### 15.7.6 Transaction Scheduling

`InnoDB` uses the Contention-Aware Transaction Scheduling (CATS) algorithm to prioritize transactions that are waiting for locks. When multiple transactions are waiting for a lock on the same object, the CATS algorithm determines which transaction receives the lock first.

 The CATS algorithm prioritizes waiting transactions by assigning a scheduling weight, which is computed based on the number of transactions that a transaction blocks. For example, if two transactions are waiting for a lock on the same object, the transaction that blocks the most transactions is assigned a greater scheduling weight. If weights are equal, priority is given to the longest waiting transaction.

Note

 Prior to MySQL 8.0.20, `InnoDB` also uses a First In First Out (FIFO) algorithm to schedule transactions, and the CATS algorithm is used under heavy lock contention only. CATS algorithm enhancements in MySQL 8.0.20 rendered the FIFO algorithm redundant, permitting its removal. Transaction scheduling previously performed by the FIFO algorithm is performed by the CATS algorithm as of MySQL 8.0.20. In some cases, this change may affect the order in which transactions are granted locks.

 You can view transaction scheduling weights by querying the `TRX_SCHEDULE_WEIGHT` column in the [`INFORMATION_SCHEMA.INNODB_TRX`](information-schema-innodb-trx-table.html) table. Weights are computed for waiting transactions only. Waiting transactions are those in a `LOCK WAIT` transaction execution state, as reported by the `TRX_STATE` column. A transaction that is not waiting for a lock reports a NULL `TRX_SCHEDULE_WEIGHT` value.

[`INNODB_METRICS`](information-schema-innodb-metrics-table.html) counters are provided for monitoring of code-level transaction scheduling events. For information about using [`INNODB_METRICS`](information-schema-innodb-metrics-table.html) counters, see [Section 15.15.6, “InnoDB INFORMATION\_SCHEMA Metrics Table”](innodb-information-schema-metrics-table.html).

* `lock_rec_release_attempts`

   The number of attempts to release record locks. A single attempt may lead to zero or more record locks being released, as there may be zero or more record locks in a single structure.

* `lock_rec_grant_attempts`

   The number of attempts to grant record locks. A single attempt may result in zero or more record locks being granted.

* `lock_schedule_refreshes`

   The number of times the wait-for graph was analyzed to update the scheduled transaction weights.

