Prefer the nearest data center

Below are examples of setting the "prefer the nearest data center" balancing algorithm in different YDB SDKs.

package main

import (
  "context"
  "os"

  "github.com/ydb-platform/ydb-go-sdk/v3"
  "github.com/ydb-platform/ydb-go-sdk/v3/balancers"
)

func main() {
  ctx, cancel := context.WithCancel(context.Background())
  defer cancel()
  db, err := ydb.Open(ctx,
    os.Getenv("YDB_CONNECTION_STRING"),
    ydb.WithBalancer(
      balancers.PreferLocalDC(
        balancers.RandomChoice(),
      ),
    ),
  )
  if err != nil {
    panic(err)
  }
  defer db.Close(ctx)
  // ...
}

Client-side balancing in the YDB database/sql driver happens only when opening a new connection (in database/sql terms), which corresponds to a YDB session on a specific node. After the session is created, all queries on that session go to that node. Queries on the same YDB session are not balanced across nodes.

Example for "prefer the nearest data center" balancing:

package main

import (
  "context"
  "database/sql"
  "os"

  "github.com/ydb-platform/ydb-go-sdk/v3"
  "github.com/ydb-platform/ydb-go-sdk/v3/balancers"
)

func main() {
  ctx, cancel := context.WithCancel(context.Background())
  defer cancel()
  nativeDriver, err := ydb.Open(ctx,
    os.Getenv("YDB_CONNECTION_STRING"),
    ydb.WithBalancer(
      balancers.PreferLocalDC(
        balancers.RandomChoice(),
      ),
    ),
  )
  if err != nil {
    panic(err)
  }
  defer nativeDriver.Close(ctx)

  connector, err := ydb.Connector(nativeDriver)
  if err != nil {
    panic(err)
  }

  db := sql.OpenDB(connector)
  defer db.Close()
  // ...
}

The YDB C++ SDK uses the prefer_local_dc (prefer nearest data center) algorithm by default.

#include <ydb-cpp-sdk/client/driver/driver.h>

int main() {
  auto connectionString = std::string(std::getenv("YDB_CONNECTION_STRING"));

  auto driverConfig = NYdb::TDriverConfig(connectionString)
    .SetBalancingPolicy(NYdb::TBalancingPolicy::UsePreferableLocation());

  NYdb::TDriver driver(driverConfig);
  // ...
  driver.Stop(true);
  return 0;
}

import os
import ydb

driver_config = ydb.DriverConfig(
    endpoint=os.environ["YDB_ENDPOINT"],
    database=os.environ["YDB_DATABASE"],
    credentials=ydb.credentials_from_env_variables(),
    use_all_nodes=False,  # prefer the nearest data center
)

with ydb.Driver(driver_config) as driver:
    driver.wait(timeout=5)
    # ...

import os
import ydb
import asyncio

async def ydb_init():
    driver_config = ydb.DriverConfig(
        endpoint=os.environ["YDB_ENDPOINT"],
        database=os.environ["YDB_DATABASE"],
        credentials=ydb.credentials_from_env_variables(),
        use_all_nodes=False,  # prefer the nearest data center
    )
    async with ydb.aio.Driver(driver_config) as driver:
        await driver.wait()
        # ...

asyncio.run(ydb_init())

import os
import sqlalchemy as sa

engine = sa.create_engine(
    os.environ["YDB_SQLALCHEMY_URL"],
    connect_args={
        "driver_config_kwargs": {
            "use_all_nodes": False,  # prefer the nearest data center
        }
    },
)
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