# Unleash
`Unleash` is a client for the
[Unleash Toggle Service](https://unleash.github.io/).
The primary use is `Unleash.enabled?/2` or `Unleash.enabled?/3` to check whether
or not the given feature flag is enabled.
```elixir
iex> Unleash.enabled?(:my_feature)
false
iex> Unleash.enabled?(:my_feature, true)
true
iex> context = %{user_id: "ID", custom_field: "Unleash"}
iex> Unleash.enabled?(:my_feature, context)
true
iex> Unleash.enabled?(:my_feature, context, false)
true
```
Context fields are transformed internally when validated against constraints,
e.g. `:user_id` becomes `"userId"`.
## Installation
If [available in Hex](https://hex.pm/docs/publish), the package can be installed
by adding `unleash_ex` to your list of dependencies in `mix.exs`:
```elixir
def deps do
[
{:unleash, "~> 1.9"}
]
end
```
## Configuration
There are many configuration options available, and they are listed below with
their defaults. These go into the relevant `config/*.exs` file.
```elixir
config :unleash, Unleash,
url: "", # The URL of the Unleash server to connect to, should include up to http://base.url/api
appname: "", # The app name, used for registration
instance_id: "", # The instance ID, used for metrics tracking
metrics_period: 10 * 60 * 1000, # Send metrics every 10 minutes, in milliseconds
features_period: 15 * 1000, # Poll for new flags every 15 seconds, in milliseconds
strategies: Unleash.Strategies, # Which module to request for toggle strategies
backup_file: nil, # Backup file in the event that contacting the server fails
custom_http_headers: [], # A keyword list of custom headers to send to the server
disable_client: false, # Whether or not to enable the client
disable_metrics: false, # Whether or not to send metrics,
retries: -1 # How many times to retry on failure, -1 disables limit
app_env: :dev # Which environment we're in
```
`:custom_http_headers` should follow the format prescribed by
`t:Mojito.headers/0`
`:strategies` should be a module that implements
`c:Unleash.Strategies.strategies/0`. See [Extensibility](#extensibility)
for more information.
## Extensibility
If you need to create your own strategies, you can extend the `Unleash` client
by implementing the callbacks in both `Unleash.Strategy` and
`Unleash.Strategies` as well as passing your new `Strategies` module in as
configuration:
1. Create your new strategy. See `Unleash.Strategy` for details on the correct
API and `Unleash.Strategy.Utils` for helpful utilities.
```elixir
defmodule MyApp.Strategy.Environment do
use Unleash.Strategy
def enabled?(%{"environments" => environments}, _context) do
with {:ok, environment} <- MyApp.Config.get_environment(),
environment = List.to_string(environment) do
{Utils.in_list?(environments, environment, &String.downcase/1),
%{environment: environment, environments: environments}}
end
end
end
```
1. Create a new strategies module. See `Unleash.Strategies` for details on the correct
API.
```elixir
defmodule MyApp.Strategies do
@behaviour Unleash.Strategies
def strategies do
[{"environment", MyApp.Strategy.Environment}] ++ Unleash.Strategies.strateges()
end
end
```
1. Configure your application to use your new strategies list.
```elixir
config :unleash, Unleash, strategies: MyApp.Strategies
```
## Telemetry events
From Unleash 1.9, telemetry events are emitted by the Unleash client
library. You can attach to these events and collect metrics or use the `Logger`,
for example:
```elixir
# An example of checking if Unleash server is reachable during the periodic
# features fetch.
:ok =
:telemetry.attach_many(
:duffel_core_feature_heatbeat_metric,
[
[:unleash, :client, :fetch_features, :stop],
[:unleash, :client, :fetch_features, :exception]
],
fn [:unleash, :client, :fetch_features, action],
_measurements,
metadata,
_config ->
require Logger
http_status = metadata[:http_response_status]
if action == :stop and http_status in [200, 304] do
Logger.info("Fetching features are ok")
else
Logger.info("Error on fetching features!!!")
end
end,
%{}
)
```
The following events are emitted by the Unleash library:
* `[:unleash, :feature, :enabled?, :start]` - dispatched by `Unleash` whenever
a feature state has been requested.
* Measurement: `%{system_time: system_time, monotonic_time: monotonic_time}`
* Metadata: `%{appname: String.t(), instance_id: String.t(), feature: String.t()}`
* `[:unleash, :feature, :enabled?, :stop]` - dispatched by `Unleash` whenever
a feature check has successfully returned a result.
* Measurement: `%{duration: native_time, monotonic_time: monotonic_time}`
* Metadata: `%{appname: String.t(), instance_id: String.t(), feature: String.t(), result: boolean(), reason: atom(), strategy_evaluations: [{String.t(), boolean()}], feature_enabled: boolean()}`
* `[:unleash, :feature, :enabled?, :exception]` - dispatched by `Unleash` after
exceptions on fetching a feature's activation state.
* Measurement: `%{duration: native_time, monotonic_time: monotonic_time}`
* Metadata: `%{appname: String.t(), instance_id: String.t(), feature: String.t(), kind: :throw | :error | :exit, reason: term(), stacktrace: Exception.stacktrace()}`
* `[:unleash, :client, :fetch_features, :start]` - dispatched by `Unleash.Client` whenever
it start to fetch features from a remote Unleash server.
