Add new replicate module API to bypass command validation

[sungming2]

Issue #1175

Problem

Performance degradation occurs due to the sanity check(lookupCommandByCString) in the VM_Replicate function, which converts const char* into sds and free them for command dictionary lookup. This check is mainly used for debugging purpose, but it is unnecessary for trusted modules. The user seeks a way to bypass this check for performance gains.

Solution

Adding a module API ReplicateWithoutValidation allows trusted modules to bypass validation for performance improvements while retaining flexibility for handling failure scenarios on replicas.

Test

Unit test

Follow-up work

Valkey document update required: https://valkey.io/topics/modules-api-ref/#section-commands-replication-api

[sungming2]

Can I get some suggestions if this naming is too general

[hwware]

Is there benefit set the flag as enum, more flags later? Can we just use int skip (0 for non-skip, 1 for skip) ?

[zuiderkwast]

It's always complicated to have types like this in the module API. I believe we support backward and forward compatiblity for modules. I mean, a module built with the valkeymodule.h from Valkey 10 can run on Valkey 9 and vice versa. A module can check if some function exists in the current server by checking if the function is NULL, like if (ValkeyModule_ReplicateWithFlag != NULL) ..., but to check which flags are valid in an enum, we don't have a way for modules to check that.

I think Wen's suggestion with an int is simpler, but an even simpler API is to just add ValkeyModule_ReplicateWithoutValidataion. WDYT?

Internally, we can use an enum or int, but that's doesn't have to be part of the API.

[sungming2]

I think ValkeyModule_ReplicateWithoutValidataion is also good idea

[codecov]

Codecov Report

Attention: Patch coverage is 3.84615% with 25 lines in your changes missing coverage. Please review.

Project coverage is 70.90%. Comparing base (2a1a65b) to head (1f41ddd).
Report is 1 commits behind head on unstable.

Files with missing lines	Patch %	Lines
src/module.c	3.84%	25 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1357      +/-   ##
============================================
+ Coverage     70.78%   70.90%   +0.11%     
============================================
  Files           120      120              
  Lines         65046    65071      +25     
============================================
+ Hits          46045    46136      +91     
+ Misses        19001    18935      -66     

Files with missing lines	Coverage Δ
src/module.c	9.59% <3.84%> (-0.01%)	⬇️

... and 12 files with indirect coverage changes

[sungming2]

Open a pr for documentation of this: valkey-io/valkey-doc#192

[hwware]

Is there benefit set the flag as enum, more flags later? Can we just use int skip (0 for non-skip, 1 for skip) ?

[hwware]

If ValkeyModuleFlag can be set as int, here we can pass int value;

[hwware]

Pls move this function to just above the VM_Replicate, reference the function zsetInitLexRange. It is a helper function too.

[hwware]

The PR comes from a performance issue about current VM_Replicated, then could you pls provide the result how much performance improvement on this PR?

[sungming2]

I don't see notable performance improvement in the multiple tests (1. benchmark 2. CPU utilization, 3. CPU profiling with perf) so I wonder if it is worth introducing this new API. @hwware @hpatro

1. Benchmark

With validation

====== eval return redis.call("propagate-test.validation") 0 ======                                                     
  100000 requests completed in 0.71 seconds
  50 parallel clients
  74 bytes payload
  keep alive: 1
  host configuration "save": 3600 1 300 100 60 10000
  host configuration "appendonly": no
  multi-thread: no

Latency by percentile distribution:
0.000% <= 0.111 milliseconds (cumulative count 1)
50.000% <= 0.295 milliseconds (cumulative count 54440)
75.000% <= 0.335 milliseconds (cumulative count 75261)
87.500% <= 0.383 milliseconds (cumulative count 88850)
93.750% <= 0.407 milliseconds (cumulative count 94702)
96.875% <= 0.423 milliseconds (cumulative count 96981)
98.438% <= 0.479 milliseconds (cumulative count 98634)
99.219% <= 0.503 milliseconds (cumulative count 99282)
99.609% <= 0.599 milliseconds (cumulative count 99611)
99.805% <= 0.791 milliseconds (cumulative count 99811)
99.902% <= 0.999 milliseconds (cumulative count 99903)
99.951% <= 1.367 milliseconds (cumulative count 99952)
99.976% <= 1.615 milliseconds (cumulative count 99976)
99.988% <= 1.679 milliseconds (cumulative count 99989)
99.994% <= 1.687 milliseconds (cumulative count 99995)
99.997% <= 1.799 milliseconds (cumulative count 99997)
99.998% <= 1.831 milliseconds (cumulative count 99999)
99.999% <= 1.895 milliseconds (cumulative count 100000)
100.000% <= 1.895 milliseconds (cumulative count 100000)

