

Comment by TheMiller | Hacker News Reader

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To elaborate further on what saurik has already pointed out: Hickey's "update-in-place" characterization of relational databases places blame in the wrong place. This is more about how people think about the data models; they think of their primary keys as identifying places, and updating rows as modifying those places. The relational model itself does not encourage this mode of thinking, although SQL arguably does, unless you think of UPDATE as shorthand for DELETE followed by INSERT. It's not that uncommon to build data models, or parts of models, that have the characteristic of never deleting facts. It's true that time-based as-of queries in such models are unwieldy, but that's a problem of the query languages that can be addressed by how queries are constructed, without redesigning the entire database system. There is research on "temporal databases" that addresses this, although I'm not up to date on it.

What I could not understand from the talk or from googling for more information on Datomic afterwards, is how it supposedly simplifies anything about the consistency issues that he talks about, with respect to the read/decide/write sequence. You read; time passes; other people make changes; when you write, the real world (modeled in your "transactor" as the single common store and arbiter of what is) will have changed.

Re: read/decide/write - Datomic supports transaction functions (written in Java or Clojure) which run within the transaction, on the transactor, and are true functions of the prior state of the database (with full access to the query API). They can be used to do all of the transformative work (traditional read/modify/write), or merely to confirm that preconditions of work done prior to transaction submission still hold true (optimistic CAS). The model is simple - there are no complexities of isolation levels etc. All transactions are serialized, period. Completely ACID.

The critical thing is that queries and reads not part of a transformation never create a transaction, and don't interact with the transactor at all.

Thanks for the reply. In the talk, I missed the point that transaction functions are able to implement "optimistic CAS" (which I assume is the same as what is more traditionally but inaccurately called "optimistic locking"). To clarify, though: Wouldn't precondition testing have to be done /inside/ the transaction, not before, in order to be effective?

That's a nice model, I suppose, but doesn't strike me as particularly novel. As has been pointed out elsewhere, the idea that read-only operations can avoid creating a transaction is not particularly important per se. What's more important is avoiding locking, and this is already available in several DBMSes. Indeed, in the sense that reads are consistent "as of" a certain point in time, Datomic really does have a notion of a read transaction. I saw in another of your replies here that you consider these to be more flexible than transactions because they are in effect immutable snapshots of the database state, and independent of a single process. I guess for some applications that could be important, but for many applications the process independence is irrelevant (perhaps it helps with scaling a middle tier?), and the eventual storage cost of true immutability will become intractable (although perhaps you'll eventually have ways to release state prior to a point in time). Coordinating different parts of code within a single process over a specific span of time doesn't strike me as any easier with Datomic's model than with traditional transactions.

This talk just didn't focus on the kinds of data modeling problems that are important to me and which I consider difficult, which mostly have to do with maintainability of the data model - except, possibly, for the ease of dealing with temporal queries. And I share others' unease with what seemed to be inaccurate characterizations of current DBMS implementations.

That's the whole point, there are never any changes. You only ever have new versions, in new transactions. It's much more similar to a DVCS than a relational db.

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