Logging

Configure Mellea’s console logging and export logs to OTLP collectors.

Prerequisites: Telemetry introduces the environment variables and telemetry architecture. This page covers logging configuration in detail.

Mellea provides two logging layers: a built-in console logger for local development and an optional OTLP exporter for centralized log aggregation. Both work simultaneously when enabled.

Console logging

Mellea uses MelleaLogger, a color-coded singleton logger built on Python’s logging module. All internal Mellea modules obtain their logger via MelleaLogger.get_logger().

Configuration

Variable	Description	Default
MELLEA_LOG_ENABLED	Master switch. Set to false / 0 / no to suppress all handlers (useful in test environments)	true
MELLEA_LOG_LEVEL	Log level name (e.g. DEBUG, INFO, WARNING)	INFO
MELLEA_LOG_JSON	Set to any truthy value (1, true, yes) to emit structured JSON instead of colour-coded output	unset
MELLEA_LOG_CONSOLE	Set to false / 0 / no to disable the console (stdout) handler	true
MELLEA_LOG_FILE	Absolute or relative path for rotating file output (e.g. /var/log/mellea.log). When unset no file handler is attached	unset
MELLEA_LOG_FILE_MAX_BYTES	Maximum size in bytes before the log file is rotated	10485760 (10 MB)
MELLEA_LOG_FILE_BACKUP_COUNT	Number of rotated backup files to keep	5
MELLEA_LOG_OTLP	Set to true / 1 / yes to export logs via OTLP. Requires opentelemetry-sdk and a configured OTLP endpoint	false
MELLEA_LOG_WEBHOOK	HTTP(S) URL to forward log records to via HTTP POST. Supersedes the deprecated MELLEA_FLOG and FLOG variables	unset
MELLEA_FLOG	Deprecated. Activates a webhook handler pointed at http://localhost:8000/api/receive. Use MELLEA_LOG_WEBHOOK instead	unset

Note: If MELLEA_LOG_FILE is set but the path cannot be opened (for example due to a permissions error or an invalid path), Mellea emits a UserWarning and continues without file logging. The application is never crashed by a misconfigured log path.

By default, MelleaLogger logs at INFO level with color-coded output to stdout. Set MELLEA_LOG_LEVEL to change the level:

export MELLEA_LOG_LEVEL=DEBUG
python your_script.py

Log format

Console output uses ANSI color codes by log level:

• Cyan — DEBUG
• Grey — INFO
• Yellow — WARNING
• Red — ERROR
• Bold red — CRITICAL

Each message is formatted as:

=== HH:MM:SS-LEVEL ======
message

Sample output

Console format (default)

Running m.instruct(...) inside a session produces lines like:

=== 11:11:25-INFO ======
SUCCESS

JSON format (MELLEA_LOG_JSON=1)

With structured JSON output enabled, the same SUCCESS record looks like:

{
  "timestamp": "2026-04-08T11:11:25",
  "level": "INFO",
  "message": "SUCCESS",
  "module": "base",
  "function": "sample",
  "line_number": 258,
  "process_id": 73738,
  "thread_id": 6179762176,
  "session_id": "550e8400-e29b-41d4-a716-446655440000",
  "backend": "OllamaModelBackend",
  "model_id": "granite4:micro",
  "strategy": "RejectionSamplingStrategy",
  "loop_budget": 3
}

The session_id, backend, model_id, strategy, and loop_budget fields are injected automatically when the call runs inside a with session: block. They appear on every log record within that scope.

Adding custom context fields

Use log_context to attach your own fields for the duration of a block:

from mellea.core import log_context

with log_context(request_id="req-abc", user_id="usr-42"):
    result = m.instruct("Summarise this document")
    # Every log record emitted here will include request_id and user_id

OTLP log export

When the [telemetry] extra is installed, Mellea can export logs to an OTLP collector alongside the existing console output. This is useful for centralizing logs from distributed services.

Note: OTLP logging is disabled by default. When disabled, there is zero overhead — no OTLP handler is created.

Enable OTLP logging

export MELLEA_LOG_OTLP=true
export OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4317

# Optional: log-specific endpoint (overrides general endpoint)
export OTEL_EXPORTER_OTLP_LOG_ENDPOINT=http://localhost:4318

# Optional: set service name
export OTEL_SERVICE_NAME=my-mellea-app

How it works

When MELLEA_LOG_OTLP=true, MelleaLogger adds an OpenTelemetry LoggingHandler alongside its existing handlers:

• Console handler — continues to work normally (color-coded output)
• REST handler — continues to work normally (when MELLEA_LOG_WEBHOOK is set)
• OTLP handler — exports logs to the configured OTLP collector

Logs are exported using OpenTelemetry’s Logs API with batched processing for efficiency.

Programmatic access

Use get_otlp_log_handler() to add OTLP log export to your own loggers:

import logging
from mellea.telemetry import get_otlp_log_handler

logger = logging.getLogger("my_app")
handler = get_otlp_log_handler()
if handler:
    logger.addHandler(handler)
    logger.info("This log will be exported via OTLP")

The function returns None when OTLP logging is disabled or not configured, so the if handler check is always safe.

OTLP collector setup example

cat > otel-collector-config.yaml <<EOF
receivers:
  otlp:
    protocols:
      grpc:
        endpoint: 0.0.0.0:4317

exporters:
  debug:
    verbosity: detailed
  file:
    path: ./mellea-logs.json

service:
  pipelines:
    logs:
      receivers: [otlp]
      exporters: [debug, file]
EOF

docker run -p 4317:4317 \
  -v $(pwd)/otel-collector-config.yaml:/etc/otelcol/config.yaml \
  -v $(pwd):/logs \
  otel/opentelemetry-collector:latest

Integration with observability platforms

OTLP logs work with any OTLP-compatible platform:

• Grafana Loki — log aggregation and querying
• Elasticsearch — log storage and analysis
• Datadog — unified logs, traces, and metrics
• New Relic — centralized logging
• Splunk — log analysis and monitoring

Performance

• Zero overhead when disabled: No OTLP handler is created, no performance impact.
• Batched export: Logs are batched and exported asynchronously.
• Non-blocking: Log export never blocks application code.
• Minimal overhead when enabled: OpenTelemetry’s efficient batching minimizes impact.

Troubleshooting

Logs not appearing in OTLP collector:

1. Verify MELLEA_LOG_OTLP=true is set.
2. Check that an OTLP endpoint is configured (OTEL_EXPORTER_OTLP_ENDPOINT or OTEL_EXPORTER_OTLP_LOG_ENDPOINT).
3. Verify the OTLP collector is running and configured to receive logs.
4. Check collector logs for connection errors.

Warning about missing endpoint:

WARNING: OTLP logs exporter is enabled but no endpoint is configured

Set either OTEL_EXPORTER_OTLP_ENDPOINT or OTEL_EXPORTER_OTLP_LOG_ENDPOINT.

Connection refused:

1. Verify the OTLP collector is running: docker ps | grep otel
2. Check the endpoint URL is correct (default: http://localhost:4317).
3. Verify network connectivity: curl http://localhost:4317

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See also:

• Telemetry — overview of all telemetry features and configuration.
• Tracing — distributed traces with Gen-AI semantic conventions.
• Metrics — metrics, exporters, and custom instruments.