Add admin-port to let administrator connect to the server even maxclient number is reached

[hwware]

Background:

The parameter "maxclient" default value is 10000. Client administrators could set higher value to accept more client connections.
However, there is one situation: If the connection number of client reaches the maxclient, then even administrators have no chance to connect to the server to process some urgent issues or update some configurations.

To solve above case, by connecting to the admin port, an administrator or cloud provider has the chance to connect to the server even the max number of connected clients reaches

Of course, there is one risk by introducing the admin port: hacker has one more chance to invade the Valkey server, but
I think this kind of risky is less than what we thought. Because in standlone mode, primary node could expose ports to multiply replicas and sentinel nodes, and in cluster mode. primary node has port to connect with other nodes. Thus one more port to connect with administrator is not a big issue.

[codecov]

Codecov Report

Attention: Patch coverage is 65.21739% with 8 lines in your changes missing coverage. Please review.

Project coverage is 70.68%. Comparing base (6b3e122) to head (8ac9019).
Report is 1 commits behind head on unstable.

Files with missing lines	Patch %	Lines
src/networking.c	36.36%	7 Missing ⚠️
src/server.c	91.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1120      +/-   ##
============================================
- Coverage     70.83%   70.68%   -0.16%     
============================================
  Files           118      118              
  Lines         63561    63583      +22     
============================================
- Hits          45025    44943      -82     
- Misses        18536    18640     +104     

Files with missing lines	Coverage Δ
src/config.c	77.63% <ø> (ø)
src/server.h	100.00% <ø> (ø)
src/server.c	87.40% <91.66%> (-0.02%)	⬇️
src/networking.c	88.29% <36.36%> (-0.24%)	⬇️

... and 13 files with indirect coverage changes

[soloestoy]

connections from admin-port can consume clients out of maxclients, but the eventloop's size is fixed, do we need to make eventloop to be auto resizing?

[hwware]

Hi @soloestoy,

Yes, you’re correct—the event loop size is fixed. However, since the port is known only to the administrator and accepts only local connections, we don't anticipate a large number of clients on the port. Additionally, we've reserved 96 extra FDs in the event loop, which I believe should be sufficient to handle the expected client load on the port. ref:

valkey/src/server.c

Line 2634 in 9b8a061

server.el = aeCreateEventLoop(server.maxclients + CONFIG_FDSET_INCR);

That said, I think I overlooked this. Do you think we should track and add a condition for the number of clients connected to the port?

[zuiderkwast]

We can add a fixed number and limit the number of admin connections. We don't need to count the admin connections separately. We can just accept connections on the admin port as long as the total number of connections is less than for example server.maxclients + 20.

[hwware]

Added a check for the number of connections accepted on the port. Currently, it's set to 20, but I believe 10 would be sufficient. Additionally, the event loop size also been updated based on the maximum number of clients accepted on the port.

[PingXie]

I think we will also need to track admin clients separately from server.clients or we are eating into the actual end-user client quote, something like below, assuming server.clients is the total

if (port != server.admin_port && 
    (listLength(server.clients) - listLength(server.admin_clients) + getClusterConnectionsCount() >= server.maxclients)) 

[zuiderkwast]

@PingXie Why is that a problem? Do you think clients are surprised they can make fewer connections because an admin is connected?

Or do you think the admins need to be completely invisible to other clients, like not shown in CLIENT LIST, etc.?

I think the admins can be visible and I don't see why it's a problem that a admin eating the users' quota is a problem.

[hwware]

Hello @PingXie, We have fixed the number of admin clients to 20(it can be adjusted) and could not see how it will impact the client quota. As Viktor pointed, do you want to hide admins from end user clients?

[madolson]

At amazon we have a fixed number of clients ontop of what is requested as well (although I believe we chose 256). It's worth mentioning that clusterbus connections also eat into the overhead.

[PingXie]

Should we close the discussion at valkey-io/valkey-rfc#3 (comment)?

[PingXie]

I think we will also need to track admin clients separately from server.clients or we are eating into the actual end-user client quote, something like below, assuming server.clients is the total

if (port != server.admin_port && 
    (listLength(server.clients) - listLength(server.admin_clients) + getClusterConnectionsCount() >= server.maxclients))