A Simple Example

Say we have a Datalog file example.dl, whose contents are as shown.

.decl Source(id: number)
.input Source(IO="file", filename="Source.csv", delimiter=",")

.decl Arc(x: number, y: number)
.input Arc(IO="file", filename="Arc.csv", delimiter=",")

.decl Reach(id: number)
.printsize Reach

Reach(y) :- Source(y).
Reach(y) :- Reach(x), Arc(x, y).

We see that Source and Arc are .input relations, so their data will be read from disk. Reach plays the role of the output relation (add .output Reach if you want FlowLog to materialize Reach.csv).

The last two rules state that (1) every tuple in Source is also in Reach, and (2) if we already know a tuple (x, z) is in Reach and there is an Arc from z to y, then (x, y) must also be reachable.

For instance, if the tab-separated input files are

Source|Arc
------|---
1     |1,2
      |2,3

then FlowLog derives the three tuples shown below. The extra (1,3) row appears because rule (2) chains the two edges together.

Reach
-----
1,2
2,3
1,3

We can compile this program by running

$ flowlog examples/example.dl -o example_flowlog -F . -D .

We get an example_flowlog rust crate, note that no reach.csv has been produced, as the program has not yet been evaluated.

The -F and -D options specify the directories for input and output files respectively. So in this case, Source.csv and Arc.csv is in the generated rust crate working directory ., and Reach.csv will be produced here also.

If instead Source.csv and Arc.csv were in a subdirectory called input, and we wanted to have Reach.csv in a subdirectory called output, we could do

$ flowlog examples/example.dl -o example_flowlog -F ./input -D ./output

To evaluate our program, we have to run the compiled rust crate

# Enter generated rust crate 
$ cd example_flowlog
$ cargo run --release -- -w 4

This compiles the generated FlowLog workspace in release mode and executes it with four worker threads. Use fewer workers on laptops with limited cores or drop --release for faster rebuilds while editing.

There are many other useful options available to FlowLog, so be sure to explore them too!