Documentation
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Overview ¶
Package feature provides a tiny, allocation-light feature-flag / rollout evaluator that piggybacks on FastConf's strongly-typed configuration. Rules live in the same YAML / overlay tree as the rest of the config; the runtime evaluator (Rule.Evaluate) is a pure function over a map[string]string context, so it composes naturally with FastConf's lock-free read path.
The design borrows the shape used by ConfigCat / LaunchDarkly / Unleash / OpenFeature: a default value, an ordered list of targeted overrides (first match wins), and an optional percentage rollout keyed on a context attribute. Unlike those SDKs, feature does not require a separate service — every rule is just a value in your config tree.
Index ¶
Constants ¶
This section is empty.
Variables ¶
This section is empty.
Functions ¶
Types ¶
type EvalContext ¶
EvalContext is the per-request bag of attributes used by Rule.Evaluate to pick targeted overrides or compute rollout buckets. Keep keys short and stable across services (e.g. "user.id", "region", "tier").
type Rollout ¶
type Rollout struct {
Percent int `json:"percent" yaml:"percent"`
HashKey string `json:"hashKey" yaml:"hashKey"`
Value any `json:"value" yaml:"value"`
}
Rollout deterministically buckets a context attribute into a 0-99 space. When HashKey is missing from ctx the rollout is skipped (it cannot decide deterministically without an anchor).
type Rule ¶
type Rule struct {
// Key is the dotted name of this rule (e.g. "features.darkMode").
// It is informational — Evaluate does not consult Key.
Key string `json:"key,omitempty" yaml:"key,omitempty"`
// Default is the value returned when no Target / Rollout matches.
Default any `json:"default" yaml:"default"`
// Targets are deterministic equality matches evaluated in order.
// The first Target whose When clauses all match wins.
Targets []Target `json:"targets,omitempty" yaml:"targets,omitempty"`
// Rollouts evaluate in order after Targets. A request lands in a
// rollout bucket when HashKey is present in ctx and its hash
// modulo 100 falls below Percent.
Rollouts []Rollout `json:"rollouts,omitempty" yaml:"rollouts,omitempty"`
}
Rule is one feature flag entry. Unmarshal it from YAML/JSON via the usual codec round-trip:
features:
darkMode:
default: false
targets:
- when: { region: "eu-west" }
value: true
rollouts:
- percent: 30
hashKey: "user.id"
value: true
func (Rule) Evaluate ¶
func (r Rule) Evaluate(ctx EvalContext) any
Evaluate returns Value for the first matching Target or Rollout, or Default when nothing matches. Evaluation is pure and deterministic for the same (Rule, ctx) pair.
Source Files