typeorm/docs/multiple-connections.md

Multiple data sources, databases, schemas and replication setup

• Using multiple data sources
• Using multiple databases in a single data source
• Using multiple schemas in a single data source
• Replication

Using multiple data sources

To use multiple data sources connected to different databases, simply create multiple DataSource instances:

import { DataSource } from "typeorm"

const db1DataSource = new DataSource({
    type: "mysql",
    host: "localhost",
    port: 3306,
    username: "root",
    password: "admin",
    database: "db1",
    entities: [__dirname + "/entity/*{.js,.ts}"],
    synchronize: true,
})

const db2DataSource = new DataSource({
    type: "mysql",
    host: "localhost",
    port: 3306,
    username: "root",
    password: "admin",
    database: "db2",
    entities: [__dirname + "/entity/*{.js,.ts}"],
    synchronize: true,
})

Using multiple databases within a single data source

To use multiple databases in a single data source, you can specify database name per-entity:

import { Entity, PrimaryGeneratedColumn, Column } from "typeorm"

@Entity({ database: "secondDB" })
export class User {
    @PrimaryGeneratedColumn()
    id: number

    @Column()
    firstName: string

    @Column()
    lastName: string
}

import { Entity, PrimaryGeneratedColumn, Column } from "typeorm"

@Entity({ database: "thirdDB" })
export class Photo {
    @PrimaryGeneratedColumn()
    id: number

    @Column()
    url: string
}

User entity will be created inside secondDB database and Photo entity inside thirdDB database. All other entities will be created in a default database defined in the data source options.

If you want to select data from a different database you only need to provide an entity:

const users = await dataSource
    .createQueryBuilder()
    .select()
    .from(User, "user")
    .addFrom(Photo, "photo")
    .andWhere("photo.userId = user.id")
    .getMany() // userId is not a foreign key since its cross-database request

This code will produce following SQL query (depend on database type):

SELECT * FROM "secondDB"."user" "user", "thirdDB"."photo" "photo"
    WHERE "photo"."userId" = "user"."id"

You can also specify a table path instead of the entity:

const users = await dataSource
    .createQueryBuilder()
    .select()
    .from("secondDB.user", "user")
    .addFrom("thirdDB.photo", "photo")
    .andWhere("photo.userId = user.id")
    .getMany() // userId is not a foreign key since its cross-database request

This feature is supported only in mysql and mssql databases.

Using multiple schemas within a single data source

To use multiple schemas in your applications, just set schema on each entity:

import { Entity, PrimaryGeneratedColumn, Column } from "typeorm"

@Entity({ schema: "secondSchema" })
export class User {
    @PrimaryGeneratedColumn()
    id: number

    @Column()
    firstName: string

    @Column()
    lastName: string
}

import { Entity, PrimaryGeneratedColumn, Column } from "typeorm"

@Entity({ schema: "thirdSchema" })
export class Photo {
    @PrimaryGeneratedColumn()
    id: number

    @Column()
    url: string
}

User entity will be created inside secondSchema schema and Photo entity inside thirdSchema schema. All other entities will be created in a default database defined in the data source options.

If you want to select data from a different schema you only need to provide an entity:

const users = await dataSource
    .createQueryBuilder()
    .select()
    .from(User, "user")
    .addFrom(Photo, "photo")
    .andWhere("photo.userId = user.id")
    .getMany() // userId is not a foreign key since its cross-database request

This code will produce following SQL query (depend on database type):

SELECT * FROM "secondSchema"."question" "question", "thirdSchema"."photo" "photo"
    WHERE "photo"."userId" = "user"."id"

You can also specify a table path instead of entity:

const users = await dataSource
    .createQueryBuilder()
    .select()
    .from("secondSchema.user", "user") // in mssql you can even specify a database: secondDB.secondSchema.user
    .addFrom("thirdSchema.photo", "photo") // in mssql you can even specify a database: thirdDB.thirdSchema.photo
    .andWhere("photo.userId = user.id")
    .getMany()

This feature is supported only in postgres and mssql databases. In mssql you can also combine schemas and databases, for example:

import { Entity, PrimaryGeneratedColumn, Column } from "typeorm"

@Entity({ database: "secondDB", schema: "public" })
export class User {
    @PrimaryGeneratedColumn()
    id: number

    @Column()
    firstName: string

    @Column()
    lastName: string
}

Replication

You can set up read/write replication using TypeORM. Example of replication options:

{
  type: "mysql",
  logging: true,
  replication: {
    master: {
      host: "server1",
      port: 3306,
      username: "test",
      password: "test",
      database: "test"
    },
    slaves: [{
      host: "server2",
      port: 3306,
      username: "test",
      password: "test",
      database: "test"
    }, {
      host: "server3",
      port: 3306,
      username: "test",
      password: "test",
      database: "test"
    }]
  }
}

All schema update and write operations are performed using master server. All simple queries performed by find methods or select query builder are using a random slave instance. All queries performed by query method are performed using the master instance.

If you want to explicitly use master in SELECT created by query builder, you can use the following code:

const masterQueryRunner = dataSource.createQueryRunner("master")
try {
    const postsFromMaster = await dataSource
        .createQueryBuilder(Post, "post")
        .setQueryRunner(masterQueryRunner)
        .getMany()
} finally {
    await masterQueryRunner.release()
}

If you want to use slave in raw queries, you also need to explicitly specify the query runner.

const slaveQueryRunner = dataSource.createQueryRunner("slave")
try {
    const userFromSlave = await slaveQueryRunner.query(
        "SELECT * FROM users WHERE id = $1",
        [userId],
        slaveQueryRunner,
    )
} finally {
    return slaveQueryRunner.release()
}

Note that connection created by a QueryRunner need to be explicitly released.

Replication is supported by mysql, postgres and sql server databases.

Mysql supports deep configuration:

{
  replication: {
    master: {
      host: "server1",
      port: 3306,
      username: "test",
      password: "test",
      database: "test"
    },
    slaves: [{
      host: "server2",
      port: 3306,
      username: "test",
      password: "test",
      database: "test"
    }, {
      host: "server3",
      port: 3306,
      username: "test",
      password: "test",
      database: "test"
    }],

    /**
    * If true, PoolCluster will attempt to reconnect when connection fails. (Default: true)
    */
    canRetry: true,

    /**
     * If connection fails, node's errorCount increases.
     * When errorCount is greater than removeNodeErrorCount, remove a node in the PoolCluster. (Default: 5)
     */
    removeNodeErrorCount: 5,

    /**
     * If connection fails, specifies the number of milliseconds before another connection attempt will be made.
     * If set to 0, then node will be removed instead and never re-used. (Default: 0)
     */
     restoreNodeTimeout: 0,

    /**
     * Determines how slaves are selected:
     * RR: Select one alternately (Round-Robin).
     * RANDOM: Select the node by random function.
     * ORDER: Select the first node available unconditionally.
     */
    selector: "RR"
  }
}