Ambassador

About 4 min

Intent

Provide a helper service instance on a client and offload common functionality away from a shared resource.

Also known as

Sidecar

Explanation

Real world example

A remote service has many clients accessing a function it provides. The service is a legacy application and is impossible to update. Large numbers of requests from users are causing connectivity issues. New rules for request frequency should be implemented along with latency checks and client-side logging.

In plain words

With the Ambassador pattern, we can implement less-frequent polling from clients along with latency checks and logging.

Microsoft documentation states

An ambassador service can be thought of as an out-of-process proxy which is co-located with the client. This pattern can be useful for offloading common client connectivity tasks such as monitoring, logging, routing, security (such as TLS), and resiliency patterns in a language agnostic way. It is often used with legacy applications, or other applications that are difficult to modify, in order to extend their networking capabilities. It can also enable a specialized team to implement those features.

Programmatic Example

With the above introduction in mind we will imitate the functionality in this example. We have an interface implemented by the remote service as well as the ambassador service:

interface RemoteServiceInterface {
    long doRemoteFunction(int value) throws Exception;
}

A remote services represented as a singleton.


@Slf4j
public class RemoteService implements RemoteServiceInterface {
    private static RemoteService service = null;

    static synchronized RemoteService getRemoteService() {
        if (service == null) {
            service = new RemoteService();
        }
        return service;
    }

    private RemoteService() {
    }

    @Override
    public long doRemoteFunction(int value) {
        long waitTime = (long) Math.floor(Math.random() * 1000);

        try {
            sleep(waitTime);
        } catch (InterruptedException e) {
            LOGGER.error("Thread sleep interrupted", e);
        }

        return waitTime >= 200 ? value * 10 : -1;
    }
}

A service ambassador adding additional features such as logging, latency checks


@Slf4j
public class ServiceAmbassador implements RemoteServiceInterface {
    private static final int RETRIES = 3;
    private static final int DELAY_MS = 3000;

    ServiceAmbassador() {
    }

    @Override
    public long doRemoteFunction(int value) {
        return safeCall(value);
    }

    private long checkLatency(int value) {
        var startTime = System.currentTimeMillis();
        var result = RemoteService.getRemoteService().doRemoteFunction(value);
        var timeTaken = System.currentTimeMillis() - startTime;

        LOGGER.info("Time taken (ms): " + timeTaken);
        return result;
    }

    private long safeCall(int value) {
        var retries = 0;
        var result = (long) FAILURE;

        for (int i = 0; i < RETRIES; i++) {
            if (retries >= RETRIES) {
                return FAILURE;
            }

            if ((result = checkLatency(value)) == FAILURE) {
                LOGGER.info("Failed to reach remote: (" + (i + 1) + ")");
                retries++;
                try {
                    sleep(DELAY_MS);
                } catch (InterruptedException e) {
                    LOGGER.error("Thread sleep state interrupted", e);
                }
            } else {
                break;
            }
        }
        return result;
    }
}

A client has a local service ambassador used to interact with the remote service:


@Slf4j
public class Client {
    private final ServiceAmbassador serviceAmbassador = new ServiceAmbassador();

    long useService(int value) {
        var result = serviceAmbassador.doRemoteFunction(value);
        LOGGER.info("Service result: " + result);
        return result;
    }
}

Here are two clients using the service.

public class App {
    public static void main(String[] args) {
        var host1 = new Client();
        var host2 = new Client();
        host1.useService(12);
        host2.useService(73);
    }
}

Here's the output for running the example:

Time taken(ms):111
        Service result:120
        Time taken(ms):931
        Failed to reach remote:(1)
        Time taken(ms):665
        Failed to reach remote:(2)
        Time taken(ms):538
        Failed to reach remote:(3)
        Service result:-1

Class diagram

Applicability

Cloud Native and Microservices Architectures: Especially useful in distributed systems where it's crucial to monitor, log, and secure inter-service communication.
Legacy System Integration: Facilitates communication with newer services by handling necessary but non-core functionalities.
Performance Enhancement: Can be used to cache results or compress data to improve communication efficiency.

Typical use cases include:

Control access to another object
Implement logging
Implement circuit breaking
Offload remote service tasks
Facilitate network connection

Consequences

Benefits:

Separation of Concerns: Offloads cross-cutting concerns from the service logic, leading to cleaner, more maintainable code.
Reusable Infrastructure Logic: The ambassador pattern allows the same logic (e.g., logging, monitoring) to be reused across multiple services.
Improved Security: Centralizes security features like SSL termination or authentication, reducing the risk of misconfiguration.
Flexibility: Makes it easier to update or replace infrastructure concerns without modifying the service code.

Trade-offs:

Increased Complexity: Adds another layer to the architecture, which can complicate the system design and debugging.
Potential Performance Overhead: The additional network hop can introduce latency and overhead, particularly if not optimized.
Deployment Overhead: Requires additional resources and management for deploying and scaling ambassador services.

Known uses

Service Mesh Implementations: In a service mesh architecture, like Istio or Linkerd, the Ambassador pattern is often employed as a sidecar proxy that handles inter-service communications. This includes tasks such as service discovery, routing, load balancing, telemetry (metrics and tracing), and security (authentication and authorization).
API Gateways: API gateways can use the Ambassador pattern to encapsulate common functionalities like rate limiting, caching, request shaping, and authentication. This allows backend services to focus on their core business logic without being burdened by these cross-cutting concerns.
Logging and Monitoring: An Ambassador can aggregate logs and metrics from various services and forward them to centralized monitoring tools like Prometheus or ELK Stack (Elasticsearch, Logstash, Kibana). This simplifies the logging and monitoring setup for each service and provides a unified view of the system's health.
Security: Security-related functionalities such as SSL/TLS termination, identity verification, and encryption can be managed by an Ambassador. This ensures consistent security practices across services and reduces the likelihood of security breaches due to misconfigurations.
Resilience: The Ambassador can implement resilience patterns like circuit breakers, retries, and timeouts. For instance, Netflix's Hystrix library can be used within an Ambassador to prevent cascading failures in a microservices ecosystem.
Database Proxy: Ambassadors can act as proxies for database connections, providing functionalities like connection pooling, read/write splitting for replicas, and query caching. This offloads significant complexity from the application services.
Legacy System Integration: In scenarios where modern microservices need to communicate with legacy systems, an Ambassador can serve as an intermediary that translates protocols, handles necessary transformations, and implements modern security practices, easing the integration process.
Network Optimization: For services deployed across different geographical locations or cloud regions, Ambassadors can optimize communication by compressing data, batching requests, or even implementing smart routing to reduce latency and costs.
Kubernetes-native API gateway for microservicesopen in new window

Proxyopen in new window: Shares similarities with the proxy pattern, but the ambassador pattern specifically focuses on offloading ancillary functionalities.
Sidecar: A similar pattern used in the context of containerized applications, where a sidecar container provides additional functionality to the main application container.
Decoratoropen in new window: The decorator pattern is used to add functionality to an object dynamically, while the ambassador pattern is used to offload functionality to a separate object.