Add architecture design doc (#1366)

* Create Architecture.md

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* Update Architecture.md

* Update Architecture.md

* Update Architecture.md

* Update README.md

Add link in menu. Also related to #1360

* Update Architecture.md

* Update Architecture.md

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* Update Architecture.md

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docs/README.md

@@ -11,6 +11,7 @@
         * [Trace Spring beans](en/agent-optional-plugins/Spring-bean-plugins.md)
         * [Trace Oracle and Resin](en/agent-optional-plugins/Oracle-Resin-plugins.md)
         * [[**Incubating**] Filter traces through custom services](../apm-sniffer/optional-plugins/trace-ignore-plugin/README.md)
+  * [Architecture Design](en/Architecture.md)
   * Advanced Features
     * [Override settings through System.properties](en/Setting-override.md)
     * [Direct uplink and disable naming discovery](en/Direct-uplink.md)

docs/en/Architecture.md

+# Architecture Design
+## Background
+For APM, agent or SDKs are just the technical details about how to instrument the libs. 
+Manual or auto are nothing about the architecture, so in this document, we will consider them as a client lib only.
+
+<img src="https://skywalkingtest.github.io/page-resources/5.0/architecture.png"/>
+
+## Basic Principles
+The basic design principles of SkyWalking architecture are **easy to maintain, controllable and streaming**. 
+
+In order to achieve these goals, SkyWalking backend provides the following designs.
+1. Modulization design.
+1. Multiple connection ways for client side.
+1. Collector cluster discovery mechanism
+1. Streaming mode.
+1. Swtichable storage implementors.
+
+## Modulization
+SkyWalking collector is based on pure **modulization design**. End user can switch or assemble the collector features by their
+own requirements.
+
+### Module
+
+Module defines a collection of features, which could include techenical implementors(such as: gRPC/Jetty server managements), 
+trace analysis(such as: trace segment or zipkin span parser), or aggregation feature. Totally decided by the module definition
+and its implementors.
+
+Each module could define their services in Java Interface, 
+and every providers of the module must provide implementors for these services. 
+And the provider should define the dependency modules based its own implementation. 
+So it means, even two different implementors of the module, could depend different modules.
+
+Also the collector modulization core checks the startup sequences, if cycle dependency or dependency not found occurs, 
+collector should be terminated by core.
+
+The collector startup all modules, which are decleared in `application.yml`. 
+In this yaml file
+- Root level is the module name, such as `cluster`, `naming`
+- Secondary level is the implementor name of the module, such as `zookeeper` is the `cluster` module
+- Third level are attributes of the implementors. Such as `hostPort` and `sessionTimeout` are required attributes of `zookepper`.
+
+_The example part of the yaml definitation_
+```yml
+cluster:
+  zookeeper:
+    hostPort: localhost:2181
+    sessionTimeout: 100000
+naming:
+  jetty:
+    #OS real network IP(binding required), for agent to find collector cluster
+    host: localhost
+    port: 10800
+    contextPath: /
+```
+
+## Multiple connection ways
+First of all, the collector provides two types of connections, also two protocols(HTTP and gRPC). These two are
+1. Naming service in HTTP, which returns the all available collectors in the backend cluster.
+1. Uplink service in gRPC(primary in SkyWalking native agents) and HTTP, which uplinks traces and metrics to collector.
+Each client will only send monitoring data(traces and metrics) to a single collector. Attempt to connect other if the connected one offline.
+
+Such as in SkyWalking Java agent
+1. `collector.servers` means the naming service, which maps to `naming/jetty/ip:port` of collector, in HTTP. 
+1. `collector.direct_servers` means setting Uplink service directly, and using gRPC to send monitoring data.
+
+
+_Example of the process flow between client lib and collector cluster_
+```
+         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----------------------------------->|
+```
+
+
+## Collector Cluster Discovery
+When collectors are running in cluster mode, collector must discovery each other in some way. In default, SkyWalking uses
+Zookeeper to coordinate and as register center for instance discovery.
+
+Through the above section([Multiple connection ways](#multiple-connection-ways)), client lib will not use the Zookeeper to find cluster. And we suggest the client shouldn't do it in that way. Because the cluster discovery mechanism is switchable, 
+provided by modulization core. Relying on that breaks the switchable capability.
+
+We hope the community provides more implementor to do cluster discovery, such as Eureka, Consul, Kubernate.
+
+
+## Streaming Mode
+Streaming mode likes a lightweight storm/spark implementation, which allows using APIs to build streaming process graph(DAG),
+and the input/output data contracts of each node.
+
+New module can find and extend the existed process graph. 
+
+There are three cases in processing
+1. Synchronizing process. Tranditional method invocation.
+1. Asynchronizing process, a.k.a batch process based on Queue buffer.
+1. Remote process. Aggregate metrices across collector. In that way, selector is defined in node to decide 
+how to find the collector in cluster. (HashCode, Rolling, ForeverFirst are the three ways supported)
+
+By having these features, collector cluster runs like as a streaming net, to aggregate the metrics and don't rely on the
+storage implementor to support writing the same metric id concurrently.
+
+## Swtichable Storage Implementors
+Because streaming mode takes care of the concurrent, storage implementor responsibilities are provide high speed write,
+and group query.
+
+Right now, we supported ElasticSearch as primary implementor, H2 for preview, and MySQL Relational Database cluster managed
+by ShardingShpere project. 
+
+# Web UI
+Besides the principles in collector design, UI is another core component in SkyWalking. It is based on React, Antd and Zuul
+proxy to provide collector cluster discovery, query dispatch and visualziation.
+
+Web UI shares the similiar process flow as client's `1.naming then 2.uplink` mechanism in [Multiple connection ways](#multiple-connection-ways) section. The only difference is that, replace the uplink with GraphQL query protocol in HTTP binding at the host and port under `ui/jetty/` in yaml definition(default:`localhost:12800`).
+