Depending on the selected routing method, load balancer and QoS setting, a Replica will automatically be chosen when the host connects. The maximum allowed latency can be set to limit the connection to only use a Replica that is within the specified maximum applied latency limit.
This can be specified in the connection string, and enables Replica selection based on the Replica which has a latency within the specified limit. For example, using the connection string:
Will specify that a host with a replication latency of less than 5 seconds should be selected.
./tools/tpm update alpha --connector-max-slave-latency=10
The Connector computes latency by polling the Replicator every 3 seconds for the current replication-view latency.
This gives the Connector an accuracy of +/- 3 seconds, which means that values of 3 or less will not function as expected.
For any queries that have a very low tolerance for replication latency, we strongly suggest you read directly from the Primary database server only. This ensures the latest data is being read.
flag does not ensure the Replica has applied the latest sequence number,
just that its latency at the last commit was under the specified
number. This behavior can be adjusted by specifying
--use-relative-latency=true in the
a cluster-wide setting, cctrl and trepctl will also report relative
latency based on this setting.
The appliedLatency is the latency between the commit time of the source event and the time the last committed transaction reached the end of the corresponding pipeline within the replicator. Within a Primary, this indicates the latency between the transaction commit time and when it was written to the THL. In a Replica, it indicates the latency between the commit time on the Primary database and when the transaction has been committed to the destination database. Clocks must be synchronized across hosts for this information to be accurate. appliedLatency : 0.828 The latency is measured in seconds. Increasing latency may indicate that the destination database is unable to keep up with the transactions from the Primary. In replicators that are operating with parallel apply, appliedLatency indicates the latency of the trailing channel. Because the parallel apply mechanism does not update all channels simultaneously, the figure shown may trail significantly from the actual latency.
See Section E.2.8, “Terminology: Fields
The relativeLatency is the latency between now and timestamp of the last event written into the local THL. This information gives an indication of how fresh the incoming THL information is. On a Primary, it indicates whether the Primary is keeping up with transactions generated on the Primary database. On a Replica, it indicates how up to date the THL read from the Primary is. A large value can either indicate that the database is not busy, that a large transaction is currently being read from the source database, or from the Primary replicator, or that the replicator has stalled for some reason. An increasing relativeLatency on the Replica may indicate that the replicator may have stalled and stopped applying changes to the dataserver.
See Section E.2.70, “Terminology: Fields
for more information.
Comparing Relative and Applied Latencies
Both relative and applied latency are visible via the trepctl status. Relative indicates the latency since the last time the appliedLastSeqno advanced; for example:
trepctl statusProcessing status command... NAME VALUE ---- ----- appliedLastEventId : mysql-bin.000211:0000000020094766;0 appliedLastSeqno : 78022 appliedLatency : 0.571 ... relativeLatency : 8.944 Finished status command...
In this example the last transaction had an applied latency (time to write to the Replica DB) of 0.571 seconds from the time it committed on the Primary, and last committed something to the Replica DB 8.944 seconds ago.
If relative latency increases significantly in a busy system, it may be a sign that replication is stalled. This is a good parameter to check in monitoring scripts.
For more information,see:
To troubleshoot the latency-based routing decisions the connector
makes, and uncomment the below lines in
#log4j.logger.com.continuent.tungsten.router.resource.loadbalancer=debug, stdout #log4j.additivity.com.continuent.tungsten.router.resource.loadbalancer=false
The log will then show and explain what node the connector choose and why.
No connector restart is required to enable logging. If you
re-comment the lines, a restart will be required. To disable without
a connector restart, replace the word