mellea-logging

Development

Best-practices guide for adding or reviewing logging in the Mellea codebase. Covers when to use log_context() vs a dedicated logger call, canonical field names, reserved attribute constraints, async/thread safety, and what events deserve dedicated log lines. Use when: adding a new log call; reviewing a PR that touches MelleaLogger; deciding where to inject context fields; debugging why a field is missing from a log record; or ensuring consistency with the project logging conventions.

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Mellea Logging Best Practices

All logging in Mellea flows through MelleaLogger.get_logger(), defined in mellea/core/utils.py. This skill documents the conventions for adding and reviewing log instrumentation.

Quick reference

from mellea.core import MelleaLogger, log_context, set_log_context, clear_log_context

logger = MelleaLogger.get_logger()

# Dedicated log call — for a discrete event
logger.info("SUCCESS")

# Context injection — attach fields to every record in a scope
with log_context(request_id="req-abc", trace_id="t-1"):
    logger.info("Starting generation")  # includes request_id, trace_id
    # ... all nested calls inherit these fields automatically

When to add a dedicated log call

Use logger.info/warning/error(...) for discrete, named events:

Event type	Level	Example
Phase transition	INFO	`"SUCCESS"`, `"FAILED"`, `"Starting session"`
Loop progress	INFO	`"Running loop 2 of 3"`
Recoverable issue	WARNING	`"Warmup failed for model: ..."`
Unexpected failure	ERROR	exception tracebacks, hard failures
Verbose diagnostics	DEBUG	token counts, prompt previews

Do not add log calls for:

Values already captured in a log_context field (redundant noise)
Internal helper functions where the calling function already logs the event
State that is already reflected in telemetry spans

When to use log_context

Use log_context (or set_log_context) to attach identifiers and metadata that should appear on every log record within a scope — without threading them through every call.

Typical injection points:

Scope	Where to inject	Fields
Session lifetime	`MelleaSession.__enter__`	`session_id`, `backend`, `model_id`
Sampling loop	`BaseSamplingStrategy.sample()`	`strategy`, `loop_budget`
HTTP request handler	entry point of the handler	`request_id`, `trace_id`
Background task	top of the task coroutine	`task_id`, `job_name`

Canonical field names

Use these names consistently. Do not invent synonyms.

Field	Type	Description
`session_id`	str (UUID)	Unique ID for a `MelleaSession`
`backend`	str	Backend class name, e.g. `"OllamaModelBackend"`
`model_id`	str	Model identifier string
`strategy`	str	Sampling strategy class name
`loop_budget`	int	Max generate/validate cycles for this sampling call
`request_id`	str	Caller-supplied request identifier
`trace_id`	str	Distributed trace ID (from OpenTelemetry or caller)
`span_id`	str	Span ID within a trace
`user_id`	str	End-user identifier (when applicable)

Reserved attribute names — do not use as context fields

The following names are standard logging.LogRecord attributes. Passing them to log_context() or set_log_context() raises ValueError. See RESERVED_LOG_RECORD_ATTRS in mellea/core/utils.py for the full set.

args, created, exc_info, exc_text, filename, funcName, levelname, levelno, lineno, message, module, msecs, msg, name, pathname, process, processName, relativeCreated, stack_info, thread, threadName

Prefer the context manager over set/clear

# Preferred — guaranteed cleanup even on exceptions
with log_context(trace_id="abc"):
    do_work()

# Acceptable only when lifetime equals __enter__/__exit__
# (e.g. MelleaSession, where the CM already guarantees cleanup)
set_log_context(session_id=self.id)
# ... later in __exit__ ...
clear_log_context()

The context manager uses a ContextVar token to restore the previous state on exit. This means nesting works correctly — inner calls can add fields without clobbering the outer scope's values.

Async and thread safety

log_context uses contextvars.ContextVar, which is safe for concurrent asyncio tasks:

Each asyncio.Task gets its own copy of the context.
Fields set in one task do not bleed into sibling tasks.

Plugin hooks: Mellea hooks (AUDIT, SEQUENTIAL, CONCURRENT) are awaited in the same asyncio task as the call site. ContextVar state IS inherited — fields set around a strategy.sample() call will appear on records emitted inside hook handlers automatically.

Checklist before committing

New log calls use MelleaLogger.get_logger(), not logging.getLogger(...).
Context fields use canonical names from the table above.
No reserved attribute names passed to log_context.
Scoped fields use with log_context(...), not set_log_context (unless managing an __enter__/__exit__ pair).
Hook handlers that need context set it internally — they do not inherit the caller's context.
New events that span multiple log records inject fields via context, not by repeating them on every logger.info(...) call.