fastotel
A Rust-backed drop-in for the OpenTelemetry Python SDK that takes tracing overhead off the request path.
Status: pre-alpha.
OTLPSpanProcessorsends every field of a span over OTLP/HTTP, encoded asOTLPSpanExporterencodes it, and batches asBatchSpanProcessordoes, but with no retries, gzip orOTEL_EXPORTER_OTLP_*configuration yet, and a process forked after the first span exports nothing from the child. The road to 1.0 is in #5.
Why
Instrumentation packages (FastAPI, httpx, SQLAlchemy, aiokafka, …) depend only on opentelemetry-api.
fastotel replaces parts of opentelemetry-sdk behind that API, so existing instrumentation keeps working.
In the pure-Python SDK, the BatchSpanProcessor worker thread encodes OTLP protobuf, gzips and sends spans
while holding the GIL, so it competes for CPU with request handling. In Rust, batching, encoding, compression
and export run on a thread that never takes the GIL.
Installation
pip install fastotel
Wheels are built for Linux (glibc and musl), macOS and Windows, x86_64 and arm64:
| Python | Wheel | Tested in CI |
|---|---|---|
| CPython 3.11, 3.12, 3.13, 3.14 | one abi3 wheel per platform, which also covers later versions |
yes |
| CPython 3.14t, free-threaded | cp314t; the GIL stays off after import |
yes |
| CPython 3.15 (pre-release) | the abi3 wheel |
yes |
| CPython 3.15t (pre-release) | none yet: builds from the sdist; a wheel ships with 3.15.0 | yes, from source |
CPython 3.10 reached end of life on 2026-10-01 and is not supported. Free-threaded 3.13t is not supported either: it was experimental, and PyO3 builds free-threaded extensions from 3.14 on.
Usage
fastotel replaces BatchSpanProcessor with OTLPSpanExporter; the SDK keeps creating spans:
from opentelemetry import trace
from opentelemetry.sdk.trace import TracerProvider
from fastotel import OTLPSpanProcessor
provider = TracerProvider()
provider.add_span_processor(OTLPSpanProcessor(endpoint="http://localhost:4318/v1/traces"))
trace.set_tracer_provider(provider)
endpoint is the URL spans are posted to, /v1/traces included, as for OTLPSpanExporter; the one above is the
default. on_end copies the span into Rust and returns; a native thread, started by the first span, batches,
encodes and sends spans without taking the GIL. As with BatchSpanProcessor, only sampled spans are exported. The
requests decode to what OTLPSpanExporter sends for the same spans; where they differ is in
ADR 0003.
Batching
The arguments and variables of BatchSpanProcessor, with its defaults, parsing and checks: an argument overrides
its variable, a variable that is not an integer gives the default and an error log on the fastotel logger, and a
value out of range raises ValueError from the constructor.
| Argument | Variable | Default | What it does |
|---|---|---|---|
max_queue_size |
OTEL_BSP_MAX_QUEUE_SIZE |
2048 | spans waiting for export; a span ended while the queue is full is dropped and counted (the count is public with #14), and on_end never blocks |
max_export_batch_size |
OTEL_BSP_MAX_EXPORT_BATCH_SIZE |
512 | a batch leaves as soon as it is full; at most max_queue_size |
schedule_delay_millis |
OTEL_BSP_SCHEDULE_DELAY |
5000 | otherwise, what is queued leaves this long after the previous export |
export_timeout_millis |
OTEL_BSP_EXPORT_TIMEOUT |
30000 | how long shutdown() waits for the last export |
Where this differs from BatchSpanProcessor (a full queue drops the newest span rather than the oldest, among
others) is in ADR 0004.
Development
Needs uv and a Rust toolchain (rustup).
make install # build the extension, install the dev dependencies
make test # run the tests, those of the Rust export pipeline (crates/export) too
make lint # ruff, mypy, cargo fmt, clippy
make bench # what the stock SDK costs an application, on a GIL and a free-threaded build (~30 min)
Every change that reaches master is a release: a pull request bumps the version in pyproject.toml
(uv version --bump patch) and adds its section to CHANGELOG.md.
License
1 maintainer
Metadata
1 maintainer
Release files for fastotel 0.6.0
For a detailed explanation of source distributions (sdists) and built distributions (wheels), please see the package formats documentation.
Source distribution (sdist)
| File | Size | Uploaded | |
|---|---|---|---|
| fastotel-0.6.0.tar.gz | 124.8 kB | Details |
Built distributions (wheels)
Total release size: 23.9 MB
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