[15] The Modules API introduced finer control over scaling, versioning, and resource allocation, allowing developers to run different parts of an application with distinct performance settings.

[18] While other services let users install and configure nearly any *NIX compatible software, App Engine requires developers to use only its supported languages, APIs, and frameworks.

Current APIs allow storing and retrieving data from the document-oriented Google Cloud Datastore database, making HTTP requests, sending e-mail, manipulating images, and caching.

Google Cloud SQL[20] can be used for App Engine applications requiring a relational MySQL compatible database backend.

[22] In response, there are a number of projects to create open-source back-ends for the various proprietary/closed APIs of the app engine, especially the datastore.

AppScale automatically deploys and scales unmodified Google App Engine applications over popular public and private cloud systems and on-premises clusters.

[23] It can run Python, Java, PHP, and Go applications on EC2, Google Compute Engine, Azure, and other cloud vendors.