Create DEVELOPER_GUIDE.md

DEVELOPER_GUIDE.md

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+# OpenSearch SDK Developer Guide
+- [Introduction](#introduction)
+- [Getting Started](#getting-started)
+	- [Git Clone IndependentPlugin Repo](#git-clone-independentplugin-repo)
+	- [Run Independent Plugin](#run-independent-plugin)
+	- [Run Tests](#run-tests)
+	- [Send Message using Telnet](#send-message-using-telnet)
+
+## Introduction
+Opensearch plugins have allowed the extension and ehancements of various core features however, current plugin architecture carries the risk of fatally impacting clusters should they fail. In order to ensure that plugins may run safely without impacting the system, our goal is to effectively isolate plugin interactions with OpenSearch by modularizing the [extension points](https://opensearch.org/blog/technical-post/2021/12/plugins-intro/) to which they hook onto. 
+
+Read more about extensibility [here](https://github.com/opensearch-project/OpenSearch/issues/1422)
+
+## Getting Started
+
+### Git Clone IndependentPlugin Repo
+Fork [IndependentPlugin](https://github.com/owaiskazi19/IndependentPlugin) and clone locally, e.g. `git clone https://github.com/[your username]/IndependentPlugin.git`.
+
+### Run Independent Plugin
+
+Run main script using `gradlew run`.
+
+```
+./gradlew run
+```
+
+This will execute the main script set within the root `build.gradle` file :
+
+```
+mainClassName = 'transportservice.RunPlugin'
+```
+Bound addresses will then be logged to the terminal :
+
+```bash
+[main] INFO  transportservice.TransportService - publish_address {127.0.0.1:3333}, bound_addresses {[::1]:3333}, {127.0.0.1:3333}
+[main] INFO  transportservice.TransportService - profile [test]: publish_address {127.0.0.1:5555}, bound_addresses {[::1]:5555}, {127.0.0.1:5555}
+```
+
+### Run Tests
+
+Run tests :
+```
+./gradlew clean build test
+```
+
+### Send Message using Telnet
+
+To send a message, first run the IndependentPlugin :
+
+```
+./gradlew run
+```
+In another terminal, run : 
+```
+telnet localhost 5555
+```
+Once Telnet Client is connected, the terminal will print out :
+```
+Trying 127.0.0.1...
+Connected to localhost.
+Escape character is '^]'.
+```
+The original terminal used to run the independent plugin will log the connection request :
+```
+[opensearch[NettySizeHeaderFrameDecoderTests][transport_worker][T#5]] TRACE transportservice.TcpTransport - Tcp transport channel accepted: Netty4TcpChannel{localAddress=/127.0.0.1:5555, remoteAddress=/127.0.0.1:57302}
+[opensearch[NettySizeHeaderFrameDecoderTests][transport_worker][T#5]] TRACE transportservice.netty4.OpenSearchLoggingHandler - [id: 0x8c1cc239, L:/127.0.0.1:5555 - R:/127.0.0.1:57302] REGISTERED
+[opensearch[NettySizeHeaderFrameDecoderTests][transport_worker][T#5]] TRACE transportservice.netty4.OpenSearchLoggingHandler - [id: 0x8c1cc239, L:/127.0.0.1:5555 - R:/127.0.0.1:57302] ACTIVE
+```
+Messages sent through the Telnet Client must begin with an 'ES', for example : 
+```
+ES1234SHDDF
+```
+The original terminal will then log the recieved message if format is validated :
+```
+         +-------------------------------------------------+
+         |  0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f |
++--------+-------------------------------------------------+----------------+
+|00000000| 45 53 31 32 33 34 53 48 44 44 46 0d 0a          |ES1234SHDDF..   |
++--------+-------------------------------------------------+----------------+
+MESSAGE RECEIVED:ES1234SHDDF
+
+REFERENCE LENGTH 13 ES1234SHDDF
+```
+
+