--- title: "Transaction priorities" url: "https://docs.yugabyte.com/stable/architecture/transactions/transaction-priorities/" --- # Transaction priorities Details about Transaction priorities in YSQL - [YSQL](/stable/architecture/transactions/transaction-priorities/) When using the [Fail-on-Conflict](/stable/architecture/transactions/concurrency-control/#fail-on-conflict "Fail-on-Conflict") concurrency control policy, transactions are assigned priorities that help decide which transactions should be aborted in case of conflict. There are two priority buckets, each having a priority range of [reals](https://www.postgresql.org/docs/15/datatype.html "reals") in \[0, 1] as follows: 1. `High-priority` bucket: if the first statement in a transaction takes a `FOR UPDATE/ FOR SHARE/ FOR NO KEY UPDATE` explicit row lock using SELECT, it will be assigned a priority from this bucket. 2. `Normal-priority` bucket: all other transactions are assigned a priority from this bucket. Note that a transaction with any priority P1 from the high-priority bucket can abort a transaction with any priority P2 from the normal-priority bucket. For example, a transaction with priority 0.1 from the high-priority bucket can abort a transaction with priority 0.9 from the normal-priority bucket. Priorities are randomly chosen from the applicable bucket. However, you can use the following two YSQL parameters to control the priority assigned to transactions in a specific session: - `yb_transaction_priority_lower_bound` - `yb_transaction_priority_upper_bound` These parameters help set lower and upper bounds on the randomly-assigned priority that a transaction should receive from the applicable bucket. These parameters accept a value of `real` datatype in the range \[0, 1]. Also note that the same bounds apply to both buckets. All single shard transactions have a priority of 1 in the normal-priority bucket. The `yb_get_current_transaction_priority` function can be used to fetch the transaction priority of the current active transaction. It outputs a pair ` (bucket)`, where `` is of a real datatype between \[0, 1] with 9 decimal units of precision, and `` is either `Normal` or `High`. Note As an exception, if a transaction is assigned the highest priority possible, that is, a priority of 1 in the high-priority bucket, the function returns `highest priority transaction` without any real value. A transaction's priority is `0.000000000 (normal-priority transaction)` until a transaction is really started. ## Examples The following examples demonstrate how to set priorities for your transactions and get the current transaction priority. 1. Create a table and insert some data. ```sql CREATE TABLE test (k INT PRIMARY KEY, v INT); INSERT INTO test VALUES (1, 1); ``` 2. Set the lower and upper bound values for your transactions as follows: ```sql SET yb_transaction_priority_lower_bound = 0.4; SET yb_transaction_priority_upper_bound = 0.6; ``` 3. Create a transaction in the normal-priority bucket as follows: ```sql BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ; SELECT yb_get_current_transaction_priority(); -- 0 due to an optimization which doesn't really start a real transaction internally unless a write occurs ``` ```output yb_get_current_transaction_priority ------------------------------------------- 0.000000000 (Normal priority transaction) (1 row) ``` ```sql SELECT * FROM test; ``` ```output k | v ---+--- 1 | 1 (1 row) ``` ```sql SELECT yb_get_current_transaction_priority(); -- still 0 due to the optimization which doesn't really start a real transaction internally unless a write occurs ``` ```output yb_get_current_transaction_priority ------------------------------------------- 0.000000000 (Normal priority transaction) (1 row) ``` ```sql INSERT INTO test VALUES (2, '2'); -- perform a write which starts a real transaction SELECT yb_get_current_transaction_priority(); -- non-zero now ``` ```output yb_get_current_transaction_priority ------------------------------------------- 0.537144608 (Normal priority transaction) (1 row) ``` ```sql COMMIT; ``` 4. Create a transaction in the high-priority bucket as follows: ```sql BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ; SELECT * FROM test WHERE k = 1 FOR UPDATE; -- starts a transaction in a high-priority bucket ``` ```output k | v ---+--- 1 | 1 (1 row) ``` ```sql SELECT yb_get_current_transaction_priority(); ``` ```output yb_get_current_transaction_priority ----------------------------------------- 0.412004009 (High priority transaction) (1 row) ``` ```sql COMMIT; ``` 5. Create a transaction with the highest priority ```sql SET yb_transaction_priority_upper_bound = 1; SET yb_transaction_priority_lower_bound = 1; BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ; SELECT * FROM test WHERE k = 1 FOR UPDATE; ``` ```output k | v ---+--- 1 | 1 (1 row) ``` ```sql SELECT yb_get_current_transaction_priority(); ``` ```output yb_get_current_transaction_priority ------------------------------------- Highest priority transaction (1 row) ``` ```sql COMMIT; ``` Internal representation of priorities Internally, both the normal and high-priority buckets are mapped to a `uint64_t` space. The 64 bit range is used by the two priority buckets as follows: 1. Normal-priority bucket: `[yb::kRegularTxnLowerBound, yb::kRegularTxnUpperBound]`, that is, 0 to `uint32_t_max`-1 2. High-priority bucket: `[yb::kHighPriTxnLowerBound, yb::kHighPriTxnUpperBound]`, that is, `uint32_t_max` to `uint64_t_max` For ease of use, the bounds are expressed as a \[0, 1] real range for each bucket in the lower or upper bound YSQL parameters and the `yb_get_current_transaction_priority` function. The \[0, 1] real range map proportionally to the integer ranges for both buckets. In other words, the \[0, 1] range in the normal-priority bucket maps to `[0, uint32_t_max-1]` and the \[0, 1] range in the high-priority bucket maps to `[uint32_t_max, uint64_t_max]`.