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The Reemergence of Datalog

Authors: Michael Fogus
Time:12:40 pm - 1:20 pm

Also known as "Return of the Living Datalog". Michael likes turtles.

A common way we look at data is as a "rectangle" (table). Rectangulation falls down a bit when describing relationships, sparse data, multi-valued, and leads to Place-Oriented Programming (PLOP).

Dealing with data in Java often involves lots of code that obscures what the data actually is: you write a lot of code to accomplish what you want, and mistake the menu for the meal. So Java programmers try to make the data more familiar/comfortable, by mapping it into what they understand: classes. "ORMG!" This reinforces a false dychotomy between code and data.

So how do we unify code and data? Unification. With unification you "punch holes" in your data and provide places for variables. Then you can try to fit data in, like a key in a lock. Unification diverges from pattern matching because it can leave variables there: you don't have to match everything, or the unification can introduce new variables.

Unification closes the gap between data and code, but doesn't quite get us to the point where you can use the data in what we'd normally consider a program. Prolog is one of the ways we try to close that gap completely. Prolog programs assert facts, and use rules to reason: X spawned Y, and if A spawned B, B is a descendant of A. This is great: our data can now sort of be treated as code. But it has a few problems: clause-order dependence, non-termination, and imperative infection.

Datalog is a query language (not general purpose, not Turing complete), very explicit with its bindings, and relatively simple. Datalog began its life in 1977, and work was done until 1995 when it was declared "not relevant". In 2002, though, it began to gain use as a way to describe security and topologies.

Datalog is a logic programming language with recursive queries and recursive joins. Datalog works off a simplified Entity Attribute Value (EAV) model. This EAV model means Datalog is suitable for querying sparse datasets.

Datomic is an implementation of Datalog which removes the need for a database, and introduces the notion of "time travel". Instead of always specifying a database, Datomic allows you to pass in a set of raw data. This makes it useful to test your queries. Keeping track of time in a relational database can be tricky. Datomic allows you to specify a fourth field in the tuple as a time field (actually transaction). This allows Datomic to perform total ordering of transactions, and allows you to bound queries by time.

Daedalus is also a Datalog implementation with a notice of time, although it's different from Datomic. Because it's designed to support distributed processing, Daedulus is based on a tick model (with some accommodation for unreliable network connections).

A third implementation, Cascalog, is written in Clojure, and provides map/reduce processing. This also means you have order independence.

Bacwn is another Datalog (also Clojure based?) which provides negation. Negation utilizes a NOT predicate, which lets you take the same query and return the logical "inverse". [Shows example using MST3K characters, querying first for all characters on the SoL, then those not on the SoL.]

So what about query time? Query plans provide one way to do things, but we don't get any guarantee that that's what the engine will do. You can hint your query, but that's a black art. Prolog requires you order for termination, but Datalog requires that you order for speed (many naive Datalog queries will run slowly). Pluggable optimizers may be the way forward: plug in an optimizer than knows your own data without impacting other Datalog optimization techniques.