Adding custom report filter

[JJ-Jasmin]

Sometimes we need a custom filter to filter the reported information to reduce the storage pressure

[wu-sheng]

Storage pressure should be taken care of by backend OAP capability.
If you don't want to trace at the beginning, a sampler makes a more sense before creating context.

[JJ-Jasmin]

for example: only submit requests below 100ms

[wu-sheng]

That is actually very tricky. You could make a lot of VNode when visuals trace.
Sampling usually is a tradeoff, but your point of reducing storage doesn't make sense to provide this feature.

[JJ-Jasmin]

some of the problems just mentioned should actually reduce the pressure on grpc to report

[JJ-Jasmin]

In the case of high QPS, grpc server needs a lot of resources. In fact, we can only collect and analyze requests with more than 100ms or errors

[wu-sheng]

But your way as post sampling actually could make things not as expected in the runtime.
When your service has performance impact, the sampler would report more and more segments, which slows the service more.
This is not a good practice in prod.

[wu-sheng]

In the case of high QPS, grpc server needs a lot of resources. In fact, we can only collect and analyze requests with more than 100ms or errors

How many QPS are you talking about? Do you really test the tracing cost VS reporting cost?

In most languages, tracing cost is almost 5-10 times than pure reporting.

[JJ-Jasmin]

In fact, the problem we encounter is that in 50000qps requests, we only need to track those wrong and slow requests, and the vast majority of requests (99.9%) do not need to be submitted to the tracking. If the full number of grpc submissions, the server received by grpc may need more resources, so we want to filter out (retain wrong and slow requests) when submitting to reduce the pressure of grpc reporting. Do you have a better approach?

[wu-sheng]

But that is not SkyWalking's typical use case. I am actually confused about your decision.
If you just need some traces of error or slow, and don't care about the analysis/metrics, why do you choose SkyWalking rather than many others out there?
SkyWalking's traces are designed for dependencies and metrics analysis, but it seems this is also not your purpose.
Then why do you choose SkyWalking, but use it like a pure tracing stack?

[wu-sheng]

Back to the codes, I think you need to update the doc at https://github.com/SkyAPM/go2sky#configuration about how to use this.

We need the doc to explain what is CustomReportStrategy. But first, this should be renamed to ReportStrategy.(Custom is not necessary).

[wu-sheng]

@arugal Could you review the codes? Although I am not supporting this use case, this seems harmless either.

I will leave the judgment for you.

[codecov-commenter]

Codecov Report

Merging #163 (282c26a) into master (6492c49) will decrease coverage by 0.11%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #163      +/-   ##
==========================================
- Coverage   64.64%   64.53%   -0.12%     
==========================================
  Files          22       22              
  Lines        1089     1094       +5     
==========================================
+ Hits          704      706       +2     
- Misses        330      334       +4     
+ Partials       55       54       -1     

Impacted Files	Coverage Δ
reporter/grpc.go	50.68% <0.00%> (-0.47%)	⬇️
reporter/grpc_opts.go	71.42% <0.00%> (-5.50%)	⬇️
sampler.go	81.96% <0.00%> (+3.27%)	⬆️

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[JJ-Jasmin]

Has been updated

[arugal]

LGTM