Writing a span processor

Span processors must implement a function called process_spans in order for rotel to execute your processor. Each time process_spans is called your processor will be handed an instance of the ResourceSpan class for you to manipulate as you like.

ResourceSpan processor example

The following is an example OTel ResourceSpan processor called append_resource_attributes_to_spans.py which adds the OS name, version, and a timestamp named rotel.process.time to the Resource Attributes of a batch of Spans. Open up your editor or Python IDE and paste the following into a file called append_resource_attributes_to_spans.py and run with the following command.

append_resource_attributes_to_spans.py

import platform
from datetime import datetime

from rotel_sdk.open_telemetry.resource.v1 import Resource
from rotel_sdk.open_telemetry.common.v1 import KeyValue, AnyValue
from rotel_sdk.open_telemetry.trace.v1 import ResourceSpans


def process_spans(resource_spans: ResourceSpans):
    resource = resource_spans.resource
    # If resource is None, we'll create a new one to store our attributes, otherwise we'll
    # append to the existing Resource
    if resource is None:
        resource = Resource()

    current_time = datetime.now().strftime("%Y-%m-%d %H:%M:%S")
    os_name = platform.system()
    os_version = platform.release()
    # Add attributes
    resource.attributes.append(KeyValue("os.name", AnyValue(os_name)))
    resource.attributes.append(KeyValue("os.version", AnyValue(os_version)))
    resource.attributes.append(KeyValue("rotel.process.time", AnyValue(current_time)))

Start rotel and the processor with the following command.

./rotel start --exporter blackhole --otlp-with-trace-processor ./append_resource_attributes_to_spans.py --debug-log traces --debug-log-verbosity detailed

If you haven't installed telemetrygen yet, use the following command to install

go install github.com/open-telemetry/opentelemetry-collector-contrib/cmd/telemetrygen@latest`

Now open a new terminal and run the following telemetrygen command

telemetrygen traces --otlp-endpoint 127.0.0.1:4317 --otlp-insecure

With debug-log enabled rotel will print out the ResourceSpans before and after executing your processor.

INFO OTLP payload before processing
ResourceSpans #0
Resource SchemaURL: https://opentelemetry.io/schemas/1.4.0
Resource attributes:
     -> service.name: Str(telemetrygen)
ScopeSpans #0
ScopeSpans SchemaURL: 
InstrumentationScope telemetrygen 
Span #0
    Trace ID       : e199fc80864d844728ebad88ccfadc67
    Parent ID      : 4e4fb0523f19a0d7
    ID             : a94838a53a9b92c0
    Name           : okey-dokey-0
    Kind           : SPAN_KIND_SERVER
Start time: 1750218567597408000
End time: 1750218567597531000
    Status code    : STATUS_CODE_UNSET
    Status message : 
Attributes:
     -> net.peer.ip: Str(1.2.3.4)
     -> peer.service: Str(telemetrygen-client)
Span #1
    Trace ID       : e199fc80864d844728ebad88ccfadc67
    Parent ID      : 
    ID             : 4e4fb0523f19a0d7
    Name           : lets-go
    Kind           : SPAN_KIND_CLIENT
Start time: 1750218567597408000
End time: 1750218567597531000
    Status code    : STATUS_CODE_UNSET
    Status message : 
Attributes:
     -> net.peer.ip: Str(1.2.3.4)
     -> peer.service: Str(telemetrygen-server)

INFO OTLP payload after processing
ResourceSpans #0
Resource SchemaURL: https://opentelemetry.io/schemas/1.4.0
Resource attributes:
     -> service.name: Str(telemetrygen)
     -> os.name: Str(Darwin)
     -> os.version: Str(23.3.0)
     -> rotel.process.time: Str(2025-06-17 20:49:27)
ScopeSpans #0
ScopeSpans SchemaURL: 
InstrumentationScope telemetrygen 
Span #0
    Trace ID       : e199fc80864d844728ebad88ccfadc67
    Parent ID      : 4e4fb0523f19a0d7
    ID             : a94838a53a9b92c0
    Name           : okey-dokey-0
    Kind           : SPAN_KIND_SERVER
Start time: 1750218567597408000
End time: 1750218567597531000
    Status code    : STATUS_CODE_UNSET
    Status message : 
Attributes:
     -> net.peer.ip: Str(1.2.3.4)
     -> peer.service: Str(telemetrygen-client)
Span #1
    Trace ID       : e199fc80864d844728ebad88ccfadc67
    Parent ID      : 
    ID             : 4e4fb0523f19a0d7
    Name           : lets-go
    Kind           : SPAN_KIND_CLIENT
Start time: 1750218567597408000
End time: 1750218567597531000
    Status code    : STATUS_CODE_UNSET
    Status message : 
Attributes:
     -> net.peer.ip: Str(1.2.3.4)
     -> peer.service: Str(telemetrygen-server)