docs: plugin service (#261)

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docs/development-guide/PluginService.md

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+## Plugin Service
+
+![](../images/plugin_service.png)
+
+The plugin service retrieves and updates the current connection and its relevant host information.
+
+It also keeps track of the host list provider in use, and notifies it to update its host list.
+
+It is expected that the plugins do not establish a connection themselves, but rather call `PluginService.connect()`
+to establish connections.
+
+## Host List Providers
+
+The plugin service uses the host list provider to retrieve the most recent host information or topology information about the database.
+
+The AWS Advanced NodeJS Wrapper has two host list providers, the `ConnectionStringHostListProvider` and the `RdsHostListProvider`.
+
+The `ConnectionStringHostListProvider` is the default provider, it parses the host parameter for cluster information and stores the information.
+The provider supports having multiple hosts in the host parameter:
+
+| Host parameter                  | Support            |
+| ------------------------------- | ------------------ |
+| `hostname1,hostname2`           | :x:                |
+| `hostname1,hostname2:8090`      | :x:                |
+| `hostname1:8090,hostname2`      | :white_check_mark: |
+| `hostname1:8090,hostname2:8090` | :white_check_mark: |
+
+The `RdsHostListProvider` provides information about the Aurora cluster.
+It uses the current connection to track the available hosts and their roles in the cluster.
+
+The `ConnectionStringHostListProvider` is a static host list provider, whereas the `RdsHostListProvider` is a dynamic host list provider.
+A static host list provider will fetch the host list during initialization and does not update the host list afterward,
+whereas a dynamic host list provider will update the host list information based on database status.
+When implementing a custom host list provider, implement either the `StaticHostListProvider` or the `DynamicHostListProvider` marker interfaces to specify its provider type.