Within reactive systems, several different terms are often used for the basic entity of a time-varying value within a data flow. These terms can imply or carry slightly different things meanings:
Stream – A stream suggests a flow of values, where each value is significant. The term “stream” has primarily been used for IO operations where we want to have a model for data, such that we can operate on it, as it is received. However, applying this term to represent the state of object that may change, is usually inappropriate, as it implies the wrong type of concerns about stateful data. A stream is a great model for IO operations, but within reactive programming, it suggests “push” reactivity which tends to suffer from inefficiency when applied to modeling state.
“Event Bus” and “Signals” are other terms that may suggest a similar concept as a stream. A signal may be a little vaguer, and more open to interpretation.
Observable – This term is used to indicate that an object’s changes can be observed. This term accurately reflects what we can do with an object, but this also why the term falls short. Rather than describing what the object is and describing it’s inherent nature (that it can vary), we are only describing one particular way that we can interact with it. This is a short-sighted description because the nature of being time-varying may give opportunity to multiple ways of interacting with an object. If it is time-varying object, not only can we observe it, we might be able to map it, modify it, or discover meta-data about how it may vary.
Variable – I have been increasingly using this term to describe the basic entity of time-varying value, because “variable” accurately conveys that it is representing something that may change or vary. One of the key aspects of an efficient reactive system is being able to lazily respond to a group of changes, rather than each intermediate step (for example, if you are incrementally changing properties on an object). A variable conveys that a value may change, but that we are not necessarily interested in every intermediate value, like with a stream. The term “variable” fits well with more efficient “pull” reactivity.