> ## Documentation Index
> Fetch the complete documentation index at: https://checklyhq.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Flask

> Learn how to instrument your Flask code with OpenTelemetry.

This guide will help you instrument your Flask application(s) with OpenTelemetry and send traces to Checkly.

## Step 1: Install the OpenTelemetry SDK

Install the relevant OpenTelemetry packages:

```bash Terminal theme={null}
pip install opentelemetry-distro \
    opentelemetry-exporter-otlp
```

Use the `opentelemetry-bootstrap` command to automatically install any OTel libraries based on your current Python app.

```bash Terminal theme={null}
opentelemetry-bootstrap --action=install
```

## Step 2: Initialize the instrumentation

Notice the `HttpHeaderSampler` class. This is a custom, head-based sampler that will only sample spans that are generated
by Checkly by inspecting the trace state. This way you only pay for the egress traffic generated by Checkly and not for
any other traffic.

```python flask-instrumentation.py theme={null}
from opentelemetry import trace
from opentelemetry.sdk.trace import TracerProvider
from opentelemetry.exporter.otlp.proto.http.trace_exporter import OTLPSpanExporter
from opentelemetry.sdk.trace.sampling import Sampler, SamplingResult, Decision
from opentelemetry.sdk.trace.export import BatchSpanProcessor
from opentelemetry.instrumentation.flask import FlaskInstrumentor
from opentelemetry.instrumentation.requests import RequestsInstrumentor
from flask import Flask, request

class HttpHeaderSampler(Sampler):
    def get_description(self) -> str:
        return "HttpHeaderSampler"
    def should_sample(
            parent_context, trace_id, name, kind=None, attributes=None, links=None, trace_state=None
    ) -> SamplingResult:
        if request.headers.get('tracestate') == 'checkly=true':
            return SamplingResult(Decision.RECORD_AND_SAMPLE)
        else:
            return SamplingResult(Decision.DROP )

trace.set_tracer_provider(TracerProvider(sampler=HttpHeaderSampler()))
trace.get_tracer_provider().add_span_processor(BatchSpanProcessor(OTLPSpanExporter()))

app = Flask(__name__)

FlaskInstrumentor().instrument_app(app, tracer_provider=trace.get_tracer_provider())
RequestsInstrumentor().instrument()

# define routes etc.
```

## Step 3: Start your app with the instrumentation

Toggle on **Import Traces** and grab your OTel API key in the **OTel API keys** section of the [Traces page in the Checkly app](https://app.checklyhq.com/settings/account/traces) and
take a note of the endpoint for the region you want to use.

<img src="https://mintcdn.com/checkly-422f444a/WJFfxOuqqqZjE4cw/images/docs/images/otel/traces_import_api_keys.png?fit=max&auto=format&n=WJFfxOuqqqZjE4cw&q=85&s=a99a491e892083bd9fd23cc116b03599" alt="Checkly OTEL API keys" width="1002" height="492" data-path="images/docs/images/otel/traces_import_api_keys.png" />

Now, export your API key in your shell by setting the `OTEL_EXPORTER_OTLP_HEADERS` environment variable.

```bash Terminal theme={null}
export OTEL_EXPORTER_OTLP_HEADERS="authorization=<your-api-key>"
```

Next, export the endpoint for the region you want to use and give your service a name.

```bash Terminal theme={null}
export OTEL_EXPORTER_OTLP_ENDPOINT="https://otel.eu-west-1.checklyhq.com"
export OTEL_SERVICE_NAME="your-service-name"
```

Then, explicitly set the protocol to use for the OTLP exporter.

```bash Terminal theme={null}
export OTEL_EXPORTER_OTLP_PROTOCOL="http/protobuf"
```

We are using the standard OpenTelemetry environment variables here to configure the OTLP exporter.

| Variable                      | Description                                                            |
| ----------------------------- | ---------------------------------------------------------------------- |
| `OTEL_EXPORTER_OTLP_HEADERS`  | The `Authorization` HTTP header containing your Checkly OTel API key.  |
| `OTEL_EXPORTER_OTLP_ENDPOINT` | The Checkly OTel API endpoint for the region you want to use.          |
| `OTEL_EXPORTER_OTLP_PROTOCOL` | The protocol to use for the OTLP exporter.                             |
| `OTEL_SERVICE_NAME`           | The name of your service to identify it among the spans in the web UI. |

Finally, start your app with the Flask CLI:

```bash Terminal theme={null}
flask run 
```

🎉 You are done. Any interactions with your app that are triggered by a Checkly synthetic monitoring check will now generate
traces, which are sent back to Checkly and displayed in the Checkly UI.