Cumulative distribution of latencies:
0.000% <= 0.103 milliseconds (cumulative count 0)
3.697% <= 0.207 milliseconds (cumulative count 2697)
60.307% <= 0.303 milliseconds (cumulative count 60307)
94.702% <= 0.407 milliseconds (cumulative count 94702)
99.282% <= 0.503 milliseconds (cumulative count 99282)
99.620% <= 0.607 milliseconds (cumulative count 99620)
99.725% <= 0.703 milliseconds (cumulative count 99725)
99.826% <= 0.807 milliseconds (cumulative count 99826)
99.878% <= 0.903 milliseconds (cumulative count 99878)
99.904% <= 1.007 milliseconds (cumulative count 99904)
99.919% <= 1.103 milliseconds (cumulative count 99919)
99.928% <= 1.207 milliseconds (cumulative count 99928)
99.939% <= 1.303 milliseconds (cumulative count 99939)
99.962% <= 1.407 milliseconds (cumulative count 99962)
99.968% <= 1.503 milliseconds (cumulative count 99968)
99.975% <= 1.607 milliseconds (cumulative count 99975)
99.995% <= 1.703 milliseconds (cumulative count 99995)
99.998% <= 1.807 milliseconds (cumulative count 99998)
100.000% <= 1.903 milliseconds (cumulative count 100000)

Summary:
  throughput summary: 141043.72 requests per second
  latency summary (msec):
          avg       min       p50       p95       p99       max
        0.305     0.104     0.295     0.415     0.495     1.895

Bypass validation

====== eval return redis.call("propagate-test.novalidation") 0 ======                                                     
  100000 requests completed in 0.69 seconds
  50 parallel clients
  76 bytes payload
  keep alive: 1
  host configuration "save": 3600 1 300 100 60 10000
  host configuration "appendonly": no
  multi-thread: no

Latency by percentile distribution:
0.000% <= 0.119 milliseconds (cumulative count 1)
50.000% <= 0.279 milliseconds (cumulative count 52272)
75.000% <= 0.327 milliseconds (cumulative count 76134)
87.500% <= 0.367 milliseconds (cumulative count 87689)
93.750% <= 0.399 milliseconds (cumulative count 95285)
96.875% <= 0.423 milliseconds (cumulative count 97098)
98.438% <= 0.471 milliseconds (cumulative count 98588)
99.219% <= 0.543 milliseconds (cumulative count 99244)
99.609% <= 0.759 milliseconds (cumulative count 99619)
99.805% <= 0.927 milliseconds (cumulative count 99810)
99.902% <= 1.087 milliseconds (cumulative count 99903)
99.951% <= 1.271 milliseconds (cumulative count 99952)
99.976% <= 1.527 milliseconds (cumulative count 99977)
99.988% <= 1.575 milliseconds (cumulative count 99989)
99.994% <= 1.623 milliseconds (cumulative count 99994)
99.997% <= 1.663 milliseconds (cumulative count 99997)
99.998% <= 1.687 milliseconds (cumulative count 99999)
99.999% <= 1.719 milliseconds (cumulative count 100000)
100.000% <= 1.719 milliseconds (cumulative count 100000)

Cumulative distribution of latencies:
0.000% <= 0.103 milliseconds (cumulative count 0)
4.562% <= 0.207 milliseconds (cumulative count 4562)
67.223% <= 0.303 milliseconds (cumulative count 67223)
96.219% <= 0.407 milliseconds (cumulative count 96219)
99.021% <= 0.503 milliseconds (cumulative count 99021)
99.438% <= 0.607 milliseconds (cumulative count 99438)
99.562% <= 0.703 milliseconds (cumulative count 99562)
99.678% <= 0.807 milliseconds (cumulative count 99678)
99.787% <= 0.903 milliseconds (cumulative count 99787)
99.858% <= 1.007 milliseconds (cumulative count 99858)
99.910% <= 1.103 milliseconds (cumulative count 99910)
99.944% <= 1.207 milliseconds (cumulative count 99944)
99.954% <= 1.303 milliseconds (cumulative count 99954)
99.956% <= 1.407 milliseconds (cumulative count 99956)
99.973% <= 1.503 milliseconds (cumulative count 99973)
99.993% <= 1.607 milliseconds (cumulative count 99993)
99.999% <= 1.703 milliseconds (cumulative count 99999)
100.000% <= 1.807 milliseconds (cumulative count 100000)

