iotdb Support a new read_consistency

This pull request introduces support for a new read consistency level, follower_read, across the IoTDB codebase. The changes ensure that configuration, logic, and documentation now recognize and correctly handle this new option alongside the existing strong and weak levels.

Read Consistency Level Enhancements:

Added FOLLOWER_READ to the ReadConsistencyLevel enum in ReadConsistencyLevel.java, allowing the system to use follower replicas for read operations.
Updated the configuration logic in ConfigNodeDescriptor.java and IoTDBConfig.java to accept and validate the new follower_read value, throwing exceptions for unknown values. [1] [2]
Enhanced the planner logic in AbstractFragmentParallelPlanner.java to select follower replicas when FOLLOWER_READ is specified, ensuring the leader is skipped and a follower is chosen based on session ID.
Updated the configuration template in iotdb-system.properties.template to document the new follower_read option for users.

Refactoring and Logic Separation:

Refactored the data node selection logic in AbstractFragmentParallelPlanner.java by extracting target index determination into a separate method, improving readability and maintainability.

Supposing we have a 3C3D cluster and data replica number is set to 3, we execute the following sqls to prepare data:

create database test;
use test;
create table t1(device_id STRING TAG, s1 float);
insert into t1 values(now(),'d1', 1.0);

Before this pr, if we do select * from t1; query, the query will always be routed to leader: f374fca129e9c9da5bb8f5e86e53384f

After this pr, if we set read_consistency_level=follower_read and then restart the cluster, then the above query will always be routed to any follower: edcf2ad9bdf44cb525d6ef196c171704

Oct 17 '25 01:10 JackieTien97

Codecov Report

:x: Patch coverage is 34.78261% with 15 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 38.67%. Comparing base (8d93384) to head (5df19d5). :warning: Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
.../planner/plan/AbstractFragmentParallelPlanner.java	46.15%	7 Missing :warning:
...ain/java/org/apache/iotdb/db/conf/IoTDBConfig.java	0.00%	5 Missing :warning:
...he/iotdb/confignode/conf/ConfigNodeDescriptor.java	0.00%	3 Missing :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #16604      +/-   ##
============================================
- Coverage     38.67%   38.67%   -0.01%     
  Complexity      207      207              
============================================
  Files          4937     4938       +1     
  Lines        326989   326996       +7     
  Branches      41503    41509       +6     
============================================
- Hits         126470   126453      -17     
- Misses       200519   200543      +24

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