You can Observe a lot by just watching

The following Observers are available:

• Observer : abstract class;
  • ErrorMinima : monitors an error variable to keep track of its minima.
    • EarlyStopper : upon reaching minima, saves Experiment and notifies Subscribers;
• AdaptiveDecay : decays learning rate when new minima on validation error isn't found;
• Logger : abstract logging class (prints reports to cmdline);
  • FileLogger : basic file-based report logging;
• CompositeObserver : a composite of observers;

[]() ## Observer ## An object that is called when events occur. Based on the Listen-Notify design pattern. Uses a mediator to publish/subscribe to channels. Observers cannot publish reports (for now). The reason for this is that a report is required of observers to do their job, thus making it impossible for them to participate to report generation. The only possibility would be to allow for Observers to modify the received report, but then the ordering of these modifications would be undefined, unless they make use of Mediator priorities. []() ### dp.Observer(channels, callbacks) ### Constructs an Observer. Arguments `channels` and `callbacks` can be strings or tables thereof. The first argument specifies the name of a [Channel](mediator.md#dp.Channel) to which the corresponding `callback` method of the Observer will be registered via a [Subscriber](mediator.md#dp.Subscriber) through the [Mediator](mediator.md#dp.Mediator). If no `callbacks` are provided, these default to taking on the same values as `channels`. []() ## ErrorMinima ## Monitors an error variable to keep track of its minima. Optionally notifies [Subscribers](mediator.md#dp.Subscriber) to the `errorMinima` of the current state of the error minima.

The monitored error variable can be obtained from an experiment report or a Mediator Channel. In the case of the report, the variable must be located at a leaf of the report tree specified by a sequence of keys (branches) : error_report. In this case, the object subscribes to doneEpoch channel in order to receive the reports at the end of each epoch.

dp.ErrorMinima{...}

Constructs an ErrorMinima Observer. Arguments should be specified as key-value pairs. Other then the following arguments, those specified in Observer also apply.

• start_epoch is a number identifying the minimum epoch after which Experiments can begin to be persisted to disk. Useful to reduce I/O at beginning of training. Defaults to 5;
• error_report is a table specifying a sequence of keys to access variable from epoch reports. Default is {'validator', 'loss', 'avgError'} (for cross-validation), unless of course an error_channel is specified.
• error_channel is a string or table specifying the Channel to subscribe to for early stopping. Should return an error value and the last experiment report. Over-rides error_report when specified.
• maximize is a boolean having a default value of false. When true, the error channel or report is negated. This is useful when the channel returns a measure like accuracy that should be maximized, instead of an error that should be minimized.
• notify is a boolean. When true (the default), notifies listeners (Subscribers) when a new minima is found.

EarlyStopper

An ErrorMinima instance that saves the version of the subject with the lowest error and terminates the experiment when no new minima is found for max_epochs. Should only be called on Experiment, Propagator or Model subjects. Error can be obtained from experiment report or mediator Channel. If obtained from experiment report via error_report, subscribes to doneEpoch channel.

dp.EarlyStopper{...}

Constructs an EarlyStopper Observer. Arguments should be specified as key-value pairs. Other then the following arguments, those specified in ErrorMinima also apply.

• save_strategy is an object with a save(subject) method for persisting the subject to disk. Defaults to dp.SaveToFile()
• max_epochs specifies the maximum number of epochs to consider after a minima has been found. After that, a terminate signal is published to the mediator. Defaults to 30.
• max_error is the maximum value for which the experiment should be stopped after min_epoch epochs. If maximize=true this is minimum value. A value of 0 (the default) means that it is ignored.
• min_epoch is a number having a default value of 1000000. See max_value for a description.

AdaptiveDecay

An Observer that decays learning rate by decay_factor when validation error doesn't reach a new minima for max_wait epochs. This object should observe in conjuction with an ErrorMinima instance, such as EarlyStopper.

As example, suppose the object is initialized with decay_factor=0.5 and max_wait=1, while the subject is initialized with learning_rate=10. If the sequence of errors is

10, 9, 8, 8, 8, 8, 9, 7, 7, 8

then the corresponding sequence of learning rates given these errors would be

10, 10, 10, 10, 1, 1, 0.1, 0.1, 0.1, 0.01

dp.AdaptiveDecay{...}

Constructs an AdaptiveDecay Observer. Arguments should be specified as key-value pairs. Other then the following arguments, those specified in Observer also apply.

• max_wait specifies the maximum number of epochs to wait for a new minima to be found. After that, the learning rate is decayed by decay_factor. Defaults to 2.
• decay_factor specifies the factor by which learning rate lr is decayed as per : lr = lr*decay_factor.

[]() ## Logger ## []() ## FileLogger ## []() ## CompositeObserver ##