Add support for command CLIENT LIST

[JyotinderSingh]

Add support for the CLIENT LIST command in DiceDB similar to the CLIENT LIST command in Redis. Please refer to the following commit in Redis to understand the implementation specifics - source.

Write unit and integration tests for the command referring to the tests written in the Redis codebase 7.2.5. For integration tests, you can refer to the tests folder. Note: they have used TCL for the test suite, and we need to port that to our way of writing integration tests using the relevant helper methods. Please refer to our tests directory.

For the command, benchmark the code and measure the time taken and memory allocs using benchmem and try to keep them to the bare minimum.

[kakdeykaushik]

please assign this to me

[swarajrb7]

@JyotinderSingh can you unassign this issue #1129 and assign this issue to me
i want to try mid-level issue

[JyotinderSingh]

please assign this to me

Assigned.

[JyotinderSingh]

@JyotinderSingh can you unassign this issue #1129 and assign this issue to me i want to try mid-level issue

Hey @swarajrb7, assigning this to @kakdeykaushik since they commented first. Will be adding more mid level issues and will tag you there.

[JyotinderSingh]

Hey, it may be good to discuss the design before you begin working on the implementation. We need to ensure this does not introduce any performance regressions on the critical path.
Feel free to let us know on discord if you'd like to discuss this in a larger group.

[kakdeykaushik]

I have been doing some research on how redis behaves and implements this command, redis's client management impl and what all syscall and socket options we can use to get required information.

Also implemented other simpler CLIENT command CLIENT ID, CLIENT GETNAME & CLIENT SETNAME to get some understanding.

Background

We don't store all the connected clients to dicedb server. Except Async server which is exclusive for clients conncted to it(connectedClients @ server.go#L36).
For HTTP server we just create a client but never store it.
For WS server post #1090 we will be creating client object but only if they have triggered QWATCH command. We would need to create object for all the clients. Again we never store it.

Approach

A global list of all the connected clients which would be modified only when client connects/disconnects and read only when CLIENT LIST command is fired.

Extra - If we implement autoincrement client id we can optimize read/write ops of this global list. Implementation - #1157.

Implementation details and impact(for critical path) for each fields are mentioned.
I have checkboxed the ones whose Redis behaviour I'm aware of and our approach that I arrived at. Descoped won't be implemented in dicedb as we don't support relevant functionalities. (please correct me if I have missed something for example last time i checked we were not going to impl transactions but i still see them around.)
For unchecked fields i have to spend more time.

• id: a unique 64-bit client ID

  • ClientIdentifierID field of Client struct
  • impact - none

• addr: address/port of the client

  • syscall.Getsockname(fd) / syscall.Getpeername(fd)
  • impact - none

• laddr: address/port of local address client connected to (bind address)

  • syscall.Getsockname(fd) / syscall.Getpeername(fd)
  • impact - none

• fd: file descriptor corresponding to the socket

  • fd for async server, for HTTP and WS server

  f, _ := conn.NetConn().(*net.TCPConn).File()
  fd := f.Fd()

  • impact - none

• name: the name set by the client with CLIENT SETNAME

  • need a varibale in Client struct - name
  • impact - none

• age: total duration of the connection in seconds

  • need a varibale in Client struct - creation_time
  • impact - none

• idle: idle time of the connection in seconds

  • need a varibale in Client struct - LastInteraction (preferred) OR
  • need to enable SO_TIMESTAMP for every FD - with this kernel will keep track of timestamp for each packet that goes through (we can do socket level monitoring with this, but overkill as of now i feel and throughput/latency would be impacted negatively)
  • impact - update LastInteraction every time read/write happens on that client (Redis also does the same)

• flags: client flags (see below)

• [-] db: current database ID

  • descoped

• [-] sub: number of channel subscriptions

  • descoped

• [-] psub: number of pattern matching subscriptions

  • descoped

• [-] ssub: number of shard channel subscriptions. Added in Redis 7.0.3

  • descoped

• [-] multi: number of commands in a MULTI/EXEC context

  • descoped

• [-] watch: number of keys this client is currently watching. Added in Redis 7.4

  • descoped

• qbuf: query buffer length (0 means no query pending)

• qbuf-free: free space of the query buffer (0 means the buffer is full)

• argv-mem: incomplete arguments for the next command (already extracted from query buffer)

• [-] multi-mem: memory is used up by buffered multi commands. Added in Redis 7.0

  • descoped

• obl: output buffer length

• oll: output list length (replies are queued in this list when the buffer is full)

• omem: output buffer memory usage

• tot-mem: total memory consumed by this client in its various buffers

• events: file descriptor events (see below)

• cmd: last command played

  • need a varibale in Client struct - LastCmd
  • impact - update LastCmd everytime a command is fired by client
  • note to self - if last command is invalid this returns null

• user: the authenticated username of the client

  • we don't have user management in place
  • default value - default
  • impact - none

• redir: client id of current client tracking redirection

• resp: client RESP protocol version. Added in Redis 7.0

[kakdeykaushik]

Values qbuf, qbuf-free, argv-mem, mutli-mem, obl, oll, omem, tot-mem are derived from querybuf, buf and relevent fields from Redis's client struct.
Primary purpose of these fields is to keep track of size of query / response to keep memory consumption in check and even terminate the client connection when consumption is too high.

Implementing these functionalities in DiceDB may impact performance since for each client we would have this buffer overhead and operating them for every query. On the plus side having these functionalities can help terminate clients which are faulty (for lack of better word).

argv-mem - args length. Example - CLIENT LIST = 10 (6+4).
multi-mem - same as argv-mem but for MULTI command.

For these fields we should have discussion, for rest of the fields I'll start the implementing them.