#!/usr/bin/env ruby

Dir.chdir(File.dirname(__FILE__)) { require 'bundler/setup' }

require 'aws-sdk'

# Look up RDS free space for the given database, return free space in Bytes, as
# the Maximum over the last hour.
#
# You can do a similar operation using the AWS CLI:
# aws cloudwatch get-metric-statistics --start-time $(gdate -u -d '-1 hour' +%FT%T) \
#   --end-time $(gdate -u +%FT%T) --period 3600 --namespace AWS/RDS --statistics Maximum \
#   --metric-name FreeStorageSpace --query 'Datapoints[*].Maximum' --output text --dimensions Name=DBInstanceIdentifier,Value=$MY_DB_NAME
#
def get_rds_free_space(db_name, debug: false)
  logger = Logger.new(STDERR)
  if debug
    logger.level = Logger::INFO
  else
    logger.level = Logger::WARN
  end

  cw = Aws::CloudWatch::Client.new(logger: logger)
  stats = cw.get_metric_statistics(
    namespace: 'AWS/RDS',
    metric_name: 'FreeStorageSpace',
    dimensions: [
      {
        name: 'DBInstanceIdentifier',
        value: db_name
      }
    ],
    start_time: Time.now - 3600,
    end_time: Time.now,
    period: 3600,
    statistics: ['Minimum', 'Maximum', 'Average']
  )

  if stats.datapoints.length != 1
    raise 'Uh oh, got unexpected stats: ' + stats.inspect
  end

  stats.datapoints.fetch(0).maximum
end

# Return allocated storage for a database, in GiB.
def get_rds_total_space(db_name)
  Aws::RDS::Resource.new.db_instance(db_name).allocated_storage
end

def b_to_gb(bytes)
  bytes / 1024.0 / 1024 / 1024
end

def get_data(db_name)
  total_gb = get_rds_total_space(db_name)
  free_b = get_rds_free_space(db_name)

  free_gb = b_to_gb(free_b)

  used_gb = total_gb - free_gb

  {
    total: total_gb,
    used: used_gb,
    free: free_gb
  }
end

def main(db_name)
  data = get_data(db_name)
  total = data.fetch(:total)
  used = data.fetch(:used)
  free = data.fetch(:free)

  puts "total: #{total.round} GiB"
  puts "used:  #{used.round(2)} GiB (#{(used / total * 100).round}%)"
  puts "free:  #{free.round(2)} GiB (#{(free / total * 100).round}%)"
end

if $0 == __FILE__
  if ARGV.empty?
    STDERR.puts "usage: #{File.basename($0)} DB_NAME"
    exit 1
  end
  main(ARGV.fetch(0))
end