Amazon Web Services Launches Local Emulator for Cloud API Testing

Hyperscale computing meets the desktop as developers gain API access for local app testing

Amazon Web Services has introduced a new emulator that comes close to letting developers run its cloud on a personal computer.

As described in the GitHub repository for the plainly named “AWS Emulator,” the tool “is a local program that serves an AWS API endpoint.” Rather than reproducing the entire AWS cloud, the emulator exposes its APIs instead.

“You use it to test your software without calling AWS, on your own machine, on a development server, or in continuous integration (CI),” the repository explains. Because these tests never reach the AWS cloud, the tool comes at no cost.

AWS built the emulator in Rust, a decision that senior specialist solutions architect Daniel Abib said helps keep the tool “light on CPU and memory on a laptop.”

The emulator does not mirror the complete AWS cloud. According to Abib, it supports the APIs of AWS services that have an API model listed in the AWS API models repository on GitHub. The Register counted more than 1,300 entries on that page.

Although the tool allows developers to exercise APIs, it falls well short of reproducing all the features of the Amazon cloud.

“It doesn’t evaluate IAM [Identity and Access Management] policies the way AWS does, and each emulator acts as a single account, with no cross-account or cross-Region behavior,” Abib wrote, noting that “how closely it matches AWS varies by service and feature.”

AWS publishes “fidelity scores” that describe how closely the emulator behaves across different services. Amazon aims for full fidelity, meaning the emulator returns the same results as AWS for valid API calls and the same errors for invalid ones.

Coding agents can also control the emulator, an option The Register highlights because AWS recommends the simplest way to confirm a project will work with the tool: use the aws-emulator skill for AI coding agents.

“With the skill, your agent finds the AWS operations and resources that your project depends on. It checks them against the fidelity tests and fidelity scores for the emulator. It then tells you which parts of your project you can rely on the emulator for, and which ones might behave differently from AWS,” according to AWS documentation.

AWS distributes the emulator as an RPM or Debian package. Mac and PC users will need to run Docker alongside an image of the emulator.

“You install the emulator, start it, and point your tools at the local endpoint,” Abib explained. “When AWS Emulator starts, it generates its own credentials, a config file, and a certificate, and writes them to ~/.aws/emulator/9297/. You set environment variables so the AWS SDK talks to your laptop instead of AWS, and your application code stays unchanged.”

“From there, your existing tests, CLI scripts, and coding agents keep working the way they already do. A unit test that creates an Amazon DynamoDB table, writes an item, and reads it back is the same test, and it runs on your machine without a deployment.”

The cloud giant’s goal appears, as always, to encourage greater use of its platform. In this case, that means reassuring developers their code will perform well once deployed to AWS, and making it easy for them to verify that ahead of time.

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