Anatomy of rules¶

At the heart of FFS are rules. Each feature flag is backed by a rule. It boils down to functions as commonly known in programming, or formulas as commonly seen in spreadsheets. Functions accept data, and return a result. That is numeric, between 0 and 1.

When it is 0, the flag is disabled. When 1, the flag is enabled. When in-between, the flag is partially enabled, meaning it is enabled for some users, proportionally to the value.

Examples¶

To build intuition, let's look at some examples:

0: the flag is disabled.
false: the flag is disabled (same as 0).
1: the flag is enabled.
true: the flag is enabled (same as 1).
0.5: the flag is enabled for 50% of users.

Let's take it up a notch, and look at more useful rules:

gte(now(), datetime("2038-01-19T04:14:07+01:00")): enabled after January 19 2038 at 03:14:07 UTC.
matches("*@example.com", env["user.email"]): enabled for emails ending in @example.com.
map(now(), datetime("2022-11-08"), datetime("2022-11-15"), 0, 1): gradual rollout between November 8 and November 16, 2022.
not(isblank(env["user.email"])): enabled for logged in users (assuming their email is set when logging in).
pow(map(now(), datetime("2021-11-08"), datetime("2021-11-15"), 0, 1), 2): gradual rollout again, but exponential instead of linear over the week.

Environment¶

All metadata supplied by the SDK can be accessed using env["<key>"]. You can send anything that makes sense to your app, and then use it in rules.

Info

Noticed env["user.email"] in the examples above? It was supplied by the SDK.

Results¶

Results are always numerical. true and false are mapped to 1 and 0, respectively. Any result that's neither a number nor boolean is treated as 0.

Functions¶

Name	Type	Syntax	Description
isblank	Info	`isblank(value)`	Checks if the value is blank.
eq	Operator	`eq(a, b)`	Checks if `a` is equal to `b`.
gt	Operator	`gt(a, b)`	Checks if `a` is greater than `b`.
gte	Operator	`gte(a, b)`	Checks if `a` is greater than or equal to `b`.
lt	Operator	`lt(a, b)`	Checks if `a` is less than `b`.
lte	Operator	`lte(a, b)`	Checks if `a` is less than or equal to `b`.
now	Date	`now()`	Returns the current date and time.
datetime	Date	`datetime(value)`	Returns a date and time. `value` must follow ISO-8601's format date and time, or date.
matches	Lookup	`matches(regex, value)`	Checks if `value` matches `regex`.
contains	Lookup	`contains(list, value)`	Checks if `list` contains `value`.
not	Logical	`not(value)`	Negates the result of `value`.
and	Logical	`and(a, b)`	Checks if both `a` and `b` are true.
or	Logical	`or(a, b)`	Checks if either `a` or `b` is true.
if	Logical	`if(condition, a, b)`	If `condition` is true, returns `a`. Otherwise, returns `b`.
plus	Operator	`plus(a, b)`	Adds `a` and `b`.
minus	Operator	`minus(a, b)`	Subtracts `b` from `a`.
times	Operator	`times(a, b)`	Multiplies `a` and `b`.
div	Operator	`div(a, b)`	Divides `a` by `b`.
rem	Operator	`rem(a, b)`	Returns the remainder of `a` divided by `b`.
log	Math	`log(a, b)`	Returns the logarithm of `a` to the base of `b`.
ln	Math	`ln(value)`	Returns the natural logarithm of `value`.
pow	Math	`pow(a, b)`	Returns `a` to the power of `b`.
exp	Math	`exp(value)`	Returns `e` to the power of `value`.
map	Math	`map(instart, inend, outstart, outend, value)`	Maps `value` over the range `[instart, inend]` and returns the result in the range `[outstart, outend]`.