feat(server): implement LATENCY command set (Redis 7.0+ compatibility)

[gongna-au]

Summary

This PR implements the LATENCY command for Kvrocks, focusing on the LATENCY HISTOGRAM subcommand introduced in Redis 7.0. It leverages the existing histogram infrastructure (histogram-bucket-boundaries config + CommandHistogram stats) to expose per-command latency distributions via the standard Redis protocol.

Changes

New File: src/commands/cmd_latency.cc

Implements CommandLatency with three subcommands:

• LATENCY HISTOGRAM [command ...] — Returns a cumulative distribution of command latencies.

  • Output uses the Redis 7.0 format: outer map keyed by command name, each value is a map with calls (integer) and histogram_usec (map of boundary → cumulative count).
  • Frequency counts from CommandHistogram::buckets are converted to cumulative sums at output time.
  • Empty leading buckets are skipped (matching Redis behavior of omitting zero-count ranges).
  • The last bucket (overflow) is rendered as "inf".
  • Returns an empty map if histogram-bucket-boundaries is not configured.

• LATENCY RESET [event ...] — Returns (integer) 0.

  • In Redis, this resets the spike-sampling event system (LATENCY LATEST/HISTORY/GRAPH), which Kvrocks does not have. Per Redis docs, histogram data is reset via CONFIG RESETSTAT, not LATENCY RESET. Returning 0 correctly indicates no event classes were reset.

• LATENCY HELP — Returns usage information for all supported subcommands.

Build System

No changes needed. The file is automatically picked up by file(GLOB_RECURSE KVROCKS_SRCS src/*.cc) in root CMakeLists.txt.

Command Registration

Registered under CommandCategory::Server via REDIS_REGISTER_COMMANDS(Server, ...), same category as cmd_server.cc. This is safe because the macro generates unique static variables per translation unit using __LINE__.

Compatibility

Subcommand	Redis	Kvrocks	Note
LATENCY HISTOGRAM [cmd ...]	7.0+	Yes	Cumulative distribution, histogram_usec field, empty buckets skipped
LATENCY HELP	7.0+	Yes
LATENCY RESET [event ...]	2.8.13+	Yes	Always returns 0 (no spike-sampling infrastructure)
LATENCY LATEST	2.8.13+	No	Requires spike-sampling event system
LATENCY HISTORY event	2.8.13+	No	Requires spike-sampling event system
LATENCY GRAPH event	2.8.13+	No	Requires spike-sampling event system
LATENCY DOCTOR	2.8.13+	No	Requires spike-sampling event system

Configuration Prerequisite

Histograms are only tracked when histogram-bucket-boundaries is set in kvrocks.conf. Example:

histogram-bucket-boundaries 1,5,10,25,50,100,250,500,1000,2500,5000,10000

If empty (default), LATENCY HISTOGRAM returns an empty map — this is expected behavior, not an error.

RESP Protocol Details

• Uses RESP3 Map (%) via conn->HeaderOfMap(), which auto-degrades to interleaved RESP2 arrays.
• Per-command entry structure:

  command_name => {
    "calls"          => (integer) N,
    "histogram_usec" => {
      boundary_1 => (integer) cumulative_count_1,
      boundary_2 => (integer) cumulative_count_2,
      ...
      "inf"      => (integer) total_calls
    }
  }
  

[git-hulk]

@gongna-au Could you please add Go tests to ensure the new command works as expected?

[gongna-au]

@git-hulk already

[jihuayu]

Greate. Please skip commands with calls == 0 in LATENCY HISTOGRAM [command ...]. Redis returns an empty reply for commands with no recorded samples, rather than an empty histogram entry.

I have a small question: why did you put this command in a separate file instead of keeping it in cmd_server?

[gongna-au]

@jihuayu Good point! I've moved CommandLatency into cmd_server.cc alongside other server diagnostic commands (SLOWLOG, PERFLOG, etc.). Also fixed the calls==0 skip for filtered histogram queries. Thanks for the review!

[sonarqubecloud]

Quality Gate passed

Issues
4 New issues
0 Accepted issues

Measures
0 Security Hotspots
74.2% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[jihuayu]

Hi @gongna-au. Would you be willing to update the command list? https://kvrocks.apache.org/docs/supported-commands

[gongna-au]

@jihuayu Sure, I can do that!💗

[gongna-au]

@jihuayu cc apache/kvrocks-website#371 done 💗