ROS 2

Record with MCAP

ROS2 supports recording directly to MCAP using the rosbag2 MCAP storage plugin. To get started, install the plugin:

$ sudo apt-get install ros-$ROS_DISTRO-rosbag2-storage-mcap

Set your storage ID to mcap when recording:

$ ros2 bag record -s mcap --all

You can also customize MCAP writer options such as compression and chunk size using storage options:

$ cat << EOF > my_storage_config.yaml
compression: "Lz4"
compressionLevel: "Fastest"
EOF
$ ros2 bag record -s mcap --all --storage-config-file my_storage_config.yaml

Convert to MCAP

Using the mcap CLI tool

To convert your existing ROS 2 db3 files into MCAP files, install the mcap CLI tool and run the following command:

$ mcap convert multiple_files_1.db3 demo.mcap

mcap will search the path stored in your $AMENT_PREFIX_PATH environment variable to locate the ROS message definitions on your hard drive.

Alternatively, you can specify a colon-separated list of directories for the CLI tool to search using the ament-prefix-path flag:

$ mcap convert ros2_input.db3 ros1_output.mcap --ament-prefix-path=/your/first/directory;/your/second/directory

You can also use the mcap CLI tool to inspect MCAP files, validate them, and even echo their messages to stdout. For a full list of possible commands, check out the mcap GitHub repo.

Using ros2 bag convert

The mcap CLI conversion support for SQLite bags works by emulating the behavior of the ROS 2 resource discovery mechanism. This may produce different results from recording the bag directly in MCAP using the rosbag2 MCAP storage plugin.

To create an MCAP exactly the way the rosbag2 MCAP storage plugin would create it, you can use ros2 bag convert from within your ROS2 workspace.

$ cat << EOF > convert.yaml
output_bags:
  - uri: ros2_output
    storage_id: mcap
    all: true
EOF
$ ros2 bag convert -i ros2_input.db3 -o convert.yaml

Read and write MCAP

If you're starting from scratch, you can write code that allows you to write your ROS 2 data to MCAP files and subsequently read your ROS 2 data from your MCAP files.

Guides

• Python

Examples

• Python - reader and writer

Inspect MCAP

Use the mcap CLI tool to inspect MCAP files, validate their contents, and even echo their messages to stdout.

For an exhaustive list of ways to interact with your MCAP data, check out the mcap GitHub repo.

Visualize MCAP

Foxglove Studio supports playing back local and remote ROS 2 db3 files, as well as local and remote MCAP files containing ROS 2 data.

With that said, we recommend MCAP files over ROS 2 db3 files, as the latter are not completely self-contained.