# Architecture Design
Here we discuss the architecture design for the mangement services on the Master node.
## Philosophy and principles
The architecture will follow the following principles:
* Micro-service: Means we decouple various functions to individual micro services. No service will crash others whatever it does.
* Fault-resilience: Means the service should be tolerant for fault, such as database crash.
* Scalability: Try best to distribute the services, to mitigate centralized bottle neck.
## Functional Layers
Following the decouple design, there are 3 layers in Cello.
* Access layer: including those Web UI dashboards operated by users.
* Orchestration layer: received the request form Access layer, and make call to correct agents to operate the blockchain resources.
* Agent layer: real workers that interact with underly infrastructures like Docker, Swarm, K8s.
Each layer should maintain stable APIs for upper layers, to achieve pluggability without changing upper layer code.
### Agent layer APIs
* Host management: create, query/list, update, delete, fillup, clean, reset
* Cluster management: create, query/list, start/stop/restart, delete, reset
## Components
* `operator dashboard`: Provide the dashboard for the pool administrator, also the core engine to automatically maintain everything.
* `engine`: Provide the restful api for other system to apply/release/list chains.
* `watchdog`: Timely checking system status, keep everything healthy and clean.
## Implementation
The restful related implementation is based on [Flask](flask.pocoo.org), a Werkzeug based micro-framework for web service.
The reasons of choosing it include:
* Lightweight
* Good enough in performance
* Flexible for extending
* Stable in code
This work is licensed under a Creative Commons Attribution 4.0 International License.