Use mcap storage compression when compression is requested

[Aposhian]

Description

You can enable compression via --compression-mode and --compression-format with -s mcap, but this produces:

• with --compression-mode=file: .mcap.zstd files that tools like Foxglove Studio does not understand
• with --compression-mode=message: Foxglove Studio cannot read .mcap files encoded like this.

However, if you make a config file like mcap-config.yaml:

compression: Zstd
compressionLevel: Default

and then run:

ros2 bag record -s mcap --storage-config-file ./mcap-config.yaml

Then you get a .mcap file that is compressed with zstd, that Foxglove Studio can also load. This also gives you control over compression level/speed.

I understand that it makes difficult for rosbag2 to treat mcap's compression as a special case, but currently if you are trying to figure out how to use compression with rosbag2, your go-to in the documentation will simply be to use the top-level flags, which gives a suboptimal result for mcap.

Related Issues

N/A

Completion Criteria

Users are not misled into producing an unreadable .mcap file.

Implementation Notes / Suggestions

Maybe rosbag2 should just give a warning or something when you specify --compression-mode with the mcap storage plugin.

Testing Notes / Suggestions

Need to test that produced .mcap files are compatible with Foxglove Studio.

[tobiastorben]

Second this. We have a lot of bags that have been recorded in the mcap format with message-level compression that we are unable to open. Is there any way to salvage these?

[Aposhian]

Second this. We have a lot of bags that have been recorded in the mcap format with message-level compression that we are unable to open. Is there any way to salvage these?

I would think you should be able to use ros2 bag convert if you specify that the input bag has message level compression, but I haven't tried that.

[MichaelOrlov]

What we can do here is to update the documentation in the readme file.

The --compression-mode and --compression-format produce different types of compression which unified and independent of the underlying storage plugins or file formats. This type of compression is done on a higher level and still could be a valid option for MCAP in some cases.

I would appreciate someone's help with updating the readme file in this regard.

[Aposhian]

The --compression-mode and --compression-format produce different types of compression which unified and independent of the underlying storage plugins or file formats. This type of compression is done on a higher level and still could be a valid option for MCAP in some cases.

I do appreciate that --compression-mode and --compression-format are storage type agnostic. However, I'm having a hard time understanding when you would want to use them over the native compression format that is still an indexable file. I think updating the README would be good, but also providing a CLI warning with this storage plugin and CLI option combination could be helpful.

[tobiastorben]

Second this. We have a lot of bags that have been recorded in the mcap format with message-level compression that we are unable to open. Is there any way to salvage these?

I would think you should be able to use ros2 bag convert if you specify that the input bag has message level compression, but I haven't tried that.

Thanks! ros2 bag convert -i <path_to_bag_dir> -o decompress.yaml was able to recover the uncompressed bag. decompress.yaml:

output_bags:
- uri: decompressed
  all: true
  storage_id: "mcap"

[defunctzombie]

@MichaelOrlov Thoughts on disabling --compression-mode=message when recording to an mcap file? MCAP does not have message level compression which is what leads to using this mode producing mcap files that other tools cannot read.

[defunctzombie]

It has been pointed out to me that this bit of code should have prevented per-message compression but I also see it is only present if ROSBAG2_STORAGE_MCAP_HAS_UPDATE_METADATA is defined. Is this always defined?

[MichaelOrlov]

@defunctzombie We added update_metadata(..) API for the storage plugin interface some time ago in the scope of the #1149.
It is present on Iron, Rolling and Jazzy ROS 2 distros. However, it is absent on Humble.

If the matter of concern to add the same exception as in the

rosbag2/rosbag2_storage_mcap/src/mcap_storage.cpp

Lines 845 to 849 in 317286c

	if (bag_metadata.compression_mode == "message") {
	throw std::runtime_error(
	"MCAP storage plugin does not support message compression, "
	"consider using chunk compression by setting `compression: 'Zstd'` in storage config");
	}

somewhere on Humble - I am fine with it.