Pause

Pause is a flexible Redis-backed rate-limiting client. Use it to track events, with rules around how often they are allowed to occur within configured time checks.

Because Pause is Redis-based, multiple ruby processes (even distributed across multiple servers) can track and report events together, and then query whether a particular identifier should be rate limited or not.

Sample applications include IP-based blocking based on HTTP request volume (see related gem "spanx"), throttling push notifications as to not overwhelm the user with too much frequency, etc.

Installation

Add this line to your application's Gemfile:

gem 'pause'

And then execute:

$ bundle

Or install it yourself as:

$ gem install pause

Usage

Configuration

Configure Pause. This could be in a Rails initializer.

• resolution - The time resolution (in seconds) defining the minimum period into which action counts are aggregated. This defines the size of the persistent store. The higher the number, the less data needs to be persisted in Redis.
• history - The maximum amount of time (in seconds) that data is persisted

Pause.configure do |config|
  config.redis_host = "127.0.0.1"
  config.redis_port = 6379
  config.redis_db   = 1
  config.resolution = 600     # aggregate all events into 10 minute blocks
  config.history    = 86400   # discard all events older than 1 day
end

Actions

Define local actions for your application. Actions define a scope by which they are identified in the persistent store (aka "namespace"), and a set of checks. Checks define various thresholds (max_allowed) against periods of time (period_seconds). When a threshold it triggered, the action is rate limited, and stays rate limited for the duration of block_ttl seconds.

Checks

Checks are configured with the following arguments (which can be passed as an array, or a symbol hash):

• period_seconds - time window this is a time period against which an action is tested
• max_allowed - the maximum number of times an action can be incremented during this particular time period before rate limiting is triggered.
• block_ttl - amount time (seconds) an action stays rate limited after threshold is reached.

Scope

Scope is simple string used to identify this action in the Redis store, and is appended to all keys. Therefore it is advised to keep scope as short as possible to reduce memory requirements of the store.

If you are using the same Redis store to rate limit multiple actions, you must ensure that each action has a unique scope.

Resolution

Resolution is the period of aggregation. As events come in, Pause aggregates them in time blocks of this length. If you set resolution to 10 minutes, all events arriving within a 10 minute block are aggregated.

Resolution must be less than or equal to the smallest period_seconds value in your checks. In other words, if your shortest check is 1 minute, you could set resolution to 1 minute or smaller.

Example

require 'pause'

class FollowAction < Pause::Action
  scope "f"
  check period_seconds:   60, max_allowed:  100, block_ttl: 3600
  check period_seconds: 1800, max_allowed: 2000, block_ttl: 3600
end

When an event occurs, you increment an instance of your action, optionally with a timestamp and count. This saves data into a redis store, so it can be checked later by other processes. Timestamps should be in unix epoch format.

class FollowsController < ApplicationController
  def create
    action = FollowAction.new(user.id)
    if action.ok?
      # do stuff!
      # and track it...
      action.increment!
    else
      # action is rate limited, either skip
      # or show error, depending on the context.
    end
  end
end

class OtherController < ApplicationController
  def index
    action = OtherAction.new(params[:thing])
    unless action.rate_limited?
      # perform business logic
      ....
      # track it
      action.increment!(params[:count].to_i, Time.now.to_i)
    end
  end
end

If more data is needed about why the action is blocked, the analyze can be called

action = NotifyViaEmailAction.new("thing")

while true
  action.increment!

  rate_limit_event = action.analyze
  if rate_limit_event
    puts rate_limit_event.identifier               # which key got rate limited ("thing")
    puts rate_limit_event.sum                      # total count that triggered a rate limit
    puts rate_limit_event.timestamp                # timestamp when rate limiting occurred
    puts rate_limit_event.period_check             # period check object, that triggered this rate limiting event
  else
    # not rate-limited, same as action.ok?
  end

  sleep 1
end

Enabling/Disabling Actions

Actions have a built-in way by which they can be disabled or enabled.

MyAction.disable
MyAction.enable

This is persisted to Redis, so state is not process-bound, but shared across all ruby run-times using this action (assuming Redis store configuration is the same).

When disabled, Pause does not check state in any of its methods, so calls to increment! or ok? still work exactly as before. This is because adding extra Redis calls can be expensive in loops. You should check whether your action is enabled or disabled if it important to support enabling and disabling of rate limiting in your context.

while true
  if MyAction.enabled?
    Thing.all.each do |thing|
      action = MyAction.new(thing.name)
      action.increment! unless action.rate_limited?
    end
  end
  sleep 10
end

Unblocking

Actions can be unblocked manually after they have been blocked.

To unblock all blocked identifiers for a single action:

MyAction.unblock_all

To unblock a single identifier for an action:

action = MyAction.new('hello')

action.ok?
# => false

action.unblock

action.ok?
# => true

Using Pause with Twemproxy

Pause can be used with Twemproxy to shard its data among multiple redis instances. When doing so, the hash_tag configuration in Twemproxy should be set to "||". In addition, the sharded Pause configuration option should be set to true.

When sharding is used, the Redis adapter used by Pause ignores the redis_db, which is not supported.

Pause.configure do |config|
  config.redis_host = "127.0.0.1"
  config.redis_port = 6379
  config.resolution = 600     # aggregate all events into 10 minute blocks
  config.history    = 86400   # discard all events older than 1 day
  config.sharded    = true
end

With this configuration, any Pause operation that we know is not supported by Twemproxy will raise Pause::Redis::OperationNotSupported. For instance, when sharding we are unable to get a list of all tracked identifiers.

The action block list is implemented as a sorted set, so it should still be usable when sharding.

Contributing

Want to make it better? Cool. Here's how:

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create a new pull request

Authors

This gem was written by Eric Saxby, Atasay Gokkaya and Konstantin Gredeskoul at Wanelo, Inc.

Please see the LICENSE.txt file for further details.