AWS Compute Blog

Creating an Enterprise Scheduler Using AWS Lambda and Tagging

Co-authored by Felix Candelario and Benjamin F., AWS Solutions Architects

Many companies are looking to optimize their AWS usage and lower operational costs. While production environments are typically well-governed and reach optimal use through services such as Auto Scaling, sometimes development and testing need a different approach.

There are a number of ways to schedule resources to start or stop based on time of day (Auto Scaling Scheduled Scaling, for example); however, some companies are looking to hand over control of the schedule to the resource owner, without reassigning the burden of creating the scheduling infrastructure.

In this post, we discuss a proposed method for running an enterprise scheduler that queries running resources for a specific tag; the tag controls when resources should be switched on or off. Control over this tag can be handed to the resource owner, thereby providing a very simple method to govern the on/off schedule of a resource based on a weekly cycle.

Resource owners only need to describe the schedule they wish to enforce in human-friendly syntax and apply that in the form of a tag on their resources. The enterprise scheduler runs periodically using AWS Lambda, scans resources for the schedule tag, parses the schedule syntax, and then issue an On or Off command per the schedule that's assigned to each resource individually.

Lambda function schedule

To enable the enterprise scheduler, a Lambda function should be added and run on a schedule. The function describes EC2 instances within the same region, and determines if there are any On or Off operations to be executed. We recommend configuring the function to run every 10 minutes. By default, the function determines On/Off actions from the last 12 minutes.

Controlling this behavior is done in two parts:

  • The Lambda function schedule
  • The max_delta parameter

The function schedule controls how often the function is invoked and begins to search for On/Off actions to perform. The max_delta parameter controls the time window in which to search for individual schedule tags on resources. For example, you may choose to run the function every 20 minutes, and then set max_delta to 22 minutes. It is necessary to keep the max_delta value slightly higher than the rate which the function is invoked so as not to miss any On/Off actions. We recommend setting it two minutes above the function invocation rate.

Required IAM permissions

The Lambda function requires permissions to query the resource tags using DescribeInstances, and then to act on them, with either a StartInstances or StopInstances API operation call.

Install the scheduler

The following procedures walk you through creating the function and validation.

Modify the code

  1. Copy the Enterprise Scheduler Lambda function code to your computer from the following code example:
from __future__ import print_function # Python 2/3 compatibility
import boto3
import datetime
import sys

def lambda_handler(event, context):
    schedule_tag = 'EntScheduler'
    max_delta = 12
    now =
    ec2 = boto3.resource('ec2')
    client = boto3.client('ec2')
    scheduled_instances = []
    processed_instances = []
    #filter for instances with the correct tag
    instances = ec2.instances.filter(Filters=[{'Name': 'tag-key', 'Values':[schedule_tag]}])
    #grab the scheduler string
    for instance in instances:
        for tag in instance.tags:
            if tag['Key'] == schedule_tag:
                scheduled_instances.append({'instance':instance, 'schedule':tag['Value']})

    def parse_schedule(instance_hold):
        day = now.strftime('%a').lower()
        current_time = datetime.datetime.strptime(now.strftime("%H%M"), "%H%M")
        instance_hold['disabled'] = False
        #parse the schedule string into seperate tokens
        tokenized_schedule = instance_hold['schedule'].split(';')
        #make sure the schedule string contains either 4 or 5 parameters.
        if len(tokenized_schedule) < 4:
            instance_hold['disabled'] = True
            sys.exit('Schedule string improperly formed. Fewer than 4 tokens specified.')
        if len(tokenized_schedule) > 6:
            instance_hold['disabled'] = True
            sys.exit('Schedule string improperly formed. Greater than 5 tokens specified.')
        #check to make sure today is the day to execute an on action
        if day in tokenized_schedule[0]:
                #check to make sure 24 hour string parses correctly
                scheduled_time_for_on = datetime.datetime.strptime(tokenized_schedule[1], "%H%M")
                #as long as not outside of the window of execution
                delta = scheduled_time_for_on - current_time
                margin = datetime.timedelta(minutes=max_delta)
                if(current_time - margin <= scheduled_time_for_on <= current_time):
                    instance_hold['on'] = True
                    instance_hold['on'] = False
            except Exception as e:
                instance_hold['disabled'] = True
                sys.exit('Time string for the on action improperly formed. Ensure in HHMM format.')
            instance_hold['on'] = False

        #check to make sure today is the day to execute an off action
        if day in tokenized_schedule[2]:
                #check to make sure 24 hour string parses correctly
                scheduled_time_for_off = datetime.datetime.strptime(tokenized_schedule[3], "%H%M")
                delta = scheduled_time_for_off - current_time
                margin = datetime.timedelta(minutes=max_delta)
                if(current_time - margin <= scheduled_time_for_off <= current_time):
                   instance_hold['off'] = True
                    instance_hold['off'] = False
            except Exception as e:
                instance_hold['disabled'] = True
                sys.exit('Time string for the on action improperly formed. Ensure in HHMM format.')
            instance_hold['off'] = False

