The Sidecar Pattern is perhaps the most cross-cutting concern of all. When we talk about microservice architectures, we also talk about the possibility of having a polyglot system. In other words, we’ve got the freedom to choose the language, technology, or framework based on the task we need to solve.

This leads, potentially, to having microservices that speak different languages making it more difficult to maintain cross-cutting libraries of concern.

One solution to this is the sidecar pattern. The logic for cross-cutting concerns is placed in its own processor container (known as a sidecar container) and then linked to the primary application. Similar to how a motorcycle sidecar is connected to a motorcycle, the sidecar application is connected to the primary application and runs alongside it: