Сервис A в проде начал отдавать 503. В логах: ``` ERROR | service-A | upstream service-B returned 503 after 30s timeout ERROR | service-B | pool exhausted: 0 free connections out of 20 WARN | service-B | slow query: SELECT ... took 5s ``` В service-B's metrics: CPU 100%, RAM в норме, connection pool exhausted. Что произошло? Действия.