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  • Yorick Peterse's avatar
    Rework how recent push events are retrieved · 9ec66da20412
    Yorick Peterse authored
    Whenever you push to a branch GitLab will show a button to create a
    merge request (should one not exist already). The underlying code to
    display this data was quite inefficient. For example, it involved
    multiple slow queries just to figure out what the most recent push event
    was.
    
    This commit changes the way this data is retrieved so it's much faster.
    This is achieved by caching the ID of the last push event on every push,
    which is then retrieved when loading certain pages. Database queries are
    only executed if necessary and the cached data is removed automatically
    once a merge request has been created, or 2 hours after being stored.
    
    A trade-off of this approach is that we _only_ track the last event.
    Previously if you were to push to branch A and B then create a merge
    request for branch B we'd still show the widget for branch A. As of this
    commit this is no longer the case, instead we will only show the widget
    for the branch you pushed to most recently. Once a merge request exists
    the widget is no longer displayed. Alternative solutions are either too
    complex and/or too slow, hence the decision was made to settle for this
    trade-off.
    
    Performance Impact
    ------------------
    
    In the best case scenario (= a user didn't push anything for more than 2
    hours) we perform a single Redis GET per page. Should there be cached
    data we will run a single (and lightweight) SQL query to get the
    event data from the database. If a merge request already exists we will
    run an additional DEL to remove the cache key.
    
    The difference in response timings can vary a bit per project. On
    GitLab.com the 99th percentile of time spent in User#recent_push hovers
    between 100 milliseconds and 1 second, while the mean hovers around 50
    milliseconds. With the changes in this MR the expected time spent in
    User#recent_push is expected to be reduced down to just a few
    milliseconds.
    
    Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/35990
    9ec66da20412