Setting the session pool size

YDB creates an actor for each session. Consequently, the session pool size on the client affects resource consumption (memory, CPU) on the YDB server.

For example, if 1000 clients of the same database open 1000 sessions each, one million actors are created on the server, consuming a large amount of memory and CPU. Without a client-side session limit, this can slow the cluster down and put it in a degraded state.

By default, native YDB SDK drivers use a limit of 50 sessions. When using third-party libraries such as Go database/sql, no limit is set.

Set the client session limit to the minimum needed for normal application operation. Remember that a session is single-threaded on both server and client. If the application must run 1000 concurrent in-flight requests to YDB, set the limit to about 1000 sessions.

Distinguish estimated RPS (requests per second) from in-flight requests. RPS is the total number of requests completed in one second. For example, with RPS = 10000 and average latency 100 ms, a limit of 1000 sessions is enough: each session can run about 10 sequential requests per second on average.

Below are examples of setting the session pool limit in different YDB SDKs.

package main

import (
  "context"

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

func main() {
  db, err := ydb.Open(ctx,
    os.Getenv("YDB_CONNECTION_STRING"),
    ydb.WithSessionPoolSizeLimit(500),
  )
  if err != nil {
    panic(err)
  }
  defer db.Close(ctx)
  ...
}

package main

import (
  "context"
  "database/sql"

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

func main() {
  db, err := sql.Open("ydb", os.Getenv("YDB_CONNECTION_STRING"))
  if err != nil {
    panic(err)
  }
  defer db.Close()
  db.SetMaxOpenConns(100)
  db.SetMaxIdleConns(100)
  db.SetConnMaxIdleTime(time.Second) // workaround for background keep-aliving of YDB sessions
  ...
}

this.queryClient = QueryClient.newClient(transport)
        // 10 — minimum active sessions kept in the pool during cleanup
        // 500 — maximum pool size
        .sessionPoolMinSize(10)
        .sessionPoolMaxSize(500)
        .build();

spring.datasource.url=jdbc:ydb:grpc://localhost:2136/local
spring.datasource.driver-class-name=tech.ydb.jdbc.YdbDriver
spring.datasource.hikari.maximum-pool-size=100 # max JDBC connections

import os
import ydb

with ydb.Driver(
    connection_string=os.environ["YDB_CONNECTION_STRING"],
    credentials=ydb.credentials_from_env_variables(),
) as driver:
    driver.wait(timeout=5)
    with ydb.QuerySessionPool(driver, size=500) as pool:
        # ...

import os
import ydb
import asyncio

async def ydb_init():
    async with ydb.aio.Driver(
        connection_string=os.environ["YDB_CONNECTION_STRING"],
        credentials=ydb.credentials_from_env_variables(),
    ) as driver:
        await driver.wait()
        async with ydb.aio.QuerySessionPool(driver, size=500) as pool:
            # ...

asyncio.run(ydb_init())