Ambassador design pattern

Understanding the Ambassador Pattern in Cloud and Distributed Computing

Context and Problem

In the realm of cloud computing and distributed systems, applications often need to communicate with external services. This communication can involve complex tasks such as monitoring, logging, routing, security (like TLS), and resiliency patterns. These tasks can be challenging to implement directly within the application, especially when dealing with legacy systems or applications that are difficult to modify¹.

Issues and Considerations

1. Latency Overhead: Introducing an ambassador service adds some latency to the network calls. This overhead needs to be balanced against the benefits of offloading tasks from the main application¹.
2. Complexity: Managing and maintaining an additional layer of services can increase the complexity of the system.
3. Security: Ensuring that the ambassador service itself is secure and does not become a point of vulnerability.
4. Resource Management: The ambassador service consumes additional resources, which need to be managed effectively.

Solution

The Ambassador Pattern involves creating a helper service that acts as an out-of-process proxy, co-located with the client application. This proxy handles network requests on behalf of the client, offloading tasks such as monitoring, logging, routing, and security¹. The ambassador service can be deployed as a sidecar container in Kubernetes, a daemon, or a Windows service, depending on the architecture².

When to Use the Pattern

• Legacy Applications: When dealing with legacy applications that are difficult to modify but require enhanced networking capabilities.
• Microservices Architecture: In microservices architectures, where services need to communicate with each other and external services efficiently.
• Security and Compliance: When there is a need to enforce security policies, such as rate limiting and IP filtering, at a central point.
• Performance Monitoring: To standardize and extend instrumentation for performance metrics like latency and resource usage¹.

Real-World Example

Imagine a busy hotel where guests frequently request restaurant reservations, event tickets, or transportation arrangements. Instead of each guest individually contacting these services, the hotel provides a concierge. The concierge handles these tasks on behalf of the guests, ensuring that reservations are made smoothly, tickets are booked on time, and transportation is scheduled efficiently. In this analogy, the guests are the client services, the external providers (restaurants, ticket vendors, transportation) are the remote services, and the concierge represents the ambassador service³.

Sample Pseudo Code in Python

Here’s a simple pseudo code example to illustrate the Ambassador Pattern in Python:

class RemoteService:
    def do_remote_function(self, value):
        # Simulate a remote service call
        import time
        import random
        wait_time = random.uniform(0, 1)
        time.sleep(wait_time)
        if wait_time >= 0.2:
            return value * 10
        else:
            return -1

class AmbassadorService:
    def __init__(self):
        self.remote_service = RemoteService()

    def call_remote_service(self, value):
        try:
            result = self.remote_service.do_remote_function(value)
            if result == -1:
                print("Error: Remote service call failed.")
            else:
                print(f"Success: Remote service returned {result}.")
        except Exception as e:
            print(f"Exception occurred: {e}")

# Client code
if __name__ == "__main__":
    ambassador = AmbassadorService()
    ambassador.call_remote_service(5)

In this pseudo code, the AmbassadorService acts as a proxy for the RemoteService, handling the network call and any potential errors or retries.

By using the Ambassador Pattern, you can simplify the client application, offload complex tasks, and centralize the management of network-related functionalities, making your system more resilient and easier to maintain.