This feature is used to reject throughput when database has a high load so as

to keep database safe.

We introduced two system variables to help limit the database throughput. One

of the variables is 'threads_running_high_watermark', which is used to control

how many connections the database allowes. The other variable is

'threads_running_ctl_mode', which defines what type of query we would like to

block. It can be only SELECT query or all kinds queries(such as all DMLs etc.).

When connections are beyond 'thread_running_high_watermark', all of the new

sessions or the newly input queries(up to 'threads_running_ctl_mode'), will be

rejected, i.e. an error will be returned to client.

Note when 'threads_running_high_watermark' is 0, its value will be the same as

the value of system variable 'max_connections'.