How To Aggregate Downstream Test Results in Hudson

I must've googled a dozen permutations of this post's title looking for the key to my problem of... well, what it says. How on earth do I get Hudson (a top-notch CI server) to "Aggregate Downstream Test Results"??? The little help icon, which is typically very useful, provided a handy description of the feature, but didn't say anything about how to use it. My searches found several posts from other people having the same problem, but little in the way of resolution. I'm not sure if there's just not many people wanting to do this or if the "how" was just very obvious to everyone but me, but one way or the other, I finally figured it out. In retrospect, it does seem a bit obvious.

The key to getting Hudson to track test results from downstream builds is that all of the builds, upstream and down, must have a common, fingerprinted artifact. Any old artifact will do...almost. More on that later. I just had my first job--let's call it Foo--create a file with the date and time in it called "starttime".

1) Have Foo archive this file as an artifact, and either enter the file in the list of artifacts to fingerprint, or turn on fingerprinting for all artifacts.

2) In the downstream job, Bar, do exactly the same as in step one for Foo: archive and fingerprint the same file. Of course this implies that Bar has actually gotten the file in question somehow. I haven't worked out how to do this through Hudson other than by using wget to retrieve the archived one from the last build of Foo: something like "wget http://localhost:8080/job/Foo/lastBuild/artifact/starttime".

3) Of course, you'll need to have some test output being recorded by at least Bar, if not both jobs, since the whole point of this is to see aggregated test results.

4) You can always see the very latest test results at http://localhost:8080/job/Foo/lastSuccessfulBuild/aggregatedTestReport. Don't use "lastBuild" or the page will be unavailable when Foo is running or broken. When you have a lot of downstream tests, you can enable auto-refresh on this page and see the results fill in as jobs complete--very cool, especially when you have a nice test cluster with lots of downstream jobs.

5) It's handy to make Bar triggered by a build of Foo, but it's by no means required. From now on, any builds of Foo and Bar that share an identical artifact (checked by md5 checksum) are linked together, and the link appears in several places, one of which is the aggregated test results.

I emphasized the word "any" there for the same reason I said you can use almost any artifact. Remember it's the md5sum of the artifact in question that is used to determine which builds of which jobs are linked together. Say you start builds Foo #4, #5, and #6, and all create or use the same "starttime" file for some reason--maybe you're using just the date instead of date + time. Then Bar #4, #5, and #6 also all retrieve, archive, and fingerprint the same file. Since the checksum of the artifact is common in all those builds, they'll all be linked together, and Hudson can't really tell them apart. I'm not sure what this does for aggregated test results, but in other places, like where upstream and downstream builds are listed, instead of showing that Foo #4 led to Bar #4, it'll show Foo #4 led to Bar #4-#6.

So there ya go. Let 'er rip. I'm happily chugging away now with a 5-node Hudson cluster churning out test results like there's no tomorrow.