Dynamic Values

Values can be configured to be either static or dynamic. Static values are loaded once at startup (more precisely, when viperutil.LoadConfig is called), and whatever value is loaded at the point will be the result of calling Get on that value for the remainder of the process’s lifetime. Dynamic values, conversely, may respond to config changes.

In order for dynamic configs to be truly dynamic, LoadConfig must have found a config file (as opposed to pulling values entirely from defaults, flags, and environment variables). If this is the case, a second viper shim, which backs the dynamic registry, will start a watch on that file, and any changes to that file will be reflected in the Get methods of any values configured with Dynamic: true.

An important caveat is that viper on its own is not threadsafe, meaning that if a config reload is being processed while a value is being accessed, a race condition can occur. To protect against this, the dynamic registry uses a second shim, sync.Viper. This works by assigning each dynamic value its own sync.RWMutex, and locking it for writes whenever a config change is detected. Value GetFuncs are then adapted to wrap the underlying get in a m.RLock(); defer m.RUnlock() layer. This means that there’s a potential throughput impact of using dynamic values, which module authors should be aware of when deciding to make a given value dynamic.