Logging & Debugging

In this reference, you will learn how to log arbitrary inputs in your Substreams execution for debugging purposes.

For testing your Substreams modules, see the Testing Reference.

Using logs in Substreams

It is possible to log arbitrary strings to the standard output of the Substreams execution. You will see these logs when running your Substreams using substreams gui or substreams run commands.

NOTE: When using the substreams gui command, use the L key to toggle logs.

To log something in Rust, use one of the following functions:

#[substreams::handlers::map]
fn map_program_data(blk: Block) -> Data {
    substreams::log::println("My log")
}

OR

#[substreams::handlers::map]
fn map_program_data(blk: Block) -> Data {
    substreams::log::debug!("My log")
}

OR

#[substreams::handlers::map]
fn map_program_data(blk: Block) -> Data {
    substreams::log::info!("My log")
}