Added README.md

Author: alex268

jdbc/ydb-token-app/README.md

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+## YDB Token Application
+## A simple example of Spring Boot 2 Application working with YDB Database
+
+### How to build
+
+Requirements
+* Java 17 or newer
+* Maven 3.0.0 or newer
+
+To build the application as a single executable jar file, run the command:
+```
+cd ydb-java-examples/jdbc/ydb-token-app
+mvn clean package spring-boot:repackage
+```
+After that, the compiled `ydb-token-app-1.1.0-SNAPSHOT.jar` can be found in the target folder.
+
+### What this application does
+
+This application allows you to create a test table called `app_token` in the YDB database, populate it with data, and
+launch a test workload for parallel reading and writing to this table. During the test, the following operations will be
+performed in parallel in several threads:
+* Read a random token from the database - 50% of operations
+* Read and update a random token in the database - 40% of operations
+* Read and update 100 random tokens in the database - 10% of operations
+
+The statistics collected during the test include the number of operations performed, RPS (requests per second), and
+average execution time for each type of operation. There is also support for exporting application metrics in Prometheus
+format.
+
+### How to launch
+
+The application is built as a single executable jar file and can be run with the command:
+```
+java -jar ydb-token-app-1.1.0-SNAPSHOT.jar <options> <commands>
+```
+Where `options` are application parameters (see the Application Parameters section), and `commands` are the sequence of
+commands the application will execute one after the other. Currently, the following commands are supported:
+* clean - clean the database, the `app_token` table will be deleted
+* init  - prepare the database, the empty `app_token` table will be created
+* load  - load test data, the `app_token` table will be filled with initial data
+* run   - start the test workload
+
+Commands can be used individually or sequenced, for example:
+
+Recreate the `app_token` table and initialize it with initial data:
+```
+java -jar ydb-token-app-1.1.0-SNAPSHOT.jar --app.connection=grpcs://my-ydb:2135/my-database clean init load
+```
+
+Start the test and then clean the database:
+```
+java -jar ydb-token-app-1.1.0-SNAPSHOT.jar --app.connection=grpcs://my-ydb:2135/my-database run clean
+```
+
+Recreate the `app_token` table, initialize it with data, and start the test:
+```
+java -jar ydb-token-app-1.1.0-SNAPSHOT.jar --app.connection=grpcs://my-ydb:2135/my-database clean init load run
+```
+
+### Application parameters
+
+Application parameters allow you to configure different aspects of the application's operation, primarily the database connection address.
+The main parameters list:
+
+* `app.connection` - database connection address. Specified as `<schema>://<endpoint>:<port>/<database>`
+* `app.threadsCount` - number of threads the application creates. Defaults to the number of CPU cores on the host.
+* `app.recordsCount` - number of records in the table used for testing. Default is 1 million.
+* `app.load.batchSize` - batch size for loading data when running the load command. Default is 1000.
+* `app.workload.duration` - test duration in seconds when running the run command. Default is 60 seconds.
+* `app.rpsLimit` - limit on the number of operations per second during the run command. By default, there is no limit (-1).
+* `app.pushMetrics` - flag indicating whether metrics should be exported to Prometheus; disabled by default.
+* `app.prometheusUrl` - endpoint of Prometheus to export metrics to. Default is http://localhost:9091.
+
+All parameters can be passed directly when launching the application (in the format `--param_name=value`) or can be
+preconfigured in an `application.properties` file saved next to the executable jar of the application.