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dorner/cron.yaml

Last active Sep 18, 2020
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How to execute code in Elastic Beanstalk only if you're the leader
# We are not using this file to run actual cron jobs. This is because the
# built-in Elastic Beanstalk cron feature actually puts the jobs at the end
# of the SQS queue. Instead we will run them manually. However, we need to
# have a non-empty cron.yaml or the SQS daemon won't even run and leader
# election won't work.
version: 1
cron:
- name: "DoNothingJob"
url: "/periodic_tasks"
schedule: "0 0 31 2 *"
# Class that runs some code, only if the current instance is the leader.
class LeaderRunner
def initialize
@instance_id = `wget -q -O - http://169.254.169.254/latest/meta-data/instance-id`
@dynamo_client = Aws::DynamoDB::Client.new
@ec2_client = Aws::EC2::Client.new
yaml = YAML.load_file('/etc/aws-sqsd.d/default.yaml')
@table = yaml['registry_table']
end
# @return [String]
def get_leader
leader_id = @dynamo_client.get_item(
key: { 'id': 'leader-election-record'},
table_name: @table
).item['leader_id']
leader_id.split('.')[0]
end
# @param instance_id [String]
# @return [String]
def instance_state(instance_id)
@ec2_client.describe_instances(
instance_ids: [instance_id]
)[0][0].instances[0][:state][:name]
end
# @param msg [String]
def log_message(msg)
Rails.logger.info(msg)
end
# Actually run the code.
def execute
raise NotImplementedError
end
def run
leader_id = get_leader
leader_state = instance_state(leader_id)
if leader_id == @instance_id && leader_state != 'running'
log_message("We are the leader but we're not in a good state (#{leader_state}). Exiting now.")
return
end
while leader_id != @instance_id && leader_state != 'running'
# Current leader is in a bad state, e.g. shutting down.
# Wait for leader-election to kick in and check again in 2 seconds.
# At that point we may be the leader.
log_message("Not the leader but the leader is not in a good state (#{leader_state}). Checking in 2 seconds...")
sleep(2)
leader_id = get_leader
leader_state = instance_state(leader_id)
end
if leader_id != @instance_id
log_message("Leader is #{leader_id}, not us, exiting")
return
end
execute
end
end
# An example which runs an ActiveJob.
class JobRunner < LeaderRunner
# @param job_class [String]
def initialize(job_class)
super()
@job_class = job_class
end
def execute
log_message("Executing job #{@job_class}")
@job_class.constantize.perform_now
log_message("Done executing #{@job_class}")
end
end
if ARGV[0].present?
JobRunner.new(ARGV[0]).run
end
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