example plugin with aws-sdk-core

glacier_account_id.rb

require 'aws-sdk-core'

class GlacierAccountId < Seahorse::Client::Plugin
  option :account_id, '-'
  
  # :validate, :build, :sign, :send - where in the stack you want the middleware to apply
  handle_request(step: :validate, priority: 99) do |context|
    context.params[:account_id] ||= context.config.account_id 
  end
end

Aws::Glacier.new.list_vaults # fails because the plugin hasn't been applied
Aws::Glacier.add_plugin GlacierAccountId
Aws::Glacier.new.list_vaults # now works!