attempt

v0.6.0

attempt v0.6.0 Attempt.Bucket.Token

Implementation of a Token Bucket

A token bucket provides a form of rate limiting

This implmentation is designed to allow for both synchronous and asynchronous token requests. The intent is to simplify the higher level APIs by giving them a soft guarantee of token return.

Since the implementation uses timers (via Process.send_after/3) neither the timing precision not the minimum time window are likely to be useful for all applications.

The primary purpose of this token bucket is to support “longer liver” functions such as 3rd party API calls and calls to other external services like databases.

Implementation

  • A bucket is defined to hold a maximum number of tokens

  • The token count is reduced by each call to get_token/2

  • When the token count reaches 0, the request is placed in a queue.

  • Every :increment_every milliseconds a new token is created. The increment is calculated as the refill_every / bucket_size so that the number of tokens is added in time consistent manner.

  • When the timer is reached and a new token is added the pending queue is processed

Link to this section Summary

Types

t()

Functions

claim_token(bucket, budget)
claim_token!(bucket, budget)
init(budget)

Invoked when the server is started. start_link/3 or start/3 will block until it returns

new(name, config \\ %{__struct__: Attempt.Bucket.Token, burst_size: 10, fill_rate: 3, max_queue_length: 100, name: nil, queue: nil, tokens: 0})
new!(name, config)
start_link(name, bucket \\ %{__struct__: Attempt.Bucket.Token, burst_size: 10, fill_rate: 3, max_queue_length: 100, name: nil, queue: nil, tokens: 0})
state(bucket)
stop(name)

Link to this section Types

Link to this type t() 
t() :: struct()

Link to this section Functions

Link to this function claim_token(bucket, budget)
Link to this function claim_token!(bucket, budget)
Link to this function init(budget)

Invoked when the server is started. start_link/3 or start/3 will block until it returns.

args is the argument term (second argument) passed to start_link/3.

Returning {:ok, state} will cause start_link/3 to return {:ok, pid} and the process to enter its loop.

Returning {:ok, state, timeout} is similar to {:ok, state} except handle_info(:timeout, state) will be called after timeout milliseconds if no messages are received within the timeout.

Returning {:ok, state, :hibernate} is similar to {:ok, state} except the process is hibernated before entering the loop. See c:handle_call/3 for more information on hibernation.

Returning :ignore will cause start_link/3 to return :ignore and the process will exit normally without entering the loop or calling c:terminate/2. If used when part of a supervision tree the parent supervisor will not fail to start nor immediately try to restart the GenServer. The remainder of the supervision tree will be (re)started and so the GenServer should not be required by other processes. It can be started later with Supervisor.restart_child/2 as the child specification is saved in the parent supervisor. The main use cases for this are:

  • The GenServer is disabled by configuration but might be enabled later.
  • An error occurred and it will be handled by a different mechanism than the Supervisor. Likely this approach involves calling Supervisor.restart_child/2 after a delay to attempt a restart.

Returning {:stop, reason} will cause start_link/3 to return {:error, reason} and the process to exit with reason reason without entering the loop or calling c:terminate/2.

Callback implementation for GenServer.init/1.

Link to this function new(name, config \\ %{__struct__: Attempt.Bucket.Token, burst_size: 10, fill_rate: 3, max_queue_length: 100, name: nil, queue: nil, tokens: 0}) 
new(atom(), Keyword.t() | Attempt.Bucket.Token.t()) ::
  {:ok, Attempt.Bucket.Token.t()} |
  {:error, {Exception.t(), String.t()}}
new(atom(), Keyword.t() | Attempt.Bucket.Token.t()) ::
  {:ok, Attempt.Bucket.Token.t()} |
  no_return()
Link to this function new!(name, config)
Link to this function start_link(name, bucket \\ %{__struct__: Attempt.Bucket.Token, burst_size: 10, fill_rate: 3, max_queue_length: 100, name: nil, queue: nil, tokens: 0})
Link to this function state(bucket) 
state(Token.Bucket.t()) ::
  {:ok, non_neg_integer()} |
  {:error, {Exception.t(), String.t()}}
Link to this function stop(name) 
stop(atom() | Retry.Budget.t() | Attempt.Bucket.Token.t()) ::
  :ok |
  {:error, {Exception.t(), String.t()}}