stuarthalloway

In order for user code to impose invariants over the entire transaction, it must have access to the entire transaction. Entity predicates have such access (they are passed the after db, which includes the pending transaction and all other transactions to boot). Transaction functions are unsuitable, as they have access only to the before db. [2]

Use entity predicates for arbitrary functional validations of the entire transaction.

[

[Stu favorite-number 41]

;; maybe more stuff

[Stu favorite-number 42]

]

An operational reading of this data would be that early in the transaction I liked 41, and that later in the transaction I liked 42. Observers after the end of the transaction would hopefully see only that I liked 42, and we would have to worry about the conditions under which observers might see that 41.

This operational reading of intra-transaction semantics is typical of many databases, but it presumes the existence of multiple time points inside a transaction, which Datomic neither has nor wants — we quite like not worrying about what happened “in the middle of” a transaction. All facts in a transaction take place at the same point in time, so in Datomic this transaction states that I started liking both numbers simultaneously.

If you incorrectly read Datomic transactions as composed of multiple operations, you can of course find all kinds of “invariant anomalies”. Conversely, you can find “invariant anomalies” in SQL by incorrectly imposing Datomic’s model on SQL transactions. Such potential misreadings emphasize the need for good documentation. To that end, we have worked with Jepsen to enhance our documentation [1], tightening up casual language in the hopes of preventing misconceptions. We also added a tech note [2] addressing this particular misconception directly.