If your app only talks to AWS, LocalStack is fantastic. But the moment a service reaches into GCP or Azure, local development fragments: you bolt on Moto for extra AWS bits, Azurite for Azure Blob, and the gcloud emulators for Pub/Sub and Firestore. Three+ tools, three configs, no shared state, no shared console.

This post compares the options for testing a multi-cloud app locally, and introduces Vyomi — what we call a multi-cloud digital twin runtime: instead of mocking one cloud, it runs a faithful, live replica of AWS, GCP, and Azure in one appliance, on real backends. (If "simulator" is the word you searched for, that's the neighbourhood: the difference is the "real backends" part below.)

The landscape

Why a digital twin, not a mock?

Instead of in-process mocks, Vyomi runs actual engines behind the cloud APIs: Postgres and MySQL for managed SQL, MinIO for S3, DynamoDB-local, Vault for KMS/secrets, NATS for eventing, Azurite for Azure Blob, fake-gcs-server for GCS, and the official Firestore / Pub-Sub emulators. Closer-to-prod behaviour, and state that persists.

The only change to your code: the endpoint URL

# AWS (boto3)
import boto3
s3 = boto3.client("s3", endpoint_url="http://localhost:9000")

# GCP (google-cloud-storage) — point at the local endpoint
# Azure (azure-storage-blob) — same idea, one connection string

aws, gcloud, gsutil, az, and terraform work the same way.

When you're done: export to Terraform

Vyomi exports your simulated stack to Terraform HCL, so the same resources you built locally terraform apply to a real AWS/GCP/Azure account — local-to-prod without a rewrite.

Try it

brew install vyomi-cloud/tap/vyomi && vyomi up
# or: docker run -d -p 9000:9000 -p 9443:9443 vyomi/appliance:latest

Runs offline, free tier, source-available.
Full feature-by-feature comparison: https://vyomi.cloud/compare/localstack.
Source: https://github.com/vyomi-cloud/appliance.

Disclosure: this article is from the Vyomi team. Feedback and "you got LocalStack wrong here" corrections welcome.