The server_locks table is similar in structure to the cluster_locks table, and provides a subset of the information found in the latter table, but which is specific to the SQL node (MySQL server) where it resides. (The cluster_locks table provides information about all locks in the cluster.) More precisely, server_locks contains information about locks requested by threads belonging to the current mysqld instance, and serves as a companion table to server_operations. This may be useful for correlating locking patterns with specific MySQL user sessions, queries, or use cases.
The server_locks table contains the following columns:
mysql_connection_id
MySQL connection ID
node_id
ID of reporting node
block_instance
ID of reporting LDM instance
tableid
ID of table containing this row
fragmentid
ID of fragment containing locked row
rowid
ID of locked row
transid
Transaction ID
mode
Lock request mode
state
Lock state
detail
Whether this is first holding lock in row lock queue
op
Operation type
duration_millis
Milliseconds spent waiting or holding lock
lock_num
ID of lock object
waiting_for
Waiting for lock with this ID
The mysql_connection_id column shows the MySQL connection or thread ID as shown by SHOW PROCESSLIST.
block_instance refers to an instance of a kernel block. Together with the block name, this number can be used to look up a given instance in the threadblocks table.
The tableid is assigned to the table by NDB; the same ID is used for this table in other ndbinfo tables, as well as in the output of ndb_show_tables.
The transaction ID shown in the transid column is the identifier generated by the NDB API for the transaction requesting or holding the current lock.
The mode column shows the lock mode, which is always one of S (shared lock) or X (exclusive lock). If a transaction has an exclusive lock on a given row, all other locks on that row have the same transaction ID.
The state column shows the lock state. Its value is always one of H (holding) or W (waiting). A waiting lock request waits for a lock held by a different transaction.
The detail column indicates whether this lock is the first holding lock in the affected row's lock queue, in which case it contains a * (asterisk character); otherwise, this column is empty. This information can be used to help identify the unique entries in a list of lock requests.
The op column shows the type of operation requesting the lock. This is always one of the values READ, INSERT, UPDATE, DELETE, SCAN, or REFRESH.
The duration_millis column shows the number of milliseconds for which this lock request has been waiting or holding the lock. This is reset to 0 when a lock is granted for a waiting request.
The lock ID (lockid column) is unique to this node and block instance.
If the lock_state column's value is W, this lock is waiting to be granted, and the waiting_for column shows the lock ID of the lock object this request is waiting for. Otherwise, waiting_for is empty. waiting_for can refer only to locks on the same row (as identified by node_id, block_instance, tableid, fragmentid, and rowid).