docs/howto/invoke-and-discover-services

Invoke remote services

In many environments with multiple services that need to communicate with each other, developers often ask themselves the following questions:

• How do I discover and invoke different services?
• How do I handle retries and transient errors?
• How do I use distributed tracing correctly to see a call graph?

Dapr allows developers to overcome these challenges by providing an endpoint that acts as a combination of a reverse proxy with built-in service discovery, while leveraging built-in distributed tracing and error handling.

For more info on service invocation, read the conceptional documentation.

1. Choose an ID for your service

Dapr allows you to assign a global, unique ID for your app.

This ID encapsulates the state for your application, regardless of the number of instances it may have.

Setup an ID using the Dapr CLI

In Standalone mode, set the --app-id flag:

dapr run --app-id cart --app-port 5000 python app.py

Setup an ID using Kubernetes

In Kubernetes, set the dapr.io/id annotation on your pod:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: python-app
  labels:
    app: python-app
spec:
  replicas: 1
  selector:
    matchLabels:
      app: python-app
  template:
    metadata:
      labels:
        app: python-app
      annotations:
        dapr.io/enabled: "true"
        dapr.io/id: "cart"
        dapr.io/port: "5000"
...

Invoke a service in code

Dapr uses a sidecar, decentralized architecture. To invoke an applications using Dapr, you can use the invoke endpoint on any Dapr instance in your cluster/environment.

The sidecar programming model encourages each applications to talk to its own instance of Dapr. The Dapr instances discover and communicate with one another.

Note: The following is a Python example of a cart app. It can be written in any programming language

from flask import Flask
app = Flask(__name__)

@app.route('/add', methods=['POST'])
def add():
    return "Added!"

if __name__ == '__main__':
    app.run()

This Python app exposes an add() method via the /add endpoint.

Invoke with curl

curl http://localhost:3500/v1.0/invoke/cart/method/add -X POST

Since the add endpoint is a 'POST' method, we used -X POST in the curl command.

To invoke a 'GET' endpoint:

curl http://localhost:3500/v1.0/invoke/cart/method/add

To invoke a 'DELETE' endpoint:

curl http://localhost:3500/v1.0/invoke/cart/method/add -X DELETE

Dapr puts any payload return by their called service in the HTTP response's body.

Overview

The example above showed you how to directly invoke a different service running in our environment, locally or in Kubernetes. Dapr outputs metrics and tracing information allowing you to visualize a call graph between services, log errors and optionally log the payload body.

For more information on tracing, visit this link.