Sidecar

A container that runs alongside the application in the same pod, providing a cross-cutting capability out of process. It is the mechanism behind service meshes, log shippers and secret-fetching agents, and its appeal is that a platform team can ship a capability to every service without touching anyone's code. The costs are equally real: it doubles the container count, it consumes requests and limits of its own, and startup and shutdown ordering between the two becomes your problem.