How to Customize Testnet Configuration

advanced intermediate

Learn how to use the --config-changes flag with marood testnet to dynamically modify node configurations for advanced testing scenarios.

Prerequisites

Familiarity with the `marood testnet` command.
Basic understanding of CometBFT's `config.toml` file.

1. The `--config-changes` Flag

This flag accepts a single string containing one or more key-value pairs. The format is "key1=value1,key2=value2".

For nested keys in config.toml, use dot notation. For example, the timeout_commit inside the [consensus] block is referenced as consensus.timeout_commit.
For top-level keys like log_level, just use the key name.

2. Example: Creating a Fast-Block Testnet

A common use case is to reduce block times for faster feedback during development. The default timeout_commit is several seconds. Let's create a 2-node testnet where blocks are committed every 500 milliseconds.

marood testnet -v 2 -o ./fast-testnet --config-changes "consensus.timeout_commit=500ms"

Note: After running this, you can inspect `./fast-testnet/node0/config/config.toml` and you will see `timeout_commit = "500ms"` has been set.

3. Example: Combining Multiple Changes

You can apply multiple changes by separating them with a comma. Let's create a testnet with fast blocks and verbose debug logging.

marood testnet -v 2 -o ./debug-testnet --config-changes "consensus.timeout_commit=500ms,log_level=debug"

4. Supported Types

The configuration parser is smart enough to handle different data types based on the target field in the configuration struct:

Durations: Use strings like "500ms", "1s", "2m".
Integers: Use plain numbers like 100.
Booleans: Use true or false.
Strings: Enclose in quotes if they contain special characters, e.g., "info".