[doc] add basic arch thing TODO: link correctly to diagrams

content/npm/enterprise/arch.md

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+# Nodejitsu private npm
+
+* [Architecture](#architecture)
+* [Hosting Scenario](#hosting-scenario)
+* [Change handling](#change-handling)
+
+## Architecture
+
+The architecture is composed of four core components:
+
+* `npmjitsu-proxy`: Core HTTP(S) proxy that serves all of the requests from the `npm` CLI.
+* `npmjitsu-api`: Server which provides the RESTful API used
+* `npmjitsu-ui`: [`bigpipe`][bigpipe] powered front-end UI server.
+* `npmjitsu-updater`: Process that manages the state of the private npm installation and allows for triggering of notifications etc.
+* `npm-replicator` (handled by [`omnipotent`][omnipotent] and [`overwatch`][overwatch]): ensures replication for public npm replicas and for `_design` documents in customers' private npm databases.
+
+![High-level architecture](https://i.cloudup.com/imQKInYaUA-3000x3000.png)
+_Figure 1. High-level architecture_
+
+## Hosting scenario
+
+Every customer has an `/{{customer_name}}_admin` database ("admin database") and a `/{{customer_name}}_npm`.
+
+These two databases are managed by the `npm-updater` process and update the
+index database (`npm-policies`) based on their state change. The `npm-updater` ensures that `policy` documents in the `npm-polciies` database are up to date. Think about the `npm-policies` as an on disk index of the state of the `{{customer_name}}_admin` database.
+
+![architecture](https://i.cloudup.com/8Jq3bZo87y-3000x3000.png)
+_Figure 1. Architecture Picture_
+
+## Change handling
+
+* `_changes` from `customer_admin`
+  1. On changes where `{ "delete": true }` from `customer_admin` the private npm registry agent
+  removes the documents from the policy in-memory.
+  2. On all other changes from `customer_npm` the private npm registry agent attempts
+  to add the documents to the in-memory policy.
+* `_changes` from `customer_npm`
+  1. On changes where `{ "delete": true }` from `customer_npm` the private npm registry agent
+  removes the documents from the `customer_admin`
+  2. On all other changes from `customer_npm` the private npm registry agent attempts
+  to add the documents to the `customer_admin`.
+
+[overwatch]: https://github.com/nodejitsu/overwatch
+[omnipotent]: https://github.com/nodejitsu/omnipotent
+[bigpipe]: https://github.com/bigpipe/bigpipe