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Scheduling

Learning Objectives
  • Describe what `cron` is and how it is used
  • Read and interpret `cron` expressions
  • Write `cron` expressions to schedule tasks
  • Describe AWS EventBridge and list the benefits of using it
  • Schedule tasks using EventBridge

Pre work​

Set up​

If nobody has done so already, copy and paste this command into Slack.

/prework Scheduling

Case study​

Task

Read this week's case study.

What questions do you have? What issues do you anticipate?

Research​

Task

Before the workshop, research these questions and share your answers on Slack

  • What is cron?
  • What is cron used for?
  • What are the rules to form a cron expression?

Practice​

Task

Using crontab.guru as a guide, write cron expressions for the following situations:

  • Every Friday at 9am
  • 6:30pm on Wednesdays in June
  • Every half hour from 3pm to 6pm on weekdays

Submission​

Submission

Please share your answers in the Slack thread.

Workshop​

Introduction​

Case study​

Ask if trainees have any questions about the week's work/case study.

Answer questions as appropriate.

Cron​

Ask trainees to explain what cron is.

Confirm that it is a command-line utility for scheduling things.

Remind trainees of a daemon - a background service - and explain that cron is a daemon (like docker) used specifically to run scheduled tasks.

State that the cron syntax is larger than the tool itself, and will appear in more places than the command line.

Motivate trainees by pointing out how tedious it is to do everything manually, and how this isn't practical with long-running (or late-night) tasks. Identify cron -- and related technologies as the solution.

Cron syntax​

Direct trainees to crontab.guru.

Talk through the syntax, showing how just these five values can provide quite fine-grained control.

Warn that there are varieties of cron syntax, and not every trick will work everywhere.

Challenge trainees to identify the syntax for the following:

  • Every 20 minutes each Tuesday (*/20 * * * 2)
  • At 2am on the fifth day of each month for the first half of the year (0 2 5 1-6 *)
  • Every 15 minutes past each hour from midnight to 6am (*/15 0-6 * * *)

Crontab​

Introduce crontab as a tool for passing instructions to cron.

Explain the basic structure of a command: [cron expression] [task to do].

Demonstrate the power of cron with a small example:

  • crontab -e to open the jobs list for editing.
  • Add * * * * * echo "I am repeating" >> log.txt to the file.
  • Wait for one minute, and then demonstrate the contents of ~/log.txt

Stress that echo is just a small example here; this could be replaced by any shell command.

Challenge trainees to set up their own cron task: a Python script that runs every 5 minutes, logging the current time to a file.

Explain that cron jobs run from the user's home directory by default, so it's important to use absolute paths for both the script and any files it creates or accesses.

Show trainees essential crontab commands:

  • crontab -l to list all currently scheduled cron jobs
  • crontab -e to edit the jobs list
  • crontab -r to remove all cron jobs for the current user (use with caution!)

Demonstrate how to unschedule specific cron tasks: crontab -e and then delete the unwanted line.

"""Simple cron script that logs the current time."""

from datetime import datetime

if __name__ == "__main__":
    # Use absolute path to ensure cron can find the file
    log_file = "/home/ubuntu/cron_log.txt"
    
    with open(log_file, 'a') as f:
        f.write(f"{datetime.now()}\n")
# Note: Use absolute paths for both the Python interpreter and script
# Cron jobs run from the home directory, not from where the script is located
*/5 * * * * /usr/bin/python3 /home/ubuntu/script.py

# To find the full path to python3, run: which python3
# To find the full path to your script, run: realpath script.py

AWS EventBridge​

Ask trainees if cron is a good way to run a critical service.

Expect the answer 'no'; it's very dependent on a specific computer always being on.

Introduce AWS EventBridge as a tool for scheduling cloud-based tasks.

Demonstrate the process of setting up a schedule, and how similar it is to cron.

Highlight the following details:

  • The similar-to-cron (but not identical) syntax used for setting up a recurring schedule.
  • The way that almost any AWS service can be triggered by an EventBridge schedule (most relevantly including, but not limited to, ECS tasks and Lambda functions).

Practical EventBridge Example​

Explain that to see EventBridge in action, we'll create a task that creates an S3 bucket on a schedule.

Guide trainees through creating the example:

Create EventBridge Schedule to create an S3 bucket​

Navigate to EventBridge Scheduler in the AWS Console.

Create a new schedule with:

  • Schedule type: Recurring schedule
  • Cron expression: * * * * ? * (every minute)
  • Target: S3 bucket
    • Set the name of the bucket to be something like C##-bucket or something funnier / more exciting.

Wait 1 minute and then check the S3 console to see the newly created bucket.

Point out that this demonstrates:

  • How EventBridge can trigger AWS services on a schedule
  • How scheduled tasks can perform real infrastructure changes
  • How the cron-like syntax translates to actual scheduled execution

Remind trainees to clean up resources afterwards:

  • Delete the created S3 buckets
  • Delete the EventBridge schedule

Conclusion​

Ask if trainees have any lingering questions.

Answer them.

Quiz the trainees on general topics surrounding these schedules to gauge their understanding more deeply.

Stress that scheduling tasks so they happen without manual input will be important for most pipelines somewhere; the more comfortable trainees are with cron and related tech, the better.