Implemented a Resource Type: How does Concourse use the output of the check, in, and out scripts? - concourse

Question

Reading the Concourse documentation about Implementing a Resource Type, in regards to what the check, in, and out scripts must emit, it is not clear why this output is needed or how Concourse uses it. My questions are:
1) How does Concourse use the output of the check script, the in script, and the out script?
2) And, why is it required that the in and out script emit the version? What happens if you don't?
For context, here is the relevant parts of the documentation:
1) For the check script:
...[it] must print the array of new versions, in chronological order,
to stdout, including the requested version if it's still valid.
For example:
[
{ "ref": "61cbef" },
{ "ref": "d74e01" },
{ "ref": "7154fe" }
]
2) For the in script:
The script must emit the fetched version, and may emit metadata as a list of key-value pairs. This data is intended for public consumption and will make it upstream, intended to be shown on the build's page.
For example:
{
"version": { "ref": "61cebf" },
"metadata": [
{ "name": "commit", "value": "61cebf" },
{ "name": "author", "value": "Hulk Hogan" }
]
}
3) Similar to the in script, the out script:
The script must emit the resulting version of the resource. For
example, the git resource emits the sha of the commit that it just
pushed.
For example:
{
"version": { "ref": "61cebf" },
"metadata": [
{ "name": "commit", "value": "61cebf" },
{ "name": "author", "value": "Mick Foley" }
]
}

Answer 1

Concourse uses the check result to verify if there is any new resource available. According to your pipeline definition, the presence of a new resource would trigger a job. The in is therefore used to read the specific resource using parameters provided by the pipeline whilst the out would take care of writing them.
As your in is going to use the information provided by the check you may want to use a similar structure, but you're not obliged to. It is useful to echo the same version information in your check/in/out in order to be able to log it and understand each resource in your pipeline is belonging to which version.