Salsa.jl

A framework for on-demand, incremental computation via memoization, inspired by Rust lang's salsa-rs/salsa.

⏯ Youtube | JuliaCon 2020 | Salsa.jl

Salsa is:

• a memoization framework, with
• runtime dependency tracking, so that
• you can update some inputs and (performantly) automatically invalidate the affected caches.

It provides a framework for automating away the potential pitfalls of cache invalidation, by automatically detecting dependencies between parts of your code (@derived functions), and using the detected dependency graph to propagate invalidations when facts about the world have changed.

• @derived
• @declare_input
• Runtime()

julia> using Salsa

julia> @declare_input x(rt)::Int
(x, set_x!, delete_x!)

julia> @derived function x_plus_one(rt)
           println("Running x_plus_one.")
           return x(rt) + 1
       end
x_plus_one (generic function with 1 method)

julia> rt = Salsa.Runtime();

julia> set_x!(rt, 1)

julia> x_plus_one(rt)
Running x_plus_one.
2

julia> x_plus_one(rt)
2

julia> set_x!(rt, 10)

julia> x_plus_one(rt)
Running x_plus_one.
11

For maximum performance in deployed software, you can disable all runtime assertions and debug code by setting this environment variable before building Salsa: SALSA_STATIC_DEBUG=false.

Or, for a slightly smaller performance gain, you can toggle it at runtime via Salsa.Debug.disable_debug().

This package was closely modeled on the Rust salsa framework, and takes heavy inspiration from that framework and adapton.

We highly recommend this talk which motivates the need for incremental, demand-driven computation, and for packages like Salsa:

• YouTube: Responsive compilers - Nicholas Matsakis - PLISS 2019

The underlying principles are very similar to, and inspired from that package: It can be hard to write correct incremental programs by hand, so we provide macros that make it easy by automatically tracking dependencies between computations.

If you are familiar with Salsa-rs, you'll see many things that are similar, with slightly more generic names that are moved away from database-oriented naming:

• derived queries => @derived functions
• query group => Runtime