A simple Elixir.Agent for registering occurrences of different events and reporting event counts for the given time period.

Configuration can be passed to start_link/1 as a keyword list. Supported parameters are:

• :ttl - the duration, in seconds, that events should be stored for. Default is 60;
• :default_period - period to consider by default when Velocity.of/1 is called. Can be either an integer or a @time_ranges key. Default is :minute.

Anything can be used for event keys. Minimum time granularity is 1 second.

Example

Velocity.start_link(ttl: 3 * 60, default_period: :minute)

Velocity.register(:foo)
Velocity.register(:bar)
Velocity.register(:foo)

Velocity.of(:foo)
#=> {:ok, 2}

Velocity.of(:foo, :minute)
#=> {:ok, 2}

Velocity.of(:bar, :minute)
#=> {:ok, 1}

Velocity.of(:baz, :minute)
#=> {:ok, 0}

#...after 2 minutes...
Velocity.of(:foo, :minute)
#=> {:ok, 0}

Velocity.of(:foo, 5 * 60)
#=> {:ok, 2}

#...after 3 minutes...
Velocity.of(:foo, 5 * 60)
#=> {:ok, 0}

TODO: implement subscribable alerts, e.g. Velocity.alert(self(), :ato, {{:gt, 10}, :minute})