Transaction priorities in YugabyteDB YSQL - Yugabyte Db

When using the 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 in [0, 1] as follows:

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.
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.

{{< note title="All single shard transactions have a priority of 1 in the normal-priority bucket." >}} {{</note >}}

The yb_get_current_transaction_priority function can be used to fetch the transaction priority of the current active transaction. It outputs a pair <priority> (bucket), where <priority> is of a real datatype between [0, 1] with 9 decimal units of precision, and <bucket> is either Normal or High.

{{< note title="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. {{</note >}}

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.

Create a table and insert some data.

sql

CREATE TABLE test (k INT PRIMARY KEY, v INT);
INSERT INTO test VALUES (1, 1);

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;

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;

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;

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;

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:

Normal-priority bucket: [yb::kRegularTxnLowerBound, yb::kRegularTxnUpperBound], that is, 0 to uint32_t_max-1
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].