# FunWithFlags
[](https://travis-ci.org/tompave/fun_with_flags)
[](https://hex.pm/packages/fun_with_flags)
[](https://hexdocs.pm/fun_with_flags/api-reference.html)
FunWithFlags, the Elixir feature flag library.
**This library is still a work in progress and is not ready to use**
## Goals
This library is meant to be an OTP application that provides an Elixir API to toggle and query feature flags and an (authenticated) web UI for administrators.
It should store feature flag information in Redis for persistence and syncronize different nodes, but it should also maintain a local cache in an ETS table for fast lookup. When flags are added or toggled, nodes should be notified (via Redis PubSub or polling) and update their local ETS representation.
Different kind of feature flags should be supported:
* simple (on, off);
* actors (on or off for specific structs or data);
* groups (or or off for structs or data that satisfy a condition).
The planned functionality of this library is heavily inspired by the [flipper Ruby gem](https://github.com/jnunemaker/flipper), although with a focus on:
* decreasing the load on Redis (feature flags are not toggled _that_ often, there is no need to query Redis for each check);
* making it more reliable (it should keep working with the last cached values if Redis becomes unavailable, although nodes can be out of sync).
Just as Elixir and Phoenix are meant to scale better than Ruby on Rails with high levels of traffic and concurrency, FunWithFlags should aim to be more scalable than Flipper.
## Status and Roadmap
### Done
* Simple boolean flags: either ON or OFF.
* Flags are persisted in Redis and available on application restart.
* In-process ETS cache. On lookup, the library checks the cache first. If the ETS table doesn't contain a flag, it falls back to Redis and copies the value into the cache. Subsequent lookups won't hit Redis. The ETS table is empty when the application starts.
* Creating or toggling a flag will update both the ETS cache and Redis.
* Both the ETS cache and the Redis connection are in a supervision tree. The [Redix](https://hex.pm/packages/redix) adapter will try to reconnect to Redis if the connection is lost.
* If the connection to Redis is lost, the application will continue to work with the known values from the ETS cache.
* Several nodes can connect to the same Redis and share the flag settings. Each one will hit Redis the first time a flag is looked up, and then will populate its ETS cache.
### Next / Problems
* If Redis become unavailable, and an unknown flag is looked up, the fallback logic will still try to check Redis and raise an exception.
* When two or more nodes are using the same Redis, and one of them updates a flag that the others have already cached, or creates a flag that the others have already looked up (and cached as "disabled"), then the other nodes will not be notified of the changes.
** Use Redis PubSub to emit change notifications.
** Add a TTL to the ETS cache (use [ConCache](https://hex.pm/packages/con_cache)?).
** Add polling to refresh the cache from Redis every X seconds (not ideal).
* Implement other "gates": at least actors and groups.
## Usage
Still a work in progress, expect breaking changes.
```elixir
FunWithFlags.enabled?(:cool_new_feature)
false
{:ok, true} = FunWithFlags.enable(:cool_new_feature)
FunWithFlags.enabled?(:cool_new_feature)
true
{:ok, false} = FunWithFlags.disable(:cool_new_feature)
FunWithFlags.enabled?(:cool_new_feature)
false
```
## Installation
The package can be installed by adding `fun_with_flags` to your list of dependencies in `mix.exs`:
```elixir
def deps do
[{:fun_with_flags, "~> 0.0.3"}]
end
```