        #check for disabled string
        if len(tokenized_schedule) > 4:
            if 'disable' in tokenized_schedule[4]:
                instance_hold['disabled'] = True
        return instance_hold

    for instance_hold in scheduled_instances:

    for instance_hold in processed_instances:
            if(instance_hold['off']==True and instance_hold['on']==True):
                print('Both on and off actions specified for this time window. Doing nothing.')
            if(instance_hold['off']==True and instance_hold['on']==False):
                print('Turning instance off: ' + instance_hold['instance'].id + ' ' + instance_hold['instance'].instance_type)
            if(instance_hold['off']==False and instance_hold['on']==True):
                print('Turning instance on: ' + instance_hold['instance'].id + ' ' + instance_hold['instance'].instance_type)
            if(instance_hold['off']==False and instance_hold['on']==False):
                print('No action on instance: ' + instance_hold['instance'].id + ' ' + instance_hold['instance'].instance_type)
            print('Schedule disabled: ' + instance_hold['instance'].id + ' ' + instance_hold['instance'].instance_type)
  1. Edit the local copy of the code on your computer using a text editor, and make the following optional changes:
  2. Optional: Edit the _schedule_tag_ parameter if you will be using a custom tag for setting up resource schedules. By default, the value is: EntScheduler.
  3. Optional: Set _max_delta_ to a value that is two minutes higher than the rate of invocation that will be used. By default, the value is 12 mins.

  4. Save the local copy of the code with the changes made above and name it

Create the Lambda function

  1. Open the Lambda console and choose Create a Lambda function.
  2. Choose Next to skip a blueprint selection.
  3. Choose Next to skip creation of a trigger at this time.
  4. Enter a function name and note it for later use. For example: enterprise_scheduler_function.
  5. For Runtime, choose Python 2.7.
  6. For Code entry type, choose Edit code inline.
  7. Paste the function code from the local version of the scheduler that was saved in the previous section (
  8. For Handler, enter enterprise_scheduler.lambda_handler .
  9. For Role selection, choose Create a custom role.

A window pops up displaying the IAM form for creating a new IAM execution role for Lambda.

Add an IAM role

  1. In the IAM window, for Role name, enter ent_scheduler_execution_role (or similar text).
  2. Choose View Policy Document, Edit the policy, OK (to confirm that you've read the documentation).
  3. Replace the contents of the default policy with the following:
  "Version": "2012-10-17",
  "Statement": [
      "Sid": "Stmt1469047780000",
      "Effect": "Allow",
      "Action": [
      "Resource": [
      "Sid": "Stmt1469047825000",
      "Effect": "Allow",
      "Action": [
      "Resource": [
  1. Choose Allow to save the policy and close the window.
  2. For Timeout, enter 5 minutes.
  3. Choose Next.
  4. Review the configuration and choose Create Function.

Add an event schedule

  1. Choose Triggers, Add trigger.
  2. Click on the box and choose CloudWatch Events – Schedule.
  3. For Schedule expression, enter the rate at which you would like the enterprise scheduler to be invoked. We recommend a value of "rate(10 minutes)".

At this point, a tagged Amazon EC2 instance in the region where the Lambda function is running is either turned on or off, based on its schedule.

Enabling the scheduler on a resource

Assigning a schedule to a resource using a tag requires following these guidelines:

  • The resource tag must match the tag named in the Lambda function. You can modify the schedule_tag parameter to choose a tag name such as YourCompanyName Scheduler.
  • The scheduler works on a weekly basis (7 days of the week), and you may specify up to 1 set hour for turning the resource On or Off. For each day that's set in the On or Off section, the same hour is used.
  • The current time is interpreted by the function using UTC, therefor the schedule tag should use the UTC time for turning instances both On and Off.
  • Use the following scheduler syntax in the Lambda function:


  • The Days values should be noted with the first 3 letters of the day. Accepted values are: mon, tue, wed, thu, fri, sat, sun. Multiple days can be specified with comma separation and order is ignored.
  • The Hour value should be noted in 24H format HHMM.
  • Optional-Disable states that this tag will be ignored by the function, allowing the user to keep the configuration and have the scheduler skip over this resource temporarily. Note: The function can be configured to disregard Optional-Disable based on company policy.

Here are some examples of scheduler syntax:

  • mon,tue,wed,thu,fri,sat,sun;0830;mon,tue,wed,thu,fri,sat,sun;1700;
    Resource would be turned on at 8:30 am UTC daily and turned off at 5:00 pm UTC daily.

  • mon;0700;fri;1800;
    Resource would be turned on at 7:00 am on Mondays and turned off at 6:00 pm UTC on Fridays.

  • wed,thu,fri;2100;thu,fri,sat;0500;
    Resource would be turned on at 9:00 pm UTC on Wednesdays, Thursdays, and Fridays, and turned off at 5:00 am UTC on Thursdays, Fridays, and Saturdays.

  • wed,thu,fri;2100;thu,fri,sat;0500;disable;
    Resource would be left untouched and the schedule ignored due to the disable option.


This method of scheduling resource use is simple and can support several use cases, such as shutting down labs for evenings and weekends. Even this small optimization can result in significant savings when you consider that a typical workday is 8 out of 24 hours. Unlike on-premises resources, AWS resources can be managed by a quick API call.

If you have questions or suggestions, please comment below.