I am building a glass app using the PHP sample app as reference.
To my understanding, the glass app is a web app which is user facing. When the user visit the web app, they will authorize the web app (using oauth2) to access their resources, and once authorization succeeded the web app will get an access token, which is then saved in a sqlite database (there's this store_credentials() function which stores the access token).
So if I have 100 users who visit the web app and register, the database will hold 100 access tokens for these users. Let's say I have some backend code which pull from a RSS feed every hour, and whenever I find there's a new story I will push it to all registered users. Let's say I have a cron job which does this, when when this job is triggered, I will find all the access tokens in the database and use them to make mirror API calls to insert cards. But what if some of the access token is expired when I am trying to make the mirror API call? It seems I will need to ask user to re-authorized, but at this point I am not interacting with the user. If I have a refresh token, I may be able to call oauth again to get a new access token.
Thanks.
You pretty much answered your own question - when you request permission using OAuth, you should also request "offline" access. The first time you do this for each user it will give you a refresh token along with your access token, and you should store both. If you did not request offline access initially, you will need to revoke the tokens that have been granted and re-grant them with offline access.
If the sample app you're referring to is the one at https://github.com/googleglass/mirror-quickstart-php, all this should be done for you already with the libraries included. The credentials returned from $client->getAccessToken() in oauth2callback.php should include both the access and refresh tokens, and these are saved in that same file by calling store_credentials(). The client libraries should check if the access token has expired and, if so, it gets a new one with the refresh token before making the call.
You may want to take a look at the contents of the credentials object at these various points and make sure there is a refresh token. If not, try revoking all the tokens and starting again, since it sounds like the first time you authorized the client you did so without requesting offline access.
If you are doing this yourself, best practice is to either refresh the access token if it has expired or is about to expire (since there may be some delay) before use, or (even better) attempt to make the call with an access token and, if it fails with an authentication error, get a new access token via the refresh token and try again.