Jets supports prewarming your application to remedy the Lambda cold start issue. Prewarming is enabled by default. To adjust the prewarming settings, edit your config/application.rb. Example:

Jets.application.configure do
  # ...
  config.prewarm.enable = true # default: enabled
  config.prewarm.rate = "30 minutes" # default: 30 minutes
  config.prewarm.concurrency = 2 # default: 2
  config.prewarm.public_ratio = 3 # default: 3

Rate vs Concurrency

Option Explanation
rate This controls how often the prewarming job runs.
concurrent For each prewarming job run, this controls how many times in parallel to hit the functions with a prewarm request.

For example, with a rate of 2 hours and concurrent of 2, this results in the Lambda functions being called with a prewarm request 24 times after 24 hours (12 hours x 2).

Public Ratio

The prewarm.public_ratio activates extra prewarming for the internal jets/public_controller.rb. The jets/public_controller.rb handles serving static files out of the public folder. This includes css and js assets that have been packaged up. Generally, each web request usually results in additional assets file requests. The prewarm.public_ratio tells Jets to prewarm the public_controller’s lambda function a little bit extra. You can tune the extra prewarming ratio higher or lower according to your needs.

Prewarm After Deployment

After a deployment finishes, Jets automatically prewarms the app immediately. This keeps your application nice and fast.

Prewarm Custom Headers

Jets appends a x-jets-prewarm-count header to the response to help you see if the lambda function was prewarmed. The header looks like this:

We can see that the lambda function had been prewarmed once and called 4 times since the last time AWS Lambda recycled the Lambda function.

Custom Prewarming

Jets prewarms most Ruby functions in your application with the same weight. If you want to prewarm a specific function that gets a high volume of traffic, you can create a custom prewarm job. Here’s a starter example:


class PrewarmJob < ApplicationJob
  class_timeout 30
  class_memory 512
  rate '30 minutes'
  def hot_page
    function_name = "posts_controller-index"
    threads = []
    10.times do
      threads << do
    threads.each { |t| t.join }
    "Finished prewarming #{function_name}."

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