Amazon Lambda function handler in Ruby

The Lambda function handler is the method in your function code that processes events. When your function is invoked, Lambda runs the handler method. Your function runs until the handler returns a response, exits, or times out.

In the following example, the file function.rb defines a handler method named handler. The handler function takes two objects as input and returns a JSON document.

require 'json'

def handler(event:, context:)
    { event: JSON.generate(event), context: JSON.generate(context.inspect) }
end

In your function configuration, the handler setting tells Lambda where to find the handler. For the preceding example, the correct value for this setting is function.handler. It includes two names separated by a dot: the name of the file and the name of the handler method.

You can also define your handler method in a class. The following example defines a handler method named process on a class named Handler in a module named LambdaFunctions.

module LambdaFunctions
  class Handler
    def self.process(event:,context:)
      "Hello!"
    end
  end
end

In this case, the handler setting is source.LambdaFunctions::Handler.process.

The two objects that the handler accepts are the invocation event and context. The event is a Ruby object that contains the payload that's provided by the invoker. If the payload is a JSON document, the event object is a Ruby hash. Otherwise, it's a string. The context object has methods and properties that provide information about the invocation, the function, and the execution environment.

The function handler is executed every time your Lambda function is invoked. Static code outside of the handler is executed once per instance of the function. If your handler uses resources like SDK clients and database connections, you can create them outside of the handler method to reuse them for multiple invocations.

Each instance of your function can process multiple invocation events, but it only processes one event at a time. The number of instances processing an event at any given time is your function's concurrency. For more information about the Lambda execution environment, see Lambda execution environment.