The replica tools-db-2 is currently lagging 21 hours behind the primary tools-db-1 (Grafana chart). This is (correctly) triggering the alert ToolsToolsDBReplicationLagIsTooHigh.
SHOW SLAVE STATUS\g in the replica shows the replication is active, but it's taking hours to process a single transaction. Notice how Slave_IO_Running: Yes and Slave_SQL_Running: Yes, but Exec_Master_Log_Pos (indicating the last transaction that was successfully replicated from the primary) is not moving.
This happened before (T338031) and Slave_SQL_Running_State: Delete_rows_log_event::find_row(-1) seems to indicate a similar problem where a big DELETE query in the primary is taking hours to apply in the replica.
MariaDB [(none)]> show slave status\G *************************** 1. row *************************** Slave_IO_State: Waiting for master to send event Master_Host: tools-db-1.tools.eqiad1.wikimedia.cloud Master_User: repl Master_Port: 3306 Connect_Retry: 60 Master_Log_File: log.023109 Read_Master_Log_Pos: 28120756 Relay_Log_File: tools-db-2-relay-bin.027178 Relay_Log_Pos: 18624967 Relay_Master_Log_File: log.023018 Slave_IO_Running: Yes Slave_SQL_Running: Yes Replicate_Do_DB: Replicate_Ignore_DB: Replicate_Do_Table: Replicate_Ignore_Table: Replicate_Wild_Do_Table: Replicate_Wild_Ignore_Table: Last_Errno: 0 Last_Error: Skip_Counter: 0 Exec_Master_Log_Pos: 18624674 Relay_Log_Space: 9771520530 Until_Condition: None Until_Log_File: Until_Log_Pos: 0 Master_SSL_Allowed: No Master_SSL_CA_File: Master_SSL_CA_Path: Master_SSL_Cert: Master_SSL_Cipher: Master_SSL_Key: Seconds_Behind_Master: 91203 Master_SSL_Verify_Server_Cert: No Last_IO_Errno: 0 Last_IO_Error: Last_SQL_Errno: 0 Last_SQL_Error: Replicate_Ignore_Server_Ids: Master_Server_Id: 2886731301 Master_SSL_Crl: Master_SSL_Crlpath: Using_Gtid: Slave_Pos Gtid_IO_Pos: 0-2886731673-33522724637,2886731673-2886731673-4887243158,2886731301-2886731301-1338530527 Replicate_Do_Domain_Ids: Replicate_Ignore_Domain_Ids: Parallel_Mode: conservative SQL_Delay: 0 SQL_Remaining_Delay: NULL Slave_SQL_Running_State: Delete_rows_log_event::find_row(-1) Slave_DDL_Groups: 432265 Slave_Non_Transactional_Groups: 156747552 Slave_Transactional_Groups: 409816287 1 row in set (0.000 sec)