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