What is AWSim?

AWSim is a fully offline AWS emulator written in Rust. It runs 37 AWS services in a single binary, starts in under 500ms, and requires no internet connection, no AWS account, and no license keys.

Why AWSim?

LocalStack Community Edition was effectively discontinued as a free tool — most useful features moved behind a paid tier. AWSim fills that gap: a permissively licensed, fully offline alternative that you can run anywhere without registering.

AWSim is dual-licensed under MIT and Apache-2.0. You can use it in commercial projects.

Architecture

AWSim is built on Axum, a Rust async web framework. When a request arrives:

1. Protocol detection — the gateway inspects the Content-Type, X-Amz-Target, and path to determine which AWS wire protocol is in use (JSON, Query, REST-XML, REST-JSON).
2. Service routing — the x-amz-target header or URL path is used to identify the target service and operation.
3. Operation dispatch — the request is forwarded to the appropriate service handler, which reads/writes in-memory state protected by DashMap (a concurrent hash map).
4. Event bus — cross-service integrations (SNS fan-out, SQS→Lambda polling, etc.) use an internal async event bus.
5. Persistence — if --data-dir is set, service state is serialized to JSON snapshots on a 30-second timer and on shutdown.

All state lives in memory. Snapshots are written atomically (write to a temp file, then rename).

IAM Policy Enforcement

AWSim includes a real IAM policy evaluation engine that implements AWS authorization semantics — identity policies, resource policies, permissions boundaries, SCPs, and session policies with all 26 condition operators. Enforcement is opt-in via AWSIM_IAM_ENFORCE=true so existing tests remain unaffected. Enabling it lets you unit-test policy documents end-to-end against S3, DynamoDB, KMS, SQS, SNS, Secrets Manager, Lambda, and IAM. See the IAM Enforcement guide.

What AWSim is not

• It does not make any network calls to AWS.
• By default it does not enforce IAM policies — you must opt in with AWSIM_IAM_ENFORCE=true.
• It is not 100% API-compatible with AWS. Edge cases and rarely-used parameters may behave differently or be unimplemented.