harness 0.1.0rc1

Configuration

Harness uses Protocol Buffers as a configuration definition language (svc.proto file):

syntax = "proto3";

package cafe;

import "harness/options.proto";
import "harness/postgres.proto";
import "harness/grpc.proto";

message Configuration {
  bool debug = 1;
  harness.postgres.DSN db = 2 [(harness.input) = "python/asyncpg.v1:Connection"];
  harness.grpc.Channel taskqueue = 3 [(harness.input) = "python/grpclib.v1:Channel"];
  harness.grpc.Endpoint listen = 4 [(harness.output) = "python/grpclib.v1:Server"];
}

As you can see from this configuration, our service has:

• debug mode

• database connection

• gRPC channel to the taskqueue service

• gRPC server to serve incoming requests

We can provide values for this configuration using YAML file (config.yaml file):

db:
  value: postgres://postgres:postgres@db.cafe.svc.cluster.local:5432/coffee
taskqueue:
  host: taskqueue.platform.svc.cluster.local
  port: 50051
listen:
  host: 0.0.0.0
  port: 50051

Service Definition

Harness must be able to run your service. In order to do this you must provide a main function, it’s signature looks like this:

from svc_wires import WiresIn, WiresOut

async def main(wires_in: WiresIn) -> WiresOut:
    ...
    return WiresOut(...)

Where WiresIn and WiresOut are generated by the Harness from your service configuration:

@dataclass
class WiresIn:
    db: harness.resources.asyncpg.v1.Connection
    taskqueue: harness.resources.grpclib.v1.Channel

@dataclass
class WiresOut:
    listen: harness.resources.grpclib.v1.Server

When Harness runs your service, you receive already initialized environment in a WiresIn structure. All you have to do is to setup your request handler and return it with a WiresOut structure (svc.py file):

from harness.resources.grpclib.v1 import Server

from svc_grpc import CoffeeMachineBase

class CoffeeMachine(CoffeeMachineBase):
    ...

async def main(wires_in: WiresIn) -> WiresOut:
    print('Connection:', wires_in.db.connection)
    print('Channel:', wires_in.taskqueue.channel)
    server_resource = Server([
        CoffeeMachine(
            db=wires_in.db.connection,
            taskqueue=wires_in.taskqueue.channel,
        ),
    ])
    return WiresOut(listen=server_resource)

Files Generation

We use protoc compiler to generate boilerplate from your configuration:

$ protoc -I $HARNESS_PROTO --python_out=. --python_grpc_out=. --python_harness_out=. --mypy_out=. svc.proto

Where $HARNESS_PROTO is where to find Harness proto-files.

Runtime

Use harness command to run your service:

$ harness svc:main config.yaml