Add many new chapters of v6 document. Make the document more clear.

# Observability Analysis Platform Cluster Management

OAP(Observability Analysis Platform) backend is a distributed system, services need to find each 

other. i.e. a web service needs to find query service, level 1 aggregate service need to find 

level 2 aggregate service. Cluster management is just a client-side implementation. It must work 

together with a distributed coordination service, (i.e. Zookeeper, Consul, Kubernetes.) unless 

running in standalone mode.

## Architecture overview

<img src="https://skywalkingtest.github.io/page-resources/6.0.0-alpha/cluster_management.png"/>

### Cluster Management Plugins

By default, OAP backend provides two implementations for cluster management, which are standalone 

and zookeeper. When the scale of services, which are under monitoring, is small, you can choose 

the standalone mode. Otherwise, you must choose the cluster mode by zookeeper plugin, or you can 

implement a new service discovery plugin for your own scene.

### Cluster Management Interface

There are two interfaces defined beforehand in the OAP server core, which are Module register and 

module query, all the cluster management plugins must implement those two interfaces.

* Module Register: When any modules need to provide services for others, the module must do register 

through this interface.

* Module Query: When any module needs to find other services, it can use this interface to retrieve 

the service instance list in the certain order.

### Process Flow Between Client and Cluster Management

The client has two ways to connect the backend, one, use the direct link by a set of backend instance 

endpoint list, or you can use naming service, which considers your given list is just the seed nodes 

of the whole cluster. The following graph is showing you how naming service works. You need to check 

the probe documents to know which way(s) is(are) supported.

```

         Client lib                         Collector1             Collector2              Collector3

 (Set collector.servers=Collector2)              (Collector 1,2,3 constitute the cluster)

             |

             +-----------> naming service ---------------------------->|

                                                                       |

             |<------- receive gRPC IP:Port(s) of Collector 1,2,3---<--|

             |

             |Select a random gRPC service

             |For example collector 3

             |

             |------------------------->Uplink gRPC service----------------------------------->|

```

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1. MQ

Component IDs are defined and reserved by SkyWalking project.

For component name/ID extension, please follow [component library setting document](../../backend/Component-library-settings.md).

For component name/ID extension, please follow [component library setting document](../setup/backend/Component-library-settings.md).

## Develop a plugin

### Abstract

SkyWalking project supports many ways to extends existing features. If you are interesting in these ways,

read the following guides.

- [Java agent plugin development guide](../setup/service-agent/java-agent/Plugin-Development-Guide.md).

- [Java agent plugin development guide](Java-Plugin-Development-Guide.md).

This guide helps you to develop SkyWalking agent plugin to support more frameworks. Both open source plugin

and private plugin developer should read this. 

- [Storage extension development guide](storage-extention.md)

# Extend storage

SkyWalking has already provided H2 and ElasticSearch as storage solutions. In this document, you could 

learn how to implement a new storage easily. 

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[gRPC service define](https://github.com/apache/incubator-skywalking-data-collect-protocol/blob/v2.0/TraceSegmentService.proto)

- UniqueId represents segmentId and globalTraceId. It have 3 parts(Longs), 1) applicationInstanceId, 2) ThreadId, 3) Timestamp + 10000 + seq(seq is in [0, 100000) )

- Span data please refs to [Plugin Development Guide](../setup/service-agent/java-agent/Plugin-Development-Guide.md)

- Span data please refs to [Plugin Development Guide](../guides/Java-Plugin-Development-Guide.md)

- Id and name both exist, please use id if possible.

  - operationNameId/operationName 

  - networkAddress/networkAddressId