What is the sidecar pattern in microservices?

The sidecar pattern attaches a helper container to a main service container, running alongside it in the same pod (in Kubernetes) and sharing the same network namespace and lifecycle. The sidecar handles cross-cutting concerns that are not part of the core business logic — such as logging, monitoring, configuration management, service mesh proxy (Envoy in Istio), or TLS termination — so the main service can focus on its domain. For example, a log aggregation sidecar collects logs from the main container and ships them to Elasticsearch. This avoids duplicating infrastructure code in every service and enables you to add or upgrade capabilities without modifying service code, promoting the single responsibility principle at the infrastructure level.