What is AWSim?

AWSim is a fully offline AWS emulator written in Rust. It runs 60+ AWS services in a single binary, starts in under 500ms, and requires no internet connection, no AWS account, and no license keys. A built-in billing dashboard projects what your workload would cost on real AWS so you can spot expensive patterns before they hit your real account.

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.

Estimated Billing

AWSim ships with a billing meter that subscribes to every request that flows through the gateway, multiplies usage by canonical AWS pricing (vendored from the public AWS Pricing Bulk JSON), and surfaces a rolling estimated monthly bill at /billing in the admin console. Currently models per-request, byte-ingest, data-transfer-out, GB-month storage, GB-second compute, instance-hours, state-transition and per-character billing axes across 25+ services. See the Billing guide for what's metered and how the rates were sourced.

What AWSim is not

• It does not make any network calls to AWS at runtime.
• 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.
• The billing dashboard is a projection, not a contract. A few services' billing axes (Lambda's per-function memory, Step Functions' transition counts, EC2/RDS instance types) round to safe defaults; consult the Billing guide for known approximations.