2.0 KiB
Replication Lag Based Throttling of Transactions
Vitess supports throttling of transactions based on replication lag. When this feature is turned on, each VTTablet primary monitors the replication lag from the replicas, and based on the observed replication lag tries to rate-limit the received transactions to keep the replication lag under a configured limit.
The decision of whether to throttle a transaction is done in the BEGIN
statement rather than in the COMMIT statement to avoid having a transaction
perform a lot of work just to eventually be throttled and potentially
rolled-back if the open-transaction timeout is exceeded.
If a BEGIN statement is throttled the client receives the gRPC UNAVAILABLE error code.
The following VTTablet command line flags control the replication-lag based throttler:
- enable-tx-throttler
A boolean flag controlling whether the replication-lag-based throttling is enabled.
- tx-throttler-config
A text-format representation of the throttlerdata.Configuration protocol buffer that contains configuration options for the throttler. The most important fields in that message are target_replication_lag_sec and max_replication_lag_sec that specify the desired limits on the replication lag. See the comments in the protocol definition file for more details. If this is not specified a default configuration will be used.
- tx-throttler-healthcheck-cells
A comma separated list of datacenter cells. The throttler will only monitor the non-RDONLY replicas found in these cells for replication lag.
Caveats and Known Issues
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The throttler keeps trying to explore the maximum rate possible while keeping the replication lag under the desired limit; as such the desired replication lag limit may occasionally be slightly violated.
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Transactions are considered homogeneous. There is currently no support for specifying how
expensivea transaction is.