Logging from a MoonBit Agent

The MoonBit SDK provides a high-level @logging module (from golemcloud/golem_sdk/logging) built on top of wasi:logging/logging. It supports named loggers, log levels, and global level filtering.

Quick Start

The @logging package is already imported in the template moon.pkg. Use the module-level convenience functions:

@logging.info("order created: " + order_id)
@logging.debug("processing item: " + item_id)
@logging.warn("retry attempt " + attempt.to_string())
@logging.error("failed to connect: " + err)

Using a Named Logger

Create a logger with a context name for structured, scoped output. The context string is passed as the WASI context parameter:

let logger : @logging.Logger = @logging.with_name("order-service")

fn process_order(self : Self, order_id : String) -> Unit {
  logger.info("processing order: " + order_id)
  logger.debug("validating items")
}

Loggers are immutable values — with_name appends a /-separated segment to the context:

let db_logger = logger.with_name("database")
// context is "order-service/database"
db_logger.info("connected to database")

Log Levels

Function	WASI Level	Use for
@logging.trace(...) / logger.trace(...)	trace	Fine-grained control flow
@logging.debug(...) / logger.debug(...)	debug	Debugging information
@logging.info(...) / logger.info(...)	info	Monitoring, normal operations
@logging.warn(...) / logger.warn(...)	warn	Hazardous situations
@logging.error(...) / logger.error(...)	error	Serious errors
@logging.critical(...) / logger.critical(...)	critical	Fatal errors

Level Filtering

Set a global minimum log level to suppress noisy output:

@logging.set_min_level(INFO)  // suppresses trace and debug globally

Per-logger overrides are also supported:

let verbose_logger = logger.with_min_level(TRACE)

If a logger has no per-instance level, it uses the global minimum (default: TRACE).

Setting a Global Context

Set a default context string for all module-level logging functions:

@logging.set_context("my-agent")
@logging.info("started")  // context = "my-agent"

Using println

MoonBit's println also works and is captured as stdout:

println("stdout message")

This appears in golem agent stream output but lacks log levels and context. Prefer @logging for structured, filterable logging.

Viewing Logs

Stream live agent output:

golem agent stream '<agent-id>'

Or during an invocation:

golem agent invoke '<agent-id>' '<method>' [args]

Key Points

• No initialization needed — the @logging package is ready to use out of the box
• @logging is pre-imported in the template moon.pkg as "golemcloud/golem_sdk/logging" @logging
• Named loggers via @logging.with_name(...) provide scoped context strings
• Logs are recorded in the oplog and visible via golem agent stream and golem agent invoke
• Logging is a side effect — during replay (crash recovery), log calls from replayed operations are skipped