*: support for limiting the number of client connections

[zimulala]

What problem does this PR solve?

Limiting the number of connections is to control that there should not be too many connections on a single server, resulting in resource occupation. This feature is similar to max_connections in MySQL, but it only controls the current server.

What is changed and how it works?

Adds a check on the connection count when a connection is established.

Check List

Tests

• unit test

Code changes

• Has interface methods change

Release note

• Support for limiting the number of client connections

[crazycs520]

LGTM

[lonng]

Why 151?

[zimulala]

It's 151 in our original code and in MySQL.

[wwar]

The reason is historical. When the apache default was 150, and MySQL was 100, there were cases where web pages would contain too many connection errors. MySQL raised its limit to Apache's +1. It does not necessarily need to be followed literally.

[jackysp]

LGTM

[wwar]

This has an important behavior difference from MySQL, which is that in MySQL the max_connections cap only applies to regular users. There will always be an extra connection for super privileged users so that an administrator can log in and kill connections if there has been a thundering herd problem.

In MySQL 8.0 this was extended to provide a separate admin port, which is a better implementation: https://dev.mysql.com/worklog/task/?id=12138

[jackysp]

Hold on.

[jackysp]

This has an important behavior difference from MySQL, which is that in MySQL the max_connections cap only applies to regular users. There will always be an extra connection for super privileged users so that an administrator can log in and kill connections if there has been a thundering herd problem.

In MySQL 8.0 this was extended to provide a separate admin port, which is a better implementation: https://dev.mysql.com/worklog/task/?id=12138

Yes, it maybe works like the extra port in MariaDB or Percona. We could discuss it in another issue.

[coocood]

The connection limit applies to a single server, I think put it into the config file is better.

[jackysp]

The connection limit applies to a single server, I think put it into the config file is better.

OK, it is better to use a different name, e.g. server-max-connections

[zimulala]

@coocood @jackysp PTAL. I update it as a new config variable.

[coocood]

I think set it higher like 4096 is better for compatibility.

[zimulala]

Done. PTAL @coocood

[coocood]

LGTM

[jackysp]

/merge

[sre-bot]

/run-all-tests