Merge pull request apache#16285 from [BEAM-13492][Playground] Update …

…backend README file

* [BEAM-13492][Playground]
update README.md file

* Update README.md file for the backend

* [BEAM-13492][Playground]
add info about NUM_PARALLEL_JOBS environment variable.

* [BEAM-13492][Playground]
Add description about LAUNCH_SITE environment variable

* [BEAM-13492][Playground]
update README file

Co-authored-by: Artur Khanin <[email protected]>

playground/backend/README.md

@@ -21,47 +21,82 @@
 
 ## About
 
-Apache Beam is an open-source, unified model for defining parallel processing pipelines for batch and streaming data.
-It provides a portable API layer for building sophisticated data-parallel processing pipelines that may be executed
-across a diversity of execution engines, or runners.
-
-Beam Playground helps facilitate trying out and adopting Apache Beam by providing a very quick way for prospective Beam
-users to see and run examples of Apache Beam pipelines in a web interface that requires no setup.
+Beam Playground helps facilitate trying out and adopting [Apache Beam](https://beam.apache.org/) by providing a very
+quick way for prospective Beam users to see and run examples of Apache Beam pipelines in a web interface that requires
+no setup.
 
 ## Getting Started
 
 See [playground/README.md](../README.md) for details on requirements and setup.
 
 This section describes what is needed to run the backend application.
-- Generating models from proto file
-- Go commands to run/test application locally
+
+- Go commands to run/test the backend locally
+- Set up environment variables to run the backend locally
+- Running the backend via Docker
 
 ### Go commands to run/test application locally
 
-The following command is used to build and serve the backend locally:
+Go to the backend directory:
 
+```shell
+$ cd backend
 ```
-go run ./cmd/server/server.go
+
+The following command is used to build and serve the backend locally:
+
+```shell
+$ go run ./cmd/server/server.go
 ```
 
 Run the following command to generate a release build file:
 
-```
-go build ./cmd/server/server.go
+```shell
+$ go build ./cmd/server/server.go
 ```
 
 Playground tests may be run using this command:
 
-```
-go test ... -v
+```shell
+$ go test ... -v
 ```
 
 The full list of commands can be found [here](https://pkg.go.dev/cmd/go).
 
-## Running the server app
-
-To run the server using Docker images there are `Docker` files in the `containers` folder for Go and Java languages.
-Each of them processes the corresponding SDK, so the backend with Go SDK will work with Go examples/katas/tests only.
+### Set up environment variables to run the backend locally
+
+These environment variables should be set to run the backend locally:
+
+- `BEAM_SDK` - is the SDK which backend could process (`SDK_GO` / `SDK_JAVA` / `SDK_PYTHON` / `SDK_SCIO`)
+- `APP_WORK_DIR` - is the directory where all folders will be placed to process each code processing request
+- `PREPARED_MOD_DIR` - is the directory where prepared go.mod and go.sum files are placed. It is used only for Go SDK
+
+There are also environment variables which are needed for the deployment of Apache Beam Playground. These variables have
+default value and there is no need to set them up to launch locally:
+
+- `SERVER_IP` - is the IP address of the backend server (default value = `localhost`)
+- `SERVER_PORT` - is the PORT of the backend server (default value = `8080`)
+- `CACHE_TYPE` - is a type of the cache service which is used for the backend server. If it is set as a `remote`, then
+  the backend server will use Redis to keep all cache values (default value = `local`)
+- `CACHE_ADDRESS` - is an address of the Redis server. It is used only when `CACHE_TYPE=remote` (default value
+  = `localhost:6379`)
+- `BEAM_PATH` - it is the place where all required for the Java SDK libs are placed
+  (default value = `/opt/apache/beam/jars/*`)
+- `KEY_EXPIRATION_TIME` - is the expiration time of the keys in the cache (default value = `15 min`)
+- `PIPELINE_EXPIRATION_TIMEOUT` - is the expiration time of the code processing (default value = `15 min`)
+- `PROTOCOL_TYPE` - is the type of the backend server protocol. It could be `TCP` or `HTTP` (default value = `HTTP`)
+- `NUM_PARALLEL_JOBS` - is the max number of the code processing requests which could be processed on the backend server
+  at the same time (default value = `20`). This value is used to check the readiness of the backend server. If the
+  server reaches the max number of concurrent code-processing requests, then the load-balancer will route all other
+  incoming requests to other instances while the instance will not ready.
+- `LAUNCH_SITE` - is the value to configure log (default value = local). If developers want to use log service on the
+  App Engine then need to change this value to `app_engine`.
+
+### Running the server app via Docker
+
+To run the server using Docker images there are `Docker` files in the `containers` folder for Java, Python and Go
+languages. Each of them processes the corresponding SDK, so the backend with Go SDK will work with Go
+examples/katas/tests only.
 
 One more way to run the server is to run it locally how it is described above.
 
@@ -70,11 +105,3 @@ One more way to run the server is to run it locally how it is described above.
 To call the server from another client – models and client code should be generated using the
 `playground/api/v1/api.proto` file. More information about generating models and client's code using `.proto`
 files for each language can be found [here](https://grpc.io/docs/languages/).
-
-## Deployment
-
-TBD
-
-## How to Contribute
-
-TBD