golem-logging-rust

name	golem-logging-rust
description	Adding logging to a Rust Golem agent. Use when the user asks about logging, log messages, log levels, the log crate, or printing debug/info/error output from a Rust agent.

Logging from a Rust Agent

Golem provides structured logging via the standard log crate. The SDK initializes the logger automatically when the agent starts — no setup code is needed.

Quick Start

The log crate is already in the project's Cargo.toml (added by golem new). Use the standard macros:

log::trace!("detailed flow: entered process_item");
log::debug!("processing item id={}", item.id);
log::info!("order created: {}", order_id);
log::warn!("retry attempt {} for request {}", attempt, req_id);
log::error!("failed to connect to database: {}", err);

Log Levels

Macro	Use for
log::trace!	Fine-grained control flow, variable values
log::debug!	Debugging information
log::info!	Monitoring, normal operations
log::warn!	Hazardous situations, degraded behavior
log::error!	Serious errors

Structured Key-Value Logging

The log crate's kv feature is enabled by default. Pass structured context as key-value pairs:

log::info!(agent_id = self.name, count = self.count; "counter incremented");
log::error!(operation = "db_query", table = "orders"; "query failed: {}", err);

Key-value pairs appear before the ; separator.

Using println! / eprintln!

Standard output and error streams also work and are captured by Golem:

println!("stdout message");   // captured as stdout
eprintln!("stderr message");  // captured as stderr

These appear in golem agent stream output but lack log levels and context. Prefer log::* macros for structured, filterable logging.

Viewing Logs

Stream live agent output (stdout, stderr, and log channels):

golem agent stream '<agent-id>'

Or during an invocation:

golem agent invoke '<agent-id>' '<method>' [args]
# Logs stream automatically; use --no-stream to suppress

Key Points

• No initialization needed — golem-rust installs the logger automatically during agent startup
• log crate is pre-configured — it is already in the template Cargo.toml with the kv feature
• 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; only new invocations produce log output