chore(rag): add eval knowledge base mirror

knowledge_base/rag-eval/database/mysql-connection-pool.md

@@ -0,0 +1,29 @@
+---
+title: MySQL Connection Pool Runbook
+keywords: [MySQL connection pool, pool exhausted, max_connections, HikariCP]
+summary: Diagnose exhausted MySQL connection pools and distinguish application leaks from database limits.
+category: database
+source: mysql-connection-pool
+breadcrumb: Database > MySQL > Connection Pool
+kb_scope: rag-eval
+covers: [mysql, connection pool, database capacity]
+when_to_retrieve: Use when MySQL clients report exhausted pools, connection acquisition timeout, max_connections pressure, or HikariCP saturation.
+---
+
+# Database
+
+## MySQL
+
+### Connection Pool
+
+When MySQL connection pool is exhausted, first compare application pool usage with database `max_connections`.
+For HikariCP, check `active`, `idle`, `pending`, and connection acquisition timeout metrics.
+
+Recommended diagnosis:
+
+1. Verify whether HikariCP active connections stay near maximum while pending threads grow.
+2. Check MySQL `Threads_connected`, `Threads_running`, and `max_connections`.
+3. Inspect slow SQL and long transactions that keep connections checked out.
+4. If the database is healthy, look for application connection leaks or missing transaction boundaries.
+
+Use this runbook as evidence for connection pool, max_connections, and HikariCP incidents.