Expose progress information in a machine-readable format

[subdavis]

Something that VIAME Web and Heavy can use to show users a more friendly indication of how their training / pipeline is going.

Option 1: signals

Long-running applications like dd use signals to communicate progress. USR1 is a good option, designed for exactly this purpose. The process manager could regularly send this signal to kwiver to get progress.

Option 2: stdout

Some regularly formatted line of stdout with a prefix we can recognize.

<PREFIX> key1="value1" key2="value2"

This is a popular option.

<PREFIX> { /* json object */ }

This would also be fine.

[linus-sherrill]

Another mechanism that is available for communicating with a pipeline is the dynamic_configuration algorithm. The derived class needs to make the connection between the two parties and can then pass key/value pairs (config block). The original application for this was to allow an external GUI to change the behaviour of a pipeline element by passing new config values to that element, but it can also be used to return an updated config block to the caller. There is at least one real usage in the squad-x code base.