Summary:
  throughput summary: 144300.14 requests per second
  latency summary (msec):
          avg       min       p50       p95       p99       max
        0.294     0.112     0.279     0.399     0.503     1.719

2. CPU utilization

• while continuously calling VM_Replicate/ReplicateWithFlag with benchmark tool (benchmark -n -l)

With validation

    PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND                                                    
  33349 sungming  20   0  144672  25056   5888 R  54.0   0.2  12:51.11 valkey-server    

    PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND                                                  
   1598 sungming  20   0  144672  25084   5700 R  93.3   0.2   0:30.32 valkey-server  

Bypass validation

    PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND                                                    
  33349 sungming  20   0  144672  25064   5888 S  52.8   0.2  13:29.67 valkey-server      

    PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND                                                  
   1598 sungming  20   0  144672  25116   5700 R  92.7   0.2   1:06.09 valkey-server

3. Performance profiling with perf

with validation

-    8.10%     0.10%  valkey-server    valkey-server       [.] VM_Replicate                                                    ▒
   - 8.00% VM_Replicate                                                                                                        ▒
      - 7.65% moduleReplicate                                                                                                  ▒
         - 3.08% lookupCommandBySdsLogic                                                                                       ▒
            - 1.21% dictFetchValue                                                                                             ▒
                 0.88% dictFind                                                                                                ▒
            - 1.10% sdssplitlen.constprop.1                                                                                    ▒
                 0.56% _sdsnewlen                                                                                              ▒
         - 1.91% moduleCreateArgvFromUserFormat                                                                                ▒
              1.12% createEmbeddedStringObject                                                                                 ▒
           0.85% _sdsnewlen                                                                                                    ▒
           0.60% alsoPropagate                    

bypass validation

-    3.84%     0.14%  valkey-server    valkey-server       [.] VM_ReplicateWithFlag                                            ▒
   - 3.70% VM_ReplicateWithFlag                                                                                                ▒
      - 3.47% moduleReplicate                                                                                                  ▒
         - 2.04% moduleCreateArgvFromUserFormat                                                                                ▒
              1.14% createEmbeddedStringObject                                                                                 ▒
              0.64% valkey_realloc                                                                                             ▒
           0.59% alsoPropagate                                                    

[hwware]

@valkey-io/core-team Do you think this is a proper solution?

[sungming2]

I added another CPU utilization test to push around 90%

[sungming2]

New performance test showing performance increase.
#1175 (comment)

[enjoy-binbin]

The new module api, if the command does not exist, the server will crash, right?

Are we considering adding flags to the client to bypass some checks? For example, a trusted client flag?

[zuiderkwast]

The new module api, if the command does not exist, the server will crash, right?

@enjoy-binbin I may be wrong, but I believe the primary can replica anything. The replica receiving it will try to execute it and will fail with an error which is normally just ignored. But there is a config that makes the replica crash:

# The propagation error behavior controls how the server will behave when it is
# unable to handle a command being processed in the replication stream from a primary
# or processed while reading from an AOF file. Errors that occur during propagation
# are unexpected, and can cause data inconsistency. 
#
# If an application wants to ensure there is no data divergence, this configuration
# should be set to 'panic' instead. The value can also be set to 'panic-on-replicas'
# to only panic when a replica encounters an error on the replication stream. One of
# these two panic values will become the default value in the future once there are
# sufficient safety mechanisms in place to prevent false positive crashes.
#
# propagation-error-behavior ignore

Are we considering adding flags to the client to bypass some checks? For example, a trusted client flag?

What do you mean? The module can send stuff to the replication stream even no actually command was sent by any real client. A module is always trusted. A module can do all kinds of dangerous things that can crash the server.

[enjoy-binbin]

right, sorry about this confusing wording, i got the context mixed up (reading other discussions at the same time). anyway, what i means is that, can we add flags to the client, or like ctx flags then can bypass some internal checks? i am not sure about it though, i am not that good at module, i am ok with the new module api if needed.

[zuiderkwast]

@valkey-io/core-team can we settle on the ReplicateWithoutValidation API? No flags added now. TSC please vote 👍 or 👎 .