* Measurement: `%{system_time: system_time, monotonic_time: monotonic_time}`
* Metadata: `%{appname: String.t(), instance_id: String.t(), etag: String.t() | nil, url: String.t()}`
* `[:unleash, :client, :fetch_features, :stop]` - dispatched by `Unleash.Client` whenever
it finishes to fetch features from a remote Unleash server.
* Measurement: `%{duration: native_time, monotonic_time: monotonic_time}`
* Metadata: `%{appname: String.t(), instance_id: String.t(), etag: String.t() | nil, url: String.t(), http_response_status: pos_integer | nil, error: struct() | nil}`
* `[:unleash, :client, :fetch_features, :exception]` - dispatched by `Unleash.Client` after
exceptions on fetching features.
* Measurement: `%{duration: native_time, monotonic_time: monotonic_time}`
* Metadata: `%{appname: String.t(), instance_id: String.t(), etag: String.t() | nil, url: String.t(), kind: :throw | :error | :exit, reason: term(), stacktrace: Exception.stacktrace()}`
* `[:unleash, :client, :register, :start]` - dispatched by `Unleash.Client` whenever
it starts to register in an Unleash server.
* Measurement: `%{system_time: system_time, monotonic_time: monotonic_time}`
* Metadata: `%{appname: String.t(), instance_id: String.t(), url: String.t(), sdk_version: String.t(), strategies: [String.t()], interval: pos_integer}`
* `[:unleash, :client, :register, :stop]` - dispatched by `Unleash.Client` whenever
it finishes to register in an Unleash server.
* Measurement: `%{duration: native_time, monotonic_time: monotonic_time}`
* Metadata: `%{appname: String.t(), instance_id: String.t(), url: String.t(), sdk_version: String.t(), strategies: [String.t()], interval: pos_integer, http_response_status: pos_integer | nil, error: struct() | nil}`
* `[:unleash, :client, :register, :exception]` - dispatched by `Unleash.Client` after
exceptions on registering in an Unleash server.
* Measurement: `%{duration: native_time, monotonic_time: monotonic_time}`
* Metadata: `%{appname: String.t(), instance_id: String.t(), url: String.t(), sdk_version: String.t(), strategies: [String.t()], interval: pos_integer, kind: :throw | :error | :exit, reason: term(), stacktrace: Exception.stacktrace()}`
* `[:unleash, :client, :push_metrics, :start]` - dispatched by `Unleash.Client` whenever
it starts to push metrics to an Unleash server.
* Measurement: `%{system_time: system_time, monotonic_time: monotonic_time}`
* Metadata: `%{appname: String.t(), instance_id: String.t(), url: String.t(), metrics_payload: %{ :bucket => %{:start => String.t(), :stop => String.t(), toggles: %{
String.t() => %{ :yes => pos_integer(), :no => pos_integer() } } } }
}`
* `[:unleash, :client, :push_metrics, :stop]` - dispatched by `Unleash.Client` whenever
it finishes to push metrics to an Unleash server.
* Measurement: `%{duration: native_time, monotonic_time: monotonic_time}`
* Metadata: `%{appname: String.t(), instance_id: String.t(), url: String.t(), http_response_status: pos_integer | nil, error: struct() | nil, metrics_payload: %{ :bucket => %{:start => String.t(), :stop => String.t(), toggles: %{
String.t() => %{ :yes => pos_integer(), :no => pos_integer() } } } } }`
* `[:unleash, :client, :push_metrics, :exception]` - dispatched by `Unleash.Client` after
exceptions on pushing metrics to an Unleash server.
* Measurement: `%{duration: native_time, monotonic_time: monotonic_time}`
* Metadata: `%{appname: String.t(), instance_id: String.t(), url: String.t(), kind: :throw | :error | :exit, reason: term(), stacktrace: Exception.stacktrace(), metrics_payload: %{ :bucket => %{:start => String.t(), :stop => String.t(), toggles: %{
String.t() => %{ :yes => pos_integer(), :no => pos_integer() } } } } }`
* `[:unleash, :repo, :schedule]` - dispatched by `Unleash.Repo` when scheduling a poll to the server for metrics
* Metadata: `%{appname: String.t(), instance_id: String.t(), retries: integer(), etag: String.t(), interval: pos_integer()}`
* `[:unleash, :repo, :backup_file_update]` - dispatched by `Unleash.Repo` when it writes features to the backup file.
* Metadata: `%{appname: String.t(), instance_id: String.t(), content: String.t(), filename: String.t()}`
* `[:unleash, :repo, :disable_polling]` - dispatched by `Unleash.Repo` when polling gets
disabled due to retries running out or zero retries being specified initially.
* Metadata: `%{appname: String.t(), instance_id: String.t(), retries: integer(), etag: String.t()}`
* `[:unleash, :repo, :features_update]` - dispatched by `Unleash.Repo` when features are updated.
* Metadata: `%{appname: String.t(), instance_id: String.t(), retries: integer(), etag: String.t(), source: :remote | :cache | :backup_file}`
