Below is the template that I use to document software architecture. This is based on the experience I gained doing architecture and design work over the years.
If you prefer Google Docs then you can create a copy of the template. If you prefer Markdown version then you get it from the Github repository.
<X> System Software Architecture Document
Give a high level overview of the requirements
User Roles and their Responsibilities
Document main user personas that will use the application
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Constraints
Any constraint that you need to keep in mind for your design
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Assumptions
Any assumptions that you have made while coming up with the design
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Business Process
Important business processes that you have considered while creating the design
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Non-functional Requirements
Most important quality attributes based on the business goals
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Back of the envelope calculations
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Functional Layer Diagram
List of functional components that will make the system. Which actor will work on which functional part.
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Architecture Style
Which architecture style you are proposing to build the application
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Solution Context Diagram
This is a high level context diagram that shows system as a black box, its main user personas, and external integrations
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Solution Diagram
In this section you create the solution diagram showing different components that make the system. This is a zoom-in view of the system you are building.
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Technology Stack
The list of technologies that you will use to build the system and the reason you choose them
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Important Architecture Decisions
Important architecture decision that you want development team to follow
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Shekhar Gulati 