Read Repair: repair during read path
Describes read repair, repair during read path.
Read repair improves consistency in a Cassandra cluster with every read request.
In a read, the coordinator node sends a data request to one replica node and digest
requests to others for consistency level (CL) greater than ONE
. If all
nodes return consistent data, the coordinator returns it to the client. For a
description of how Cassandra handles inconsistency among replicas, see How are read requests accomplished?.
In read repair, Cassandra sends a digest request to each replica not directly involved in
the read. Cassandra compares all replicas and writes the most recent version to any
replica node that does not have it. If the query's consistency level
is above ONE
, Cassandra performs this process on all replica nodes in
the foreground before the data is returned to the client. Read repair repairs any node
queried by the read. This means that for a consistency level of ONE
,
no data is repaired because no comparison takes place. For QUORUM
, only
the nodes that the query touches are repaired, not all nodes.
Read repair cannot be performed on tables that use DateTieredCompactionStrategy, due to the method of checking timestamps used
in DTCS compaction. If your table uses DateTieredCompactionStrategy
,
set read_repair_chance
to zero. For other compaction strategies,
read_repair_chance
is typically set to a value of 0.2.