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Concepts

This page explores flags, variations, and allocations, the central feature flagging concepts on Eppo.

Flags

A flag is simply a fork somewhere in your code. Where originally there was a single code path, the presence of a flag creates a gate that splits that path into two or more code paths. In its most basic form a flag object can represented by a human readable name, a key, and a status (on/off). Here's a quick example:

# Before, price is $10 for everyone.

price_of_shoes = 10

The presence of a feature flag unlocks multiple paths:

# After, show different prices to different users based on a flag.

group = get_assignment(user_id, 'pricing_gate')

if (group == "round_down_to_cents"):
price_of_shoes = 9.99
else:
price_of_shoes = 10

Explore interesting flag uses cases here.

Variations

Variations are the distinct paths that can be taken from a single flag. In the simplest case a flag will have two variations, but there is no limit.

Allocations

An allocation is a logical grouping of targeting rules and variation weights associated with a flag. In Eppo, allocations are stacked in a vertical waterfall as seen below:

Allocation waterfall

In the picture above there are two allocations:

  • The first allocation targets internal users, which are identified with by their email domain. If a user's email matches the criteria, they are show Variant 1. If not, they are moved along to the next allocation (i.e down the waterfall).
  • The second allocation targets users in North America using an iPhone. 50% of users who match this criteria see Variant 1 and the remaining 50% of users see Variant 2. If users do no match, they are moved along. Since there are no more allocations to evaluate, all remaining users see the default value, which in this example is Variant 1.

Allocations can be turned on/off in the waterfall using the Enable Allocation checkbox. If an allocation is off, it is simply skipped when evaluating the waterfall logic.

Note that it is possible to reduce an allocation's traffic exposure to less than 100%. In this case, the allocation's rules are evaluated and then subsequently traffic exposure check is computed. Subjects that pass the rules test but fall outside the traffic exposure check are always served NULL. They do not continue down the waterfall.