The Google Measurement Protocol documentation claims that the cid is optional if uid is used:
This field is required if User ID (uid) is not specified in the request
So I make the following requests to measurement protocol:
GET https://www.google-analytics.com/collect?v=1&tid=<MY_TID>&aip=1&uid=<USER_ID>&ec=Foo&ea=Bar&el=Baz
But they do not get recorded in Google Analytics.
I make other requests where I specify both uid and cid and they work perfectly. Is there something I am missing? Elsewhere I have read that cid is actually mandatory.
This Answer is a correct answer to my question, so accepted. But I wanted to add that in addition to the fact that these hits were never going to show up in the Main View (only the User ID view), I had neglected to set t=event in my request (necessary for ec, ea and el). The debug endpoint didn't help diagnosing this I just stared at it (and the docs) for long enough to see it.
It is mandatory because if you don't send it but you send only uid, the hit will be appear only in USER-ID view.