Log Function

The log function emits structured log messages through the Rust log crate, with a dynamic message string and optional key-value fields evaluated against the message context. It never modifies the context and always succeeds with status 200.

{
    "function": {
        "name": "log",
        "input": {
            "level": "info",
            "message": {"cat": ["Processing order ", {"var": "data.order.id"}]},
            "fields": {
                "order_id": {"var": "data.order.id"},
                "total": {"var": "data.order.total"}
            }
        }
    }
}

Configuration

Output Format

Each log entry is formatted as message [key=value, ...] with fields sorted alphabetically by key. String values are formatted directly, while non-string values (numbers, booleans, arrays, objects) are serialized as compact JSON.

For example, given data = {"order": {"id": "ORD-7", "total": 99.5, "items": [1, 2]}, "customer": {"name": "Ada"}}:

{
    "level": "info",
    "message": {"cat": ["Processing order ", {"var": "data.order.id"}, " for ", {"var": "data.order.total"}]},
    "fields": {
        "order_id": {"var": "data.order.id"},
        "customer": {"var": "data.customer.name"},
        "total": {"var": "data.order.total"},
        "items": {"var": "data.order.items"}
    }
}

emits the following log line at info level under the dataflow::log target:

Processing order ORD-7 for 99.5 [customer=Ada, items=[1,2], order_id=ORD-7, total=99.5]

If an expression in message or fields fails to evaluate, "<eval error>" is written in its place, the evaluation error is logged at error level by the engine, and the log task still finishes with status 200:

<eval error> [x=<eval error>]

Log Levels

Zero overhead when filtered out: Before evaluating message or fields, log checks log::log_enabled! for the task's level on target dataflow::log. When that level is disabled by your logger configuration (for example, a debug task when running at info level), no expressions are evaluated.

Configuring Your Logger (dataflow::log Target)

All workflow log tasks emit under the dataflow::log target so you can filter workflow logs independently from internal engine logs.

With env_logger via environment variable:

RUST_LOG=dataflow::log=info cargo run

Or in Rust code:

#![allow(unused)]
fn main() {
env_logger::Builder::new()
    .filter_module("dataflow::log", log::LevelFilter::Debug)
    .init();
}

Examples

Static Log Message

{
    "id": "log_start",
    "function": {"name": "log", "input": {"message": "Starting order processing"}}
}

Conditional Warning

Combine log with a task condition to log only when an edge case occurs:

{
    "id": "warn_missing",
    "condition": {"!": {"var": "data.shipping_address"}},
    "function": {
        "name": "log",
        "input": {
            "level": "warn",
            "message": {"cat": ["Order ", {"var": "data.order.id"}, " has no shipping address"]}
        }
    }
}

Dumping Context State for Debugging

{
    "id": "debug_state",
    "function": {
        "name": "log",
        "input": {
            "level": "debug",
            "message": "Current state",
            "fields": {
                "data": {"var": "data"},
                "has_email": {"!!": {"var": "data.email"}}
            }
        }
    }
}

Secrets Protection

Because log lines leave the engine, neither message nor any expression in fields may reference {"secret": "name"}—compilation rejects the workflow at build time with SECRET_IN_MESSAGE_WRITE. See Secrets.