Laravelで複数データベース接続を運用する：接続定義・読み書き分離・テナント分割まで

// config/database.php（抜粋イメージ）
'connections' => [

    'mysql' => [
        'driver' => 'mysql',
        'host' => env('DB_HOST', '127.0.0.1'),
        'port' => env('DB_PORT', '3306'),
        'database' => env('DB_DATABASE', 'laravel'),
        'username' => env('DB_USERNAME', 'root'),
        'password' => env('DB_PASSWORD', ''),
        // ...
    ],

    'analytics' => [
        'driver' => 'pgsql',
        'host' => env('ANALYTICS_DB_HOST'),
        'port' => env('ANALYTICS_DB_PORT', '5432'),
        'database' => env('ANALYTICS_DB_DATABASE'),
        'username' => env('ANALYTICS_DB_USERNAME'),
        'password' => env('ANALYTICS_DB_PASSWORD'),
        // ...
    ],

    'legacy' => [
        'driver' => 'mysql',
        'host' => env('LEGACY_DB_HOST'),
        'port' => env('LEGACY_DB_PORT', '3306'),
        'database' => env('LEGACY_DB_DATABASE'),
        'username' => env('LEGACY_DB_USERNAME'),
        'password' => env('LEGACY_DB_PASSWORD'),
        // ...
    ],
],

# .env（例）
DB_CONNECTION=mysql
DB_HOST=db
DB_PORT=3306
DB_DATABASE=app
DB_USERNAME=app
DB_PASSWORD=secret

ANALYTICS_DB_HOST=analytics-db
ANALYTICS_DB_PORT=5432
ANALYTICS_DB_DATABASE=analytics
ANALYTICS_DB_USERNAME=analytics
ANALYTICS_DB_PASSWORD=secret

LEGACY_DB_HOST=legacy-db
LEGACY_DB_PORT=3306
LEGACY_DB_DATABASE=legacy
LEGACY_DB_USERNAME=legacy
LEGACY_DB_PASSWORD=secret

use Illuminate\Support\Facades\DB;

$rows = DB::connection('analytics')
    ->table('daily_metrics')
    ->whereDate('date', now()->toDateString())
    ->get();

namespace App\Models\Analytics;

use Illuminate\Database\Eloquent\Model;

class DailyMetric extends Model
{
    protected $connection = 'analytics';
    protected $table = 'daily_metrics';
    public $timestamps = false;
}

use App\Models\Analytics\DailyMetric;

$metrics = DailyMetric::whereDate('date', today())->get();

$user = (new \App\Models\User)->setConnection('legacy')
    ->newQuery()
    ->where('email', $email)
    ->first();

$users = \App\Models\User::on('legacy')->where('status', 'active')->get();

// config/database.php（概念例：mysql）
'mysql' => [
    'driver' => 'mysql',
    'read' => [
        'host' => [env('DB_READ_HOST', '127.0.0.1')],
    ],
    'write' => [
        'host' => [env('DB_WRITE_HOST', '127.0.0.1')],
    ],
    'sticky' => true,
    'database' => env('DB_DATABASE', 'laravel'),
    'username' => env('DB_USERNAME', 'root'),
    'password' => env('DB_PASSWORD', ''),
    // ...
],

use Illuminate\Support\Facades\DB;

DB::connection('mysql')->transaction(function () {
    // mysql側の更新はここで完結
});

DB::connection('analytics')->transaction(function () {
    // analytics側の更新はここで完結
});

# analytics用マイグレーションを別パスで実行する例
php artisan migrate --database=analytics --path=database/migrations/analytics

public function handle(): void
{
    $row = \DB::connection('legacy')->table('users')->where('id', $this->legacyUserId)->first();

    // 取り込んだ結果を mysql に保存
    \DB::connection('mysql')->table('users')->updateOrInsert(
        ['external_id' => $row->id],
        ['name' => $row->name]
    );
}

use Illuminate\Support\Facades\DB;

public function handle($request, \Closure $next)
{
    $tenant = $request->header('X-Tenant');

    config([
        'database.connections.tenant.database' => "tenant_{$tenant}",
        'database.default' => 'tenant',
    ]);

    DB::purge('tenant');     // 既存接続を破棄
    DB::reconnect('tenant'); // 新設定で再接続

    return $next($request);
}

try {
    DB::connection('analytics')->select('select 1');
} catch (\Throwable $e) {
    // フォールバック（キャッシュ値、別経路、機能停止）
}