Hello Service

The simplest possible Aster service: a greeting RPC. Three files, no credentials, no infrastructure. This example is Python-specific.

The service definition

Create hello_service.py with the shared type and service definitions:

from dataclasses import dataclass
from aster import service, rpc

@dataclass
class HelloRequest:
    name: str = ""

@dataclass
class HelloResponse:
    message: str = ""

@service
class HelloService:
    """Simple greeting service."""

    @rpc
    async def say_hello(self, req: HelloRequest) -> HelloResponse:
        return HelloResponse(message=f"Hello, {req.name}!")

Line by line:

@dataclass defines plain data types for the request and response. Fields have defaults so Fory can construct them during deserialization.
@service marks the class as an Aster RPC service. The service name defaults to the class name (HelloService), version defaults to 1.
@rpc marks say_hello as a unary RPC method. The framework inspects the type annotations (HelloRequest and HelloResponse) to handle serialization automatically.

Since no @wire_type is applied, the @service decorator auto-tags the types using their module-qualified names. This is fine for development; production services should use explicit @wire_type tags.

The producer

Create producer.py to host the service:

import asyncio
from hello_service import HelloService
from aster import AsterServer

async def main():
    async with AsterServer(services=[HelloService()]) as srv:
        print(f"Endpoint address: {srv.address}")
        print("Waiting for connections... (Ctrl+C to stop)")
        await srv.serve()

asyncio.run(main())

Line by line:

AsterServer(services=[HelloService()]) creates a producer that hosts one service. The server builds an iroh node, registers the aster/1 ALPN for RPC, and runs the consumer admission ALPN for discovery.
async with calls start() (which creates the QUIC endpoint) and serve() (which starts the accept loop). On exit, it calls close().
srv.address is the aster1... connection address that consumers need to connect.

The consumer

Create consumer.py to call the service:

import asyncio
from hello_service import HelloService, HelloRequest
from aster import AsterClient

async def main():
    async with AsterClient(address="<paste address from producer>") as client:
        hello = await client.client(HelloService)
        resp = await hello.say_hello(HelloRequest(name="World"))
        print(resp.message)

asyncio.run(main())

Line by line:

AsterClient(address=...) takes the aster1... address printed by the producer. Alternatively, set ASTER_ENDPOINT_ADDR as an environment variable and omit the argument.
async with calls connect(), which creates a QUIC endpoint and runs the admission handshake to discover available services.
client.client(HelloService) returns a typed stub. The stub’s say_hello method serializes the request, sends it over QUIC, deserializes the response, and returns it.

Running it

Open two terminals. No credentials or configuration files are needed.

Terminal 1 – producer:

pip install aster-rpc
python producer.py

Output:

Endpoint address: g6Jpa...kNQ==
Waiting for connections... (Ctrl+C to stop)

Terminal 2 – consumer:

# Option A: pass the address inline in the code
python consumer.py

# Option B: use an environment variable
export ASTER_ENDPOINT_ADDR=g6Jpa...kNQ==
python consumer.py

Output:

Hello, World!

What is happening under the hood

QUIC endpoint creation. Both producer and consumer create iroh QUIC endpoints. Each gets an ed25519 identity key (ephemeral by default) and connects to a relay server for NAT traversal.
ALPN negotiation. The consumer first connects on the aster.consumer_admission ALPN. The producer responds with the list of available services (in this case, HelloService v1) and its RPC channel address.
Admission. In dev mode (no root key configured), the producer auto-opens the consumer gate. The consumer is admitted without credentials.
RPC connection. The consumer opens a second QUIC connection on the aster/1 ALPN. This connection carries RPC traffic.
Call dispatch. The consumer sends a StreamHeader (service name, method name, serialization mode), followed by the serialized HelloRequest. The producer deserializes it, calls say_hello, serializes the HelloResponse, and sends it back.
Cleanup. When the async with blocks exit, both sides close their QUIC connections and endpoints.

Next steps

Add @wire_type tags for production stability: Define a Service
Connect with credentials: Dial a Service
Add streaming methods: Define a Service – Streaming