processor_sampling: new trace sampling processor

[edsiper]

This patch introduces a new trace sampling processor designed with a pluggable architecture, allowing easy extension to support multiple sampling strategies and backends.

The initial implementation includes basic probabilistic sampling, with future patches planned to add additional sampling methods such as rate-limiting, latency-based, and tail-based sampling.

The probabilistic sampler can be configured as follows:

pipeline:
  inputs:
    - name: opentelemetry
      port: 4318

      processors:
        traces:
          - name: sampling
            type: probabilistic
            debug: true
            rules:
               sampling_percentage: 40

  outputs:
    - name: stdout
      match: '*'

in this configuration:

• debug mode (debug: true) is enabled, allowing detailed logging of sampling decisions.
• sampling_percentage: 40 ensures that 40% of traces are retained, while the rest are discarded.
• traces that pass sampling will be forwarded to the stdout output for visibility.

Fluent Bit v4.0.0
* Copyright (C) 2015-2024 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

______ _                  _    ______ _ _             ___  _____
|  ___| |                | |   | ___ (_) |           /   ||  _  |
| |_  | |_   _  ___ _ __ | |_  | |_/ /_| |_  __   __/ /| || |/' |
|  _| | | | | |/ _ \ '_ \| __| | ___ \ | __| \ \ / / /_| ||  /| |
| |   | | |_| |  __/ | | | |_  | |_/ / | |_   \ V /\___  |\ |_/ /
\_|   |_|\__,_|\___|_| |_|\__| \____/|_|\__|   \_/     |_(_)___/

[2025/02/28 16:46:00] [ info] [fluent bit] version=4.0.0, commit=0e885e2d60, pid=778903 [2025/02/28 16:46:00] [ info] [storage] ver=1.5.2, type=memory, sync=normal, checksum=off, max_chunks_up=128
[2025/02/28 16:46:00] [ info] [simd    ] disabled
[2025/02/28 16:46:00] [ info] [cmetrics] version=0.9.9
[2025/02/28 16:46:00] [ info] [ctraces ] version=0.6.0
[2025/02/28 16:46:00] [ info] [input:opentelemetry:opentelemetry.0] initializing
[2025/02/28 16:46:00] [ info] [input:opentelemetry:opentelemetry.0] storage_strategy='memory' (memory only)
[2025/02/28 16:46:00] [ info] [input:opentelemetry:opentelemetry.0] listening on 0.0.0.0:4318
[2025/02/28 16:46:00] [ info] [processor:sampling:sampling.0] initializing probabilistic sampling processor
[2025/02/28 16:46:00] [ info] [sp] stream processor started
[2025/02/28 16:46:00] [ info] [output:stdout:stdout.0] worker #0 started

🔍 Debug sampling 'probabilistic' (0x779068027940): before
   ┌─────────────────────────────────────────────────────────────────┐
   │ trace_id=5b8efff798038103d269b633813fc60c                       │
   ├─────────────────────────────────────────────────────────────────┤
   │ spans:                                                          │
   │   ├── id=eee19b7ec3c1b174 name=I'm a server span                │
   │   ├── id=eee19b7ec3c1b175 name=Child span of server span        │
   │   ├── id=eee19b7ec3c1b176 name=Database query                   │
   └─────────────────────────────────────────────────────────────────┘

   ┌─────────────────────────────────────────────────────────────────┐
   │ trace_id=6a9dfff798038103d269b633813fc60d                       │
   ├─────────────────────────────────────────────────────────────────┤
   │ spans:                                                          │
   │   ├── id=fff19b7ec3c1b174 name=A span in another trace          │
   └─────────────────────────────────────────────────────────────────┘

   ┌─────────────────────────────────────────────────────────────────┐
   │ trace_id=7c8efff798038103d269b633813fc60e                       │
   ├─────────────────────────────────────────────────────────────────┤
   │ spans:                                                          │
   │   ├── id=0000000000000000 name=Slow request                     │
   └─────────────────────────────────────────────────────────────────┘

   ┌─────────────────────────────────────────────────────────────────┐
   │ trace_id=8d9efff798038103d269b633813fc60f                       │
   ├─────────────────────────────────────────────────────────────────┤
   │ spans:                                                          │
   │   ├── id=0000000000000000 name=High traffic span                │
   │   ├── id=0000000000000000 name=Load testing event               │
   └─────────────────────────────────────────────────────────────────┘

   ┌─────────────────────────────────────────────────────────────────┐
   │ trace_id=9a1bfff798038103d269b633813fc610                       │
   ├─────────────────────────────────────────────────────────────────┤
   │ spans:                                                          │
   │   ├── id=0000000000000000 name=Faulty transaction               │
   │   ├── id=0000000000000000 name=Database rollback                │
   └─────────────────────────────────────────────────────────────────┘

🔍 Debug sampling 'probabilistic' (0x779068027940): after
   ┌─────────────────────────────────────────────────────────────────┐
   │ trace_id=6a9dfff798038103d269b633813fc60d                       │
   ├─────────────────────────────────────────────────────────────────┤
   │ spans:                                                          │
   │   ├── id=fff19b7ec3c1b174 name=A span in another trace          │
   └─────────────────────────────────────────────────────────────────┘

   ┌─────────────────────────────────────────────────────────────────┐
   │ trace_id=7c8efff798038103d269b633813fc60e                       │
   ├─────────────────────────────────────────────────────────────────┤
   │ spans:                                                          │
   │   ├── id=0000000000000000 name=Slow request                     │
   └─────────────────────────────────────────────────────────────────┘

|-------------------- RESOURCE SPAN --------------------|
  resource:
     - attributes: - service.name: 'other.service'
     - dropped_attributes_count: 0 - schema_url: "" [scope_span] instrumentation scope:
        - name                    : other.library
        - version                 : 2.0.0
        - dropped_attributes_count: 0
        - attributes: undefined
    schema_url: ""
    [spans]
         [span #0 'A span in another trace']
             - trace_id                : 6a9dfff798038103d269b633813fc60d
             - span_id                 : fff19b7ec3c1b174
             - parent_span_id          : undefined
             - kind                    : 2 (server)
             - start_time              : 1544712660000000000
             - end_time                : 1544712662000000000
             - dropped_attributes_count: 0
             - dropped_events_count    : 0
             - dropped_links_count     : 0
             - trace_state             : (null)
             - status:
                 - code    : 0
             - attributes: none
             - events: none
             - [links]
|-------------------- RESOURCE SPAN --------------------|
  resource:
     - attributes:
            - service.name: 'latency.test.service'
     - dropped_attributes_count: 0
     - schema_url: ""
  [scope_span]
    instrumentation scope:
        - name                    : latency.test.library
        - version                 : 3.0.0
        - dropped_attributes_count: 0
        - attributes: undefined
    schema_url: ""
    [spans]
         [span #0 'Slow request']
             - trace_id                : 7c8efff798038103d269b633813fc60e
             - span_id                 : 0000000000000000
             - parent_span_id          : undefined
             - kind                    : 2 (server)
             - start_time              : 1544712660000000000
             - end_time                : 1544712675000000000
             - dropped_attributes_count: 0
             - dropped_events_count    : 0
             - dropped_links_count     : 0
             - trace_state             : (null)
             - status:
                 - code    : 0
             - attributes: none
             - events: none
             - [links]

Manual test

Using this JSON file: trace_sampling_extended.json , try with curl:

curl -X POST -H "Content-Type: application/json" -d @trace_sampling_extended.json -i localhost:4318/v1/traces

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.