• Yorick Peterse's avatar
    Storing of application metrics in InfluxDB · 141e946c
    Yorick Peterse authored
    This adds the ability to write application metrics (e.g. SQL timings) to
    InfluxDB. These metrics can in turn be visualized using Grafana, or
    really anything else that can read from InfluxDB. These metrics can be
    used to track application performance over time, between different Ruby
    versions, different GitLab versions, etc.
    
    == Transaction Metrics
    
    Currently the following is tracked on a per transaction basis (a
    transaction is a Rails request or a single Sidekiq job):
    
    * Timings per query along with the raw (obfuscated) SQL and information
      about what file the query originated from.
    * Timings per view along with the path of the view and information about
      what file triggered the rendering process.
    * The duration of a request itself along with the controller/worker
      class and method name.
    * The duration of any instrumented method calls (more below).
    
    == Sampled Metrics
    
    Certain metrics can't be directly associated with a transaction. For
    example, a process' total memory usage is unrelated to any running
    transactions. While a transaction can result in the memory usage going
    up there's no accurate way to determine what transaction is to blame,
    this becomes especially problematic in multi-threaded environments.
    
    To solve this problem there's a separate thread that takes samples at a
    fixed interval. This thread (using the class Gitlab::Metrics::Sampler)
    currently tracks the following:
    
    * The process' total memory usage.
    * The number of file descriptors opened by the process.
    * The amount of Ruby objects (using ObjectSpace.count_objects).
    * GC statistics such as timings, heap slots, etc.
    
    The default/current interval is 15 seconds, any smaller interval might
    put too much pressure on InfluxDB (especially when running dozens of
    processes).
    
    == Method Instrumentation
    
    While currently not yet used methods can be instrumented to track how
    long they take to run. Unlike the likes of New Relic this doesn't
    require modifying the source code (e.g. including modules), it all
    happens from the outside. For example, to track `User.by_login` we'd add
    the following code somewhere in an initializer:
    
        Gitlab::Metrics::Instrumentation.
          instrument_method(User, :by_login)
    
    to instead instrument an instance method:
    
        Gitlab::Metrics::Instrumentation.
          instrument_instance_method(User, :save)
    
    Instrumentation for either all public model methods or a few crucial
    ones will be added in the near future, I simply haven't gotten to doing
    so just yet.
    
    == Configuration
    
    By default metrics are disabled. This means users don't have to bother
    setting anything up if they don't want to. Metrics can be enabled by
    editing one's gitlab.yml configuration file (see
    config/gitlab.yml.example for example settings).
    
    == Writing Data To InfluxDB
    
    Because InfluxDB is still a fairly young product I expect the worse.
    Data loss, unexpected reboots, the database not responding, you name it.
    Because of this data is _not_ written to InfluxDB directly, instead it's
    queued and processed by Sidekiq. This ensures that users won't notice
    anything when InfluxDB is giving trouble.
    
    The metrics worker can be started in a standalone manner as following:
    
        bundle exec sidekiq -q metrics
    
    The corresponding class is called MetricsWorker.
    141e946c
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