Initial revision for system design documentation. (#107)

SYSTEM_DESIGN.md

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+# System Design
+
+## Table of Contents
+- [Components](#components)
+- [Process Flow](#process-flow)
+- [Integration](#integration)
+
+## Components
+
+The following diagram illustrates the major components in the VinylDNS ecosystem and the external systems they interact with.
+
+![VinylDNS Architecture Diagram](img/VinylDNS_overview.png)
+
+* API - RESTful endpoints to allow interaction with VinylDNS
+
+* Database - stores information that the VinylDNS application needs
+
+* DNS servers - communicates DNS changes and resolves DNS records
+
+* Message queue - temporarily stores DNS requests for processing 
+
+* LDAP Service - application protocol used to authenticate user access to the VinylDNS portal
+
+* Portal - graphical user interface to interact with the VinylDNS API
+
+* Tooling - external libraries and utilities used to interact with the VinylDNS API
+
+## Process Flow
+
+1. LDAP service authenticates user credentials and grants access to the portal.
+1. If the user is accessing the portal for the first time, VinylDNS credentials are generated and stored.
+1. User navigates portal or uses integration tooling to generate a signed API request.
+1. When the API receives a request, it loads the credentials for the calling user from the database and validates the request signature to ensure that the request was not modified in transit.
+1. The request is then validated to ensure that:
+    - the request data is correct 
+    - the request passes all validation checks
+    - the user has access to make the change
+1. Assuming the request is in good order, the request is put on a message queue for handling.
+1. One of the VinylDNS API server instances pulls the message from the queue for processing. For record changes, a DDNS message is issued to the DNS backend server.
+1. When the message completes processing, it is removed from the message queue. The changes are applied to the VinylDNS database along with an audit record for the request.
+
+## Integration
+
+Integrating with VinylDNS is simple since each API endpoint is effectively a distinct DNS operation (eg. create record, update record, delete record, etc.). The only requirement for sending a request is generating the correct AWS SIG4 signature without content length and providing the corresponding HTTP headers so that VinylDNS can verify it. See [API Authentication](https://www.vinyldns.io/api/auth-mechanism.html) for more details.
+
+The current tooling available to perform VinylDNS API requests include:
+
+* [go-vinyldns](https://github.com/vinyldns/go-vinyldns) - Golang client package
+
+* [vinyldns-cli](https://github.com/vinyldns/vinyldns-cli) - command line utility written in Golang
+
+* [vinyldns-java](https://github.com/vinyldns/vinyldns-java) - Java client
+
+* [vinyldns-python](https://github.com/vinyldns/vinyldns-python) - Python client library
+
+* [vinyldns-ruby](https://github.com/vinyldns/vinyldns-ruby) - Ruby gem