Transaction priorities
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 aFOR 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.
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 <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
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 returnshighest 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.
-
Create a table and insert some data.
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:
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:
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
yb_get_current_transaction_priority ------------------------------------------- 0.000000000 (Normal priority transaction) (1 row)
SELECT * FROM test;
k | v ---+--- 1 | 1 (1 row)
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
yb_get_current_transaction_priority ------------------------------------------- 0.000000000 (Normal priority transaction) (1 row)
INSERT INTO test VALUES (2, '2'); -- perform a write which starts a real transaction SELECT yb_get_current_transaction_priority(); -- non-zero now
yb_get_current_transaction_priority ------------------------------------------- 0.537144608 (Normal priority transaction) (1 row)
COMMIT;
-
Create a transaction in the high-priority bucket as follows:
BEGIN TRANSACTION ISOLATION LEVEL REPEATABLE READ; SELECT * FROM test WHERE k = 1 FOR UPDATE; -- starts a transaction in a high-priority bucket
k | v ---+--- 1 | 1 (1 row)
SELECT yb_get_current_transaction_priority();
yb_get_current_transaction_priority ----------------------------------------- 0.412004009 (High priority transaction) (1 row)
COMMIT;
-
Create a transaction with the highest priority
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;
k | v ---+--- 1 | 1 (1 row)
SELECT yb_get_current_transaction_priority();
yb_get_current_transaction_priority ------------------------------------- Highest priority transaction (1 row)
COMMIT;
Internal representation of priorities
Internally, both the normal and high-priority buckets are mapped to a unit64_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 touint32_t_max
-1 -
High-priority bucket:
[yb::kHighPriTxnLowerBound, yb::kHighPriTxnUpperBound]
, that is,uint32_t_max
touint64_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]
.