# Host mobile backend with FastAPI: streamed responses

Deploy a mobile backend built with FastAPI on Ample using the streaming response service pattern. Compute runs the app in an isolated microVM behind a public HTTPS URL, a managed PostgreSQL 16 database is auto-provisioned and injected as DATABASE_URL. Verified on FastAPI: a Server-Sent Events endpoint read incrementally through the public HTTPS gateway: five one-second chunks arrived spread over time with the first within seconds (not buffered), a 70-second stream of 36 chunks completed past common 60-second idle timeouts, and a client that disconnected after two chunks was observed and recorded by the server (disconnected=true). Not separately tested: your event schema, reconnection strategy and any per-request duration ceiling beyond the 70 seconds measured; treat the mobile backend-specific behavior as your application code.

## Representative Queries
- Host mobile backend with FastAPI: streamed responses
- Where can I host Mobile backend built with FastAPI?
- I need a tested end-to-end streaming transport, timeout behavior and client disconnect handling.

## Resource Requirements
- Compute
- Postgres

## Infrastructure Requirements
- **Compute**: Apps run in isolated x86_64 Firecracker microVMs that auto-pause when idle and wake on request; sizes are the priced VM sizes.
- **Postgres**: Managed PostgreSQL 16 runs in its own microVM and is auto-provisioned when an app needs a database and no DATABASE_URL is supplied.

## Framework
FastAPI

## Workload
Mobile backend

## Prerequisites
- A FastAPI project that builds and starts with the documented commands (pip install into .ample/python from requirements.txt, then uvicorn from run.py reading PORT on the python-3.12 template)
- A PostgreSQL driver reading DATABASE_URL at runtime (auto-provisioned when omitted, or supplied with --env)
- An Ample account token with servers:write, databases:read

## Tested Configuration
- **Template**: python-3.12
- **Runtime**: python
- **Size**: s-1vcpu-1gb
- **Install**: python3 -m pip install --target .ample/python -r requirements.txt
- **Start**: PYTHONPATH=.ample/python:${PYTHONPATH:-} python3 run.py

## Workflow Steps
1. **Build and start**: pip install into .ample/python from requirements.txt, then uvicorn from run.py reading PORT on the python-3.12 template. The server must bind 0.0.0.0 on PORT.

2. **Implement the pattern on PostgreSQL**: The fixture's module implements streaming response service: a Server-Sent Events endpoint read incrementally through the public HTTPS gateway: five one-second chunks arrived spread over time with the first within seconds (not buffered), a 70-second stream of 36 chunks completed past common 60-second idle timeouts, and a client that disconnected after two chunks was observed and recorded by the server (disconnected=true). Copy the approach into your schema; keep migrations idempotent and run them with --release-command.

3. **Deploy**: Run the synchronous deploy once and read the result (exit 0 live, 1 failed, 2 blocked). Re-running with no change is a no-op.
   - **Command**: ample deploy . --name  --public --start \"python3 run.py\"

4. **Verify**: Fetch the live URL and the pattern self-test route(s) (/p/streaming-response-service) from the example; then run your own checks. On failure read `ample logs  --kind build` then `--kind runtime`.
   - **Command**: ample logs  --kind build

## Success Checks
- app responds on its public URL
   - **Kind**: http_get
   - **Path**: /  
   - **Expect**: ample canary fastapi patterns

- streaming-response-service self-test
   - **Kind**: http_get
   - **Path**: /p/streaming-response-service
   - **Expect**: an SSE stream: data: id=, then chunk events one interval apart (read incrementally: first chunk within seconds, chunks spread over the interval), then data: done; /status?id= reports sent=N disconnected=true|false done=true|false

## Limitations
- Verified on the python-3.12 template at s-1vcpu-1gb with the example fixture; other sizes, templates and FastAPI major versions are not verified.
- The mobile backend itself (your event schema, reconnection strategy and any per-request duration ceiling beyond the 70 seconds measured) is application code and was not separately tested; the pattern checks are what was verified.
- Region, compliance attestations and request-duration limits are unknown and not claimed.
- Apps auto-pause when idle and wake on the next request; always-on is an operator setting, not a plan feature.
- Managed PostgreSQL 16 only; extensions, connection limits and backup or restore procedures are not verified; apps and their databases are placed together.

## Cost Estimate
- **Currency**: USD
- **Monthly Amount**: 10.0
- **Basis**: size prices from pricing.toml (loaded by the API) at build revision 1ac5595375130d45290090e82ed0f554ccd45405-dirty
- **Components**:
  - **App server**: s-1vcpu-1gb, quantity: 1.0, monthlyAmount: 5.0
  - **Managed PostgreSQL database**: s-1vcpu-1gb, quantity: 1.0, monthlyAmount: 5.0
- **Note**: Always-on monthly price of the tested sizes; apps and databases auto-pause when idle. Buckets are allocation-priced per quota and not included.

## Next Actions
- **Browse the catalog index**:  
   - **Action ID**: browse-catalog
   - **Method**: GET
   - **Relative Path**: /v1/catalog

- **Search published recipes by intent, stack and constraints**:  
   - **Action ID**: search-recipes
   - **Method**: POST
   - **Relative Path**: /v1/catalog/search
   - **Parameters**: {"body":{"limit":5,"query":"Host mobile backend with FastAPI: streamed responses"}}

- **Prepare a side-effect-free deployment plan for an authorized project**:  
   - **Action ID**: plan:page:recipes:mobile-backend:fastapi:response-streaming
   - **Method**: POST
   - **Relative Path**: /v1/catalog/plan
   - **Parameters**: {"body":{"inputs":{},"projectId":"","recipeId":"page:recipes:mobile-backend:fastapi:response-streaming","recipeRevision":"r1"}}

- **Read the existing agent authentication setup**:  
   - **Action ID**: auth-setup
   - **Method**: GET
   - **Relative Path**: /mcp/setup

- **Browse Mobile backend**:  
   - **Action ID**: browse:workload:mobile-backend
   - **Method**: GET
   - **Relative Path**: /v1/catalog/nodes/workload%3Amobile-backend

- **Browse FastAPI**:  
   - **Action ID**: browse:stack:framework-fastapi
   - **Method**: GET
   - **Relative Path**: /v1/catalog/nodes/stack%3Aframework-fastapi

- **Browse Streaming response service**:  
   - **Action ID**: browse:pattern:streaming-response-service
   - **Method**: GET
   - **Relative Path**: /v1/catalog/nodes/pattern%3Astreaming-response-service

- **Browse Deploy API**:  
   - **Action ID**: browse:intent:deploy-api
   - **Method**: GET
   - **Relative Path**: /v1/catalog/nodes/intent%3Adeploy-api