04-Read-After-Write-Consistency.md

title
Read-After-Write-Consistency

Introduction

When an application writes data to the primary node and subsequently reads it on a read-only node, WeSQL WeScale makes sure that the data that was just written to the primary node can be accessed and read from the read-only node.

The goal of this tutorial is to explain how to enable the Read-After-Write-Consistency feature in WeSQL WeScale.

Setting through the set command.

If you have already set up a cluster and want to enable Read-After-Write-Consistency, the simplest way is using the MySQL "set" command:

# session
set session read_after_write_consistency='SESSION'

# global
set global read_after_write_consistency='SESSION'

Currently, WeSQL WeScale supports these consistency levels:

policy	description
EVENTUAL	No guarantee of consistency.
SESSION	Within the same connection, ensure that subsequent read requests can read previous write operations. (recommend)
INSTANCE	Within the same instance (VTGate), ensure that subsequent read requests can read previous write operations, even if the read and write requests are not initiated by the same connection.
GLOBAL	Within the same WeSQL WeScale cluster, ensure that any read request can read previous write operations.

Setting via launch parameters

If you need to set the default value of read_write_splitting_policy, you can pass it as a startup parameter for the vtgate process: