fastsqs 1.1.0

FastAPI-like, lightweight SQS message processing for Python (routing + middleware + dependency injection)

FastSQS

A FastAPI-style router for AWS SQS on Lambda — pydantic routing, dependency injection, a middleware system, and native partial batch failure.

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FastSQS turns an SQS-triggered Lambda into a typed, declarative app. You write handlers for pydantic event models; FastSQS parses each record, routes it, validates it, runs your middleware, and returns the batchItemFailures SQS expects — so failed messages are redelivered and dead-lettered by the queue's own redrive policy, not by bespoke in-app code.

Features

• 🚀 FastAPI-style routing — @app.route(OrderCreated) dispatches by a payload discriminator (default key "type").
• 🔒 Pydantic validation — handlers receive a validated SQSEvent model; bad messages become clean batch failures.
• 💉 Dependency injection — declare Depends(...) params (powered by fast-depends); no @inject needed.
• 🧩 Typed Context — ctx.message_id, ctx.queue_type, … as typed attributes; arbitrary scratch in ctx.state.
• 🪝 Middleware — before/after hooks with balanced unwind (resources acquired in before are always released).
• 🦾 Partial batch failure — native ReportBatchItemFailures for standard and FIFO queues.
• 🔀 FIFO-aware — queue type is inferred from the event-source ARN; per-group ordering with configurable failure mode.
• 🧪 In-process test client — drive your app with synthetic events, no AWS required.
• 🐍 Typed — ships py.typed; full editor/mypy support.

Install

pip install fastsqs

Requires Python 3.10+. Depends on pydantic>=2 and fast-depends>=3.

Quick start

from fastsqs import FastSQS, SQSEvent

app = FastSQS()  # queue type auto-detected from the event-source ARN


class OrderCreated(SQSEvent):
    order_id: str
    amount: int


@app.route(OrderCreated)
async def handle_order(msg: OrderCreated):
    print("processing", msg.order_id, msg.amount)
    # raising marks this record as failed -> SQS redelivers it


# Lambda entry point (set as the function handler):
def handler(event, context):
    return app.handler(event, context)

A message is routed by its discriminator value ("type" by default), matched to the event model's name in snake_case — {"type": "order_created", "order_id": "...", "amount": 1} routes to OrderCreated.

Typed context

Annotate a handler (or middleware) param ctx: Context for typed access to the framework-owned fields. Put your own scratch data in ctx.state:

from fastsqs import FastSQS, SQSEvent, Context

app = FastSQS()


@app.route(OrderCreated)
async def handle(msg: OrderCreated, ctx: Context):
    ctx.message_id        # str
    ctx.queue_type        # QueueType enum (.value for the string)
    ctx.fifo_info         # FifoInfo | None (.message_group_id, ...)
    ctx.state.tenant = "acme"   # arbitrary scratch — never collides with a framework field

Dependency injection

Declare Depends(...) params and FastSQS wires them per invocation (no decorator):

from fastsqs import FastSQS, SQSEvent, Depends

def get_db():
    return Database(...)

app = FastSQS()


@app.route(OrderCreated)
async def handle(msg: OrderCreated, db=Depends(get_db)):
    await db.save(msg.order_id)

Sub-dependencies (a Depends that itself takes Depends) resolve automatically.

Middleware

Subclass Middleware and override before / after. after always runs for every middleware whose before completed (balanced unwind), and receives the error (or None):

from fastsqs import FastSQS, Middleware, TimingMiddleware, LoggingMiddleware

class Audit(Middleware):
    async def before(self, payload, record, context, ctx):
        ctx.state.t0 = ...
    async def after(self, payload, record, context, ctx, error):
        if error is not None:
            ...  # observe the failure

app = FastSQS()
app.add_middleware(LoggingMiddleware())
app.add_middleware(TimingMiddleware())
app.add_middleware(Audit())

Observability, idempotency and PII masking are application concerns — compose them as your own middleware (or use aws-lambda-powertools alongside FastSQS).

FIFO & partial batch failure

• Queue type is QueueType.AUTO by default: FastSQS infers FIFO from a .fifo event-source ARN. Force it with FastSQS(queue_type=QueueType.FIFO).
• fifo_failure_mode (FIFO only): "isolate_groups" (default) blocks only the failed messageGroupId's tail; "halt_batch" halts the whole batch at the first failure (AWS Powertools' default).
• partial_batch_failure (default True) reports per-record failures. Set it False to fail the entire batch (raising BatchFailedError) so SQS redelivers every message.

FastSQS only reports failures — redelivery and dead-lettering are the queue's job (visibility timeout + maxReceiveCount + redrive policy).

max_concurrent_messages (default 10) bounds concurrency on standard queues; FIFO records are processed in order per group.

debug (default False) enables verbose per-record debug logging through the registered LoggingMiddleware.

Testing

from fastsqs.testing import SQSTestClient, RecordSpec

client = SQSTestClient(app)

# one message
result = client.send({"type": "order_created", "order_id": "1", "amount": 5})
assert result == {"batchItemFailures": []}

# a FIFO batch with two message groups (a .fifo ARN is set so AUTO infers FIFO)
client.send_batch([
    RecordSpec({"type": "order_created", "order_id": "1", "amount": 1}, group_id="g1"),
    RecordSpec({"type": "order_created", "order_id": "2", "amount": 2}, group_id="g2"),
])

# a raw (malformed) body to exercise the InvalidMessageError path
client.send("{not json", message_id="bad")

Exceptions

All errors derive from FastSQSError: RouteNotFoundError, InvalidMessageError, and BatchFailedError (whose .failures holds the failed item ids).

License

MIT — see LICENSE.