This module implements the Fix Window algorithm.

The fixed window algorithm works by dividing time into fixed intervals or "windows" of a specified duration (scale). Each window tracks request counts independently.

For example, with a 60 second window:

• Window 1: 0-60 seconds
• Window 2: 60-120 seconds
• And so on...

The algorithm:

1. When a request comes in, we:
   • Calculate which window it belongs to based on current time
   • Increment the counter for that window
   • Store expiration time as end of window
2. To check if rate limit is exceeded:
   • If count <= limit: allow request
   • If count > limit: deny and return time until window expires
3. Old windows are automatically cleaned up after expiration

This provides simple rate limiting but has edge cases where a burst of requests spanning a window boundary could allow up to 2x the limit in a short period. For more precise limiting, consider using the sliding window algorithm instead.

The fixed window algorithm is a good choice when:

• You need simple, predictable rate limiting with clear time boundaries
• The exact precision of the rate limit is not critical
• You want efficient implementation with minimal storage overhead
• Your use case can tolerate potential bursts at window boundaries

Common use cases include:

• Basic API rate limiting where occasional bursts are acceptable
• Protecting backend services from excessive load
• Implementing fair usage policies
• Scenarios where clear time-based quotas are desired (e.g. "100 requests per minute")

The main tradeoff is that requests near window boundaries can allow up to 2x the intended limit in a short period. For example with a limit of 100 per minute:

• 100 requests at 11:59:59
• Another 100 requests at 12:00:01

This results in 200 requests in 2 seconds, while still being within limits. If this behavior is problematic, consider using the sliding window algorithm instead.

The fixed window algorithm supports the following options:

• :clean_period - How often to run the cleanup process (in milliseconds) Defaults to 1 minute. The cleanup process removes expired window entries.

Example configuration:

MyApp.RateLimit.start_link(
  clean_period: :timer.minutes(5),
)copy

This would run cleanup every 5 minutes and clean up old windows.