Monolith

A monolith is an application built and deployed as a single unit. All the code lives in one codebase, runs in one process, and typically shares one database.

It has been unfashionable for a decade and remains the correct default for most projects. Calls between parts of the system are function calls that cannot fail halfway, a change spanning several areas is one commit and one deployment, and the whole thing runs on a developer laptop without orchestration.

A monolith becomes a problem when it becomes a big ball of mud — when the internal boundaries have eroded so far that no change is local any more. The remedy is boundaries inside the codebase, which is the same discipline microservices enforce with a network, at a fraction of the operational cost. Keep those boundaries honest and splitting later remains possible.