Custom & external facts

go-facter extends its fact set two ways: custom facts registered in-process through the Go API, and external facts loaded from facts.d directories. Together they are the seam the Ruby custom-fact DSL binds onto.

Custom facts (in-process)

c.AddValue("datacenter", "eu-west-1")
c.AddFunc("kernel_is_recent", func(cc *facter.Collection) (any, bool) {
    v, ok := cc.Value("kernelmajversion")
    return ok && v.(string) >= "6.0", true
})

Registering a fact replaces any earlier registration of the same name and invalidates its cached value. A resolver returning (nil, false) marks the fact absent. This is the mapping target for Ruby's:

Facter.add(:datacenter) { setcode { "eu-west-1" } }

External facts (facts.d)

err := c.LoadExternalFacts("/etc/facter/facts.d", "/opt/puppetlabs/facts.d")

Each directory is scanned and every fact file is registered as a top-level fact. Four formats are understood:

File	Parsed as
*.json	a JSON object of facts
*.yaml / *.yml	a YAML mapping (nested maps, scalar sequences, scalars)
*.txt	key=value lines, with scalar type inference
any other file with an executable bit	run; its key=value stdout parsed as facts

Directories, and non-executable files with an unknown extension, are ignored. Errors reading a directory or an individual fact file are collected — the first is returned, but every readable fact is still loaded — so one malformed file never hides the rest.

# /etc/facter/facts.d/site.txt
role=web
tier=frontend

# /etc/facter/facts.d/app.yaml
app:
  name: checkout
  version: 3
tags:
  - critical
  - pci