AWS Developer Blog

Waiters

by Trevor Rowe | on | in Ruby | Permalink | Comments |  Share

We’ve added a feature called Waiters to the v2 AWS SDK for Ruby, and I am pretty excited about it. A waiter is a simple abstraction around the pattern of polling an AWS API until a desired state is reached.

Basic Usage

This simple example shows how to use waiters to block until a particular EC2 instance is running:

ec2 = Aws::EC2::Client.new
ec2.wait_until(:instance_running, instance_ids:['i-12345678'])

Waiters will not wait indefinitely and can fail. Each waiter has a default polling interval and maximum number of attempts to make. If a waiter encounters an unexpected error or fails to reach the desired condition in time it will raise an error:

begin
  ec2.wait_until(:instance_running, instance_ids:['i-12345678'])
resuce Aws::Waiters::Errors::WaiterFailed
  # oops
end

Configuration

You can modify the default interval and wait time between attempts by passing a block.

# this will wait upto ~ one hour
ec2.wait_until(:instance_running, instance_ids:['i-12345678']) do |w|

  # seconds between each attempt
  w.interval = 15

  # maximum number of polling attempts before giving up
  w.max_attempts = 240

end

Callbacks

In addition to interval and maximum attempts, you can configure callbacks to trigger before each attempt polling attempt and before sleeping between attempts.

ec2.wait_until(:instance_running, instance_ids:['i-12345678']) do |w|

  w.before_attempt do |n|
    # n - the number of attempts made
  end

  w.before_wait do |n, resp|
    # n - the number of attempts made
    # resp -the client response from the previous attempt
  end

end

You can throw :success or :failure from these callbacks to stop the waiter immediately. You can use this to write you own delay and back-off logic.

Here I am using a callback to perform exponential back-off between polling attempts:

ec2.wait_until(:instance_running, instance_ids:['i-12345678']) do |w|
  w.interval = 0 # disable normal sleep
  w.before_wait do |n, resp|
    sleep(n ** 2)
  end
end

This example gives up after one hour.

ec2.wait_until(:instance_running, instance_ids:['i-12345678']) do |w|
  one_hour_later = Time.now + 3600
  w.before_wait do |n, resp|
    throw :failure, 'waited too long' if Time.now > one_hour_later
  end
end

Waiters and Resources, Looking Ahead

You may have noticed that some waiters have already been exposed to the resource classes.

ec2 = Aws::EC2::Resource.new
instance = ec2.instance('i-12345678')
instance.stop
instance.wait_until_stopped
puts instance.id + ' is stopped'

In addition to connecting more waiters and resources, I’m excited to look into batch waiters. Imagine the following use case:

instances = ec2.create_instances(min_count: 5, ...)
instances.wait_until_running
puts "the following new instances are now running:n"
puts instances.map(&:id)

Documentation

Waiters are documented in the Ruby SDK API reference. Each service client documents the #wait_until method and provides a list of available waiter names. Here are links to the Aws::EC2::Client waiter methods:

Give waiters a try and let us know what you think!