How to document a system another developer can actually understand
Good documentation explains boundaries, decisions and operational reality. It does not attempt to narrate every line of code.
Document the map first
Start with what the system does, who uses it and the major components. Include the public domains, applications, databases, queues, storage, third-party services and authentication flow. A new developer needs the map before the details.
A simple architecture diagram plus a written description of data flow usually provides more value than dozens of screenshots.
Write down the non-obvious decisions
Explain why a particular integration, database relationship or deployment pattern exists. Code shows what the system does today; decision notes explain why changing it casually may break an assumption.
Record known trade-offs, temporary workarounds and areas deliberately deferred. Hidden compromises become expensive when future developers rediscover them through failure.
Include operational documentation
Document deployment steps, environment requirements, scheduled tasks, backup/restore procedures, log locations and how to verify key integrations. A system is not maintainable if it can only be deployed by the person who built it.
Keep secrets out of documentation. Document the variable name and purpose, not the live credential.
Maintain docs with the system
Documentation that is never updated becomes actively misleading. Put ownership on important documents and update them as part of the definition of done for changes that alter architecture, operations or user workflows.
Keep the useful part, not just the reading.
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