Add documentation for query priority support by the transaction throttler

content/en/docs/17.0/reference/programs/vttablet.md

@@ -401,6 +401,7 @@ The following global options apply to `vttablet`:
 | --twopc_coordinator_address                                      | string        | address of the (VTGate) process(es) that will be used to notify of abandoned transactions.                                                                                                                                                                                                                                                                                                                                                                                                                 |
 | --twopc_enable                                                   | boolean       | if the flag is on, 2pc is enabled. Other 2pc flags must be supplied.                                                                                                                                                                                                                                                                                                                                                                                                                                       |
 | --tx-throttler-config                                            | string        | Synonym to -tx_throttler_config (default "target_replication_lag_sec: 2\nmax_replication_lag_sec: 10\ninitial_rate: 100\nmax_increase: 1\nemergency_decrease: 0.5\nmin_duration_between_increases_sec: 40\nmax_duration_between_increases_sec: 62\nmin_duration_between_decreases_sec: 20\nspread_backlog_across_sec: 20\nage_bad_rate_after_sec: 180\nbad_rate_increase: 0.1\nmax_rate_approach_threshold: 0.9\n")                                                                                        |
+| --tx-throttler-default-priority int                              | int           | Default query priority used by the transaction throttler if it is not specified in a query comment directive. It must be a number between 0 (highest priority) and 100 (lowest priority) (default: 100)                                                                                                                                                                                                                                                                                                      |
 | --tx-throttler-healthcheck-cells                                 | strings       | Synonym to -tx_throttler_healthcheck_cells                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
 | --tx_throttler_config                                            | string        | The configuration of the transaction throttler as a text formatted throttlerdata.Configuration protocol buffer message (default "target_replication_lag_sec: 2\nmax_replication_lag_sec: 10\ninitial_rate: 100\nmax_increase: 1\nemergency_decrease: 0.5\nmin_duration_between_increases_sec: 40\nmax_duration_between_increases_sec: 62\nmin_duration_between_decreases_sec: 20\nspread_backlog_across_sec: 20\nage_bad_rate_after_sec: 180\nbad_rate_increase: 0.1\nmax_rate_approach_threshold: 0.9\n") |
 | --tx_throttler_healthcheck_cells                                 | strings       | A comma-separated list of cells. Only tabletservers running in these cells will be monitored for replication lag by the transaction throttler.                                                                                                                                                                                                                                                                                                                                                             |

content/en/docs/17.0/user-guides/configuration-advanced/comment-directives.md

@@ -119,3 +119,15 @@ As shown above, the metrics being instrumented this way are `vttablet_query_coun
 If the query lacks the `WORKLOAD_NAME` directive, the corresponding label in the metric will have the value of an empty
 string. If `vttablet` is not started with `--enable-per-workload-table-metrics`, metrics are emitted without the
 workload label (e.g. `vttablet_query_counts{plan="Select",table="customer"}`.
+
+### Priority (`PRIORITY`)
+
+Specifies the priority associated with the execution of this query, as a number between 0 (highest priority) and 100
+(lowest priority). The priority is used by the transaction throttler (see `--tx-throttler-...` and `-enable-tx-throttler`
+flags in the `vttablet` [documentation](../../reference/programs/vttablet.md))) to determine whether a particular query
+should be throttled. If the transaction throttler determines that a query may need to be throttled, it will throttle it
+with a probability equal to the provided query priority. This allows avoiding high business impact queries from being
+throttled (by setting their priority to `0`) while letting less business critical ones continue to be throttled (by
+setting their priority to more than `0`).
+
+Notice that this directive has no effect if the transaction throttler is not enabled.