Ambassador is one of the Separation of Concern (SoC) implementation. In cloud platform, functional features (e.g. Business logic) and non-functional features (e.g. retry / queue / etc) are implement in multi-containers: main application and proxy.

Problem

Most microservices might require non-functional feature such as queueing and retry, but it will become complicate when number of microservices increase and changes.

Also, some of the application might cannot be change (e.g. legacy / source code missing / external application) so it cannot be add these non-functional feature in application itself.

Solution

Setup the Ambassador proxy to handle non-functional feature, main applications are require to get request / send response via Ambassador.

Use Case

Legacy application which cannot implement such non-functional feature;
Share non-functional feature in different application;

Implementation

There are some tools which can role as ambassador, some representives as below:

Envoy
Nginx

Consideration

Compare with Sidecar

Sidecar is for each microservices and Ambassador can for group of microservices. As result, it is complex on manage each sidecar. For example, using Istio to centralize mange Envoy for each sidecar.

Compare with Adaptor

Adaptor is transform request / response between microservices and caller. Majorly it is for legacy application but it do not provide additional non-functional feature.