Use TiSpark to Read TiFlash Replicas

This document introduces how to use TiSpark to read TiFlash replicas.

Currently, you can use TiSpark to read TiFlash replicas in a method similar to the engine isolation in TiDB. This method is to configure the spark.tispark.isolation_read_engines parameter. The parameter value defaults to tikv,tiflash, which means that TiDB reads data from TiFlash or from TiKV according to CBO's selection. If you set the parameter value to tiflash, it means that TiDB forcibly reads data from TiFlash.

Notes
When this parameter is set to tiflash, only the TiFlash replicas of all tables involved in the query are read and these tables must have TiFlash replicas; for tables that do not have TiFlash replicas, an error is reported. When this parameter is set to tikv, only the TiKV replica is read.

You can configure this parameter in one of the following ways:

Add the following item in the spark-defaults.conf file:
```
spark.tispark.isolation_read_engines tiflash
```
Add --conf spark.tispark.isolation_read_engines=tiflash in the initialization command when initializing Spark shell or Thrift server.
Set spark.conf.set("spark.tispark.isolation_read_engines", "tiflash") in Spark shell in a real-time manner.
Set set spark.tispark.isolation_read_engines=tiflash in Thrift server after the server is connected via beeline.

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