how can i use laravel multi-database? [duplicate]

Define a separate database connection in config/database.php.

'mysql' => [ // default
    'driver' => 'mysql',
    'host' => env('DB_HOST', '127.0.0.1'),
    'port' => env('DB_PORT', '3306'),
    'database' => env('DB_DATABASE', 'forge'),
    'username' => env('DB_USERNAME', 'forge'),
    'password' => env('DB_PASSWORD', ''),
    ...
],
'db1' => [ // another 
    'driver' => 'mysql',
    'host' => env('DB_HOST_ONE', '127.0.0.1'),
    'port' => env('DB_PORT_ONE', '3306'),
    'database' => env('DB_DATABASE_ONE', 'forge'),
    'username' => env('DB_USERNAME_ONE', 'forge'),
    'password' => env('DB_PASSWORD_ONE', ''),
    ...
]

Note that you have to define respective config in your .env.

Then when you want to use db1 connection it, use Config::set('database.default', 'db1'). However, it only works when you have known amount of database connections(that you can define in config/database.php), if you have unknown amount of databases, then you should change the config directly instead of the name of the connection only.

Example:

Config::set('database.connections.mysql.database', 'db1')
Config::set('database.connections.mysql.username', 'admin');
Config::set('database.connections.mysql.password', 'secret');

You can see my another answer to know how does it work underhood.

In this case, I presume you are building a system that is connecting to various existing databases possibly from different applications.

You can define as many DB connections as you want in your config/database.php

'mysql' => [
    'driver' => 'mysql',
    'host' => env('DB_HOST', '127.0.0.1'),
    'port' => env('DB_PORT', '3306'),
    'database' => env('DB_DATABASE', 'forge'),
    'username' => env('DB_USERNAME', 'forge'),
    'password' => env('DB_PASSWORD', ''),
    'unix_socket' => env('DB_SOCKET', ''),
    'charset' => 'utf8mb4',
    'collation' => 'utf8mb4_unicode_ci',
    'prefix' => '',
    'strict' => true,
    'engine' => null,
],

'mysql2' => [
    'driver' => 'mysql',
    'host' => env('DB_HOST_2', '127.0.0.1'),
    'port' => env('DB_PORT_2', '3306'),
    'database' => env('DB_DATABASE_2', 'forge'),
    'username' => env('DB_USERNAME_2', 'forge'),
    'password' => env('DB_PASSWORD_2', ''),
    'unix_socket' => env('DB_SOCKET', ''),
    'charset' => 'utf8mb4',
    'collation' => 'utf8mb4_unicode_ci',
    'prefix' => '',
    'strict' => true,
    'engine' => null,
],
...

You would however also need a way to specify for each user which other external DB from which their details are going to be fetched from, for instance a column db_name on the users table.

At the point of fetching the data of say 'Alex' User model, you can do something like

$user = User::find(1); //if Alex has user_id 1 and where $user->db_name is 'mysql2' as is in the config/database.php file

$userDetails = DB::connection($user->db_name)->where('username',$user->name)->where('other_details','some details')->get();

Make sure to specify this in your .env too, to match what's defined in config/database.php

DB_CONNECTION=mysql
DB_HOST=127.0.0.1
DB_PORT=3306
DB_DATABASE=homestead
DB_USERNAME=homestead
DB_PASSWORD=secret

DB_HOST_2=127.0.0.1
DB_PORT_2=3306
DB_DATABASE_2=db2_name_here
DB_USERNAME_2=db2_username_here
DB_PASSWORD_2=db2_password_here

...