Background of my specific environment: I am trying to set up GA to track the user journeys around an iOS native app written in React Native. The app is in-house only and all users consent to being tracked in this way.
As I understand it, the measurement protocol allows either Client ID (cid) or User ID (uid). In the docs, both are optional, but at least one is required. Makes sense so far. Client ID is an anonymous UUIDv4 that tracks a browser or device, but not necessarily a user. User ID on the other hand is some uniquely-identifying value for a user, but may cross devices/browsers.
Initially I wrote my application using cid (as I hadn't yet read the docs on uid). Every time a user logged in the user's uuid would be stored and sent as the cid parameter. I also used session control sc. This worked - I saw sessions coming in, the User Explorer showed me the individual user uuids and sessions, etc. But upon rereading the docs, this seemed like an incorrect use of cid. It seems like cid is meant to be consistent and not something that changes as you log in and out.
As people on my app can log in and out (and possibly will - staff members may well share devices), using a long-lasting cid that is specific to the device won't allow me to track the habits of individual users and changing it per login-session, as mentioned above, feels wrong.
So I switched out cid for uid. The docs say both are optional and one is only required if the other is not used, so I figured I could just use uid and use the user's uuid in that field and it's otherwise act the same.
However, when doing it this way no hits seems to be registered at all - in "real time data" as well as the User Explorer.
Am I doing something wrong or is this by design? (If by design, are the docs wrong to say cid and uid are both optional or am I reading it incorrectly?)
Either way, best-practices-wise, do I:
Just go back to using an ephemeral cid that only changes when people log in and out. Individual sessions in the app don't change the cid but I do use sc to explicitly start and end sessions by navigation inference (click 'home' = sc=end, navigate from 'home', sc=start).
Use uid as I am trying to (i.e. switching out the current cid for uid), but also include a random cid that never changes (regardless of session lifetime, logging in and out, etc.).
Something else?
I have seen this question and its answer but it does not really answer my actual question regarding how cid and uid should actually be used, and if my issue is me or the way that GA works.
Oki so client-id is randomly generated value and must be sent with all hits. In a non-User-ID-enabled view, Client-ID is used to calculate unique users. In a User-ID-enabled view, User-ID is used to calculate unique users
In reference to your app, you can send the unique Device id value in place of random value for client-id. This will resolve all your problems and ensure that the client id is sent with all the hits, otherwise it wont get registered in GA