Mega Mode

Update 10/26/2023: The Rails experimental support has been broken for a while. It has been removed in Jets v5 to prepare for a version of Rails support that will be container based. There is currently no timeline. These docs are only kept around for posterity.

Jets Mega Mode supports deploying your Rails app also with little changes to your code.

With Mega Mode we start up a rack server as subprocess and Jets proxies requests to the underlying rack server. Jets Mega Mode is designed to achieve generic Rack support for frameworks like Rails. Jets Afterburner is actually implemented by Mega Mode underneath the hood. Here are some aspects of how Mega Mode works.

Separate Server Process

The rack application is started up as a separate process. This ensures isolation between the Rails project and the Jets project. This is a key point. The Mega Mode approach keeps the Jets project and its dependencies free from colliding with Rails because they are running in separate processes.

Since everything runs in the same local network within the Lambda Execution context the overhead is inconsequential after the cold start.


Since the rack sub-application is separate process it means that it’s stdout available to the main jets process. Jets accounts for this and automatically adds the rack process stdout to CloudWatch logs. A designation of Rails or Rack is prepended to the logging output to indicate that the log entry is from the rack subprocess. Example:

START RequestId: 3b95bd71-d96a-11e8-ac10-cd7b5b89e808 Version: $LATEST
Processing by Jets::RackController#process
Event: ...
Parameters: {"catchall"=>"posts"}
Rails: Started GET "/dev/posts" for at 2018-10-26 21:58:12 +0000
Rails: Processing by PostsController#index as */*
Rails: PostsController#index ""
Rails: Rendering posts/index.html.erb within layouts/application
Rails: Post Load (1.2ms) SELECT "posts".* FROM "posts"
Rails: ↳ app/views/posts/index.html.erb:14
Rails: Rendered posts/index.html.erb within layouts/application (16.0ms)
Rails: Completed 200 OK in 32ms (Views: 15.3ms | ActiveRecord: 6.8ms)
Completed Status Code 200 in 0.415373045s
END RequestId: 3b95bd71-d96a-11e8-ac10-cd7b5b89e808
REPORT RequestId: 3b95bd71-d96a-11e8-ac10-cd7b5b89e808    Duration: 229.89 ms    Billed Duration: 300 ms Memory Size: 1536 MB    Max Memory Used: 556 MB

Use of /tmp Folder

Normally, AWS Lambda runs your application on a read-only filesystem. Most Rails applications assume it has filesystem write access. For example, it might create tmp/cache folders upon starting up. Rails gems or plugins might also assume write access. Because of this, the rack application runs from the /tmp folder which AWS Lambda allows write access to.

Using the /tmp folder also increases the amount of space available to run applications to 512MB. This is important because the maximum size of your uncompressed code with Lambda Layers on AWS Lambda is currently limited to 250MB. Using the /tmp folder allows for larger rack applications.

Cold Start Overhead

Since Mega Mode starts a separate rack server process within the Lambda Execution Context, there is some overhead. The overhead depends on your application. If you have configured your Lambda function to at least 1GB of RAM, then the overhead is usually 1-3 seconds. Additional requests after the cold start usually range from 30ms to 300ms.

Why Mega Mode?

Since there is overhead associated with the Mega Mode approach, you may be wondering why take the approach. Simple. It’s the most pragmatic way to get a Rails app running on serverless.