SensESP builds on a number of interface classes and abstractions and following the code logic can feel overwhelming if you’re not familiar with the concepts. This page provides a brief overview of the internals of the library.


The Observable base class implements the Observer pattern. Any class inheriting from Observable can be observed. In practical terms, you can call observable->attach(cb) to attach the callback cb to the Observable instance. When the Observable instance value is updated, the notify() method is called, causing all the attached callbacks to be called in turn.

The value in having Observables is that you don’t have to know in advance who is interested in object value updates, or how many interested parties there are.

Usually, the attached callback will propagate the new value of the Observable to the observer object.

This simplified graph shows an Observable with three observers:

An example of Observable

This is the same graph with the callbacks shown:

Observable with callbacks

Producers and Consumers

Moving forward from Observables, we have Producers and Consumers, implemented in the ValueProducer and ValueConsumer classes respectively.

Producers are Observables that additionally define a connect_to() method that can be used to connect the Producer to a Consumer. The connect_to() method creates a standardized callback between the two objects, thus allowing you to connect the two without having to define the callback yourself.

Consumers, on their turn, are classes that implement the set_input() method that the Observable callback will call. Consumers also define a connect_from() convenience method for connecting the Consumer to a Producer. connect_from() is a bit more complex than connect_to() because it “goes against the flow”, so to speak, but it can come in handy in many situations.

It is common for a class to be both a ValueProducer and a ValueConsumer. Such classes typically receive values, perform some operations, and then notify their consumers, thus creating a transform of the input. Transform classes are described in the Transforms section.

An important difference between Observables and Producers is that Producers are typed. ValueProducer is a template class that, when instantiated, will also have its output type defined. This means that a StringProducer cannot be connected to an IntConsumer because the types don’t match.

There is nothing wrong in using the ValueProducer<T> and ValueConsumer<T> classes directly, but some pre-specialized convenience classes have been defined for common use cases:

  • FloatProducer
  • IntProducer
  • BoolProducer
  • StringProducer

The value propagation graph for Producers and Consumers is almost identical to the one for Observables. Shown here is an example of IntProducers and IntConsumers:

Producer and Consumer value propagation graph

Automatic type conversions for types that can be converter with a type cast are also available. Hence, it is possible to connect a IntProducer to an FloatConsumer, etc. Converting from String to bool, for example, is not possible because no such type cast exists. Manual conversion can be created with a LambdaTransform, for example.

ObservableValue is one of the simplest Producers. By itself, it does nothing, but it implements the ValueProducer interface, and you can connect ValueConsumer objects to it. If you update the value of the ObservableValue, all the connected consumers will be notified. This approach can be used to inject arbitrary data into SensESP processing networks.


Many SensESP objects benefit from having a configuration interface and means for storing and retriveing configuration values from a persistent storage. The Configurable class is a base class for all such objects.

Configurable objects can read and write their configuration values by defining set_configuration() and get_configuration() methods. The method naming can be a bit confusing: set_configuration() sets the values of object member variables, while get_configuration() returns an ArduinoJson object filled with the member variable values. Hence, set loads and get saves the configuration.

Configurable objects also normally define a config schema, acquired by calling get_config_schema(). The config schema is used to render the configuration page in the web interface.

Resettables and Startables

Some additional base classes exist for objects that can be reset and that have additional startup routines. These inherit from Resettable and Startable base classes.

Startables are more interesting and will be discussed first.

A Startable object is something that should be somehow enabled or started when all other objects have been initialized and we want to actually start running the program. The startup routine is defined by the start() method implemented by the inheriting class.

For example, basic WiFi networking objects should be initialized before the websocket connection to the Signal K server is attempted because the latter may utilize the former. It should be noted, however, that network connections, in particular, are established asynchronously. Returning from the start() call only signals that we have started establishing the connection, not that the connection already exists.

The Startable constructor has an optional priority parameter that can be used to control the object startup order. Higher numbers come first and negative numbers are allowed. The priority definitions are arbitrary, but some of the currently defined values are:

80WiFi networking
60SK Websocket client
50HTTP server
0Default value

A Resettable object is something that should be called when the ESP device is factory reset. Currently, the only use cases for Resettable are initializing the file system and the WiFi network settings.

Similar to Startable, the Resettable constructor has an optional priority parameter that can be used to control the object reset order.

The currently defined values are:

0WiFi networking
-100File system


Sensors are classes that read value from some real-world source and provide them to the rest of the system. Building on the concepts described above, a Sensor subclass is a ValueProducer of a certain type, a Configurable, and a Startable (even though the default start() method does nothing and is usually not overridden). In other words, it is something that you can attach Observables or Consumers to, which can store and retrieve its own configuration (and can be configured via the web interface), and can define its own startup routine.

Like ValueProducers, all Sensors inherit from the SensorT<T> template class, and there are pre-specialized classes for convenience: FloatSensor, IntSensor, BoolSensor, and StringSensor.

When creating new sensor classes, inheriting from Sensor requires writing quite a bit of boilerplate that may not be very relevant for simple sensor classes. If you are creating a new sensor that simply reads a value using an external library at given time intervals, you can use the RepeatSensor class instead.

Sensors are further discussed on the Sensors section of the Concepts page.


Transforms are classes that take a value in, perform some operation on the value, and then spit a new value out. They implement both ValueProducer and ValueConsumer interfaces. Furthermore, the Transform class is a Configurable and a Startable.

The input and output types of a Transform do not have to be the same, and often aren’t.

In many common cases, defining a new Transform subclass is a lot of work and can be avoided by using the LambdaTransform class. A LambdaTransform is a class that takes a lambda function as a parameter, conveniently avoiding the need to define a new class.

Transforms are further discussed on the Transforms section of the Concepts page.

Configuration API

The web user interface of SensESP is implemented using a RESTful configuration API that can be used independently of the web UI. It would, for example, be possible to create a Signal K plugin for configuring all connected sensors over their configuration interfaces.

Regular SensESP devices listen to HTTP port 80. The following calls are supported:

/configGETGet a list of all configuration paths
/config/PATHGETGet the Json configuration object for configuration path PATH
/config/PATHPUTUpdate the configuration for path PATH
/infoGETGet information text for the device
/device/resetGETFactory reset the device
/device/restartGETRestart the device