# ROS Bag Recorder

## Record bags from your nodes

Posted by Nick Lamprianidis on March 29, 2018

Let’s say you want to perform a test, record the events and data that present themselves, and later play them back, in order for you to take a closer look at what happened. In ROS, you can store messages, that get published to topics, to a bag file. You don’t need to know much about bags to get started, as there is a command line tool, rosbag, to help you create and playback bag files.

Now, what if you wanted to have greater control over what gets stored and when? No worries, as bags have an API that allows you to create a file programmatically from any of your nodes. Consider the following example, taken from one of the wiki pages.

A file is opened, two messages of different types are written to the file, then the file is closed, and a bag now exists in the current working directory of the node. But given this API, a few questions arise. What if you wanted to record messages from many topics? You would have to add write calls to many different places, which is not ideal. What if you didn’t know what are the topics you want to listen to at compile time? You would need to create a generic subscriber. What if you wanted to potentially save multiple bag files, but decide whether you actually do, based on an event? It would be nice if you could avoid opening all these files and writing to disk.

Fortunately, there is the rosbag::Recorder class, which offers some of this functionality, and this means that you don’t have to implement everything yourself. But the recorder has been designed to work with the record function of the rosbag tool. Even though there is a great architecture inside, it is not exposed in a way that would allow many different uses. So, in this post, I will describe how you can set the recorder up and how it works, and you can then take it and modify it as needed to fit your case.

It’s pretty easy to start a recorder. You just have to initialize some options, give them to the recorder, and ask it to run. The recorder is meant to run until the node terminates. There is no stopping it.

The options are defined in the rosbag::RecorderOptions struct. There is no documentation on these parameters, apart from a short mention to some of them, here. So, we’ll go through them one by one. But before we move forward, I’ll have to describe two terms relating to bags, chunk, and snapshot.

• Messages are written to files in chunks. When a message is written to a rosbag:::Bag, it is stored in a buffer internally, and when the size of the buffer exceeds a threshold, this chunk of messages is moved to the file.
• “Triggering a snapshot” means taking all the messages received so far from a buffer and moving them to a bag file. The rosbag::Bag is created at the moment of triggering. This means all the messages stay in memory until the last moment, and if you let the recorder run for a long time, it might fill your RAM. So, keep that in mind when using snapshots.

We’ll start with those parameters that are not used. These are trigger, quiet, and name. Considering the state of the recorder right now, I’m guessing that there was the intention to do more, but that never happened. If it works, don’t mess with it. Right?

The rest are described as follows:

• record_all: The recorder subscribes to all topics. For that, it queries the master regularly. So, it subscribes to new topics as they appear on the network.
• node: The recorder subscribes only to the topics that this node subscribes to.
• topics: If record_all is false and node is empty, the recorder subscribes to the topics specified in this list.
• regex: If true, topics is expected to be a list of regular expressions. The recorder subscribes to the topics that match these regular expressions.
• exclude_regex: Any topic that matches this regular expression is excluded.
• do_exclude: Enables exclude_regex.
• limit: Limit in the number of messages to record per topic. When the limit is exceeded, the recorder unsubscribes from the topic.
• verbose: Outputs a log when a message is received.
• prefix: The start of the name of the bag file. If you want to store the bag file in a directory other than the current working one, you need to set here an absolute path, e.g. /tmp/mybag.
• append_date: Appends the date after the prefix, e.g. mybag_2018-03-29-19-42-38.
• compression: Compression type of the bag file. Either bz2 or lz4.
• snapshot: If false, when the recorder starts, a bag file is created. Then, when a message is received, the message is put in a buffer, a thread is notified, and the thread immediately writes the message to the bag. If snapshot is true, there is no bag and the message stays in the buffer. In this case, the recorder has a subscriber to snapshot_trigger. When a message is received to snapshot_trigger, a new buffer is created, and the contents of the old buffer are stored in a bag file.
• buffer_size: The size (in bytes) of the buffer that received messages are stored in. If exceeded, old messages start getting dropped.
• chunk_size: The size (in bytes) of the bag chunks.
• split: If true, messages can be split into multiple bag files.
• max_splits: Maximum number of splits.
• max_size: Maximum size of a bag. If exceeded, the recorder terminates, or a new split is created if split is true.
• max_duration: Maximum duration of a bag. If exceeded, the recorder terminates, or a new split is created if split is true.
• min_space: If the minimum free space (in bytes) on disk is below this threshold, the recorder terminates.
• min_space_str: Just a string representation of min_space, used when writing logs.

That’s it. You bother only with the options you want to modify, give them to the recorder, and let it run. Now, what are some potentials issues or restrictions if you want to use the recorder? I’ll just mention a few:

• If you want to take snapshots, you are restrict to have only one recorder under a given namespace. Otherwise, when you want to trigger a specific recorder, you need to send a message to snapshot_trigger which will be received by all recorders under that namespace.
• If you want to take snapshots, and trigger them programmatically, you can call doTrigger. But there are two issues with this method. Even though it’s a method of the recorder, it still triggers the snapshot by publishing a message (see above). Also, after publishing the message, it blocks (and that’s not the only problem. Try it yourself and see. Hint: Two threads call ros::spin).
• When max_duration gets exceeded, the recorder is done. It would be nice if there was another parameter that worked like limit. That is, a rolling window is implemented, such that when time is exceeded, old messages start getting dropped.
• Once the recorder starts, you have no way to stop it other than to terminate the node.

By now, you should have an idea of why I messed around with the recorder in the first place. I wanted to have something that maintains the last few seconds of communication, and if an error happens, it saves that to a file. Thankfully, with all that’s already in the recorder, it’s not that hard to implement the rest.

So, the options are explained, the potential presented, and you can take that and do your own thing. See you in the next post.