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What makes Mutiny different?#

There are other reactive programming libraries out there. In the Java world, we can mention Project Reactor and Rx Java.

So, what makes Mutiny different from these two well-known libraries? The API!

Asynchronous is hard to grasp for most developers, and for good reasons. Thus, the API must not require advanced knowledge or add cognitive overload. It should help you design your logic and still be intelligible when you return to the code 6 months later.

To achieve this, Mutiny is built on three pillars:

  • Event-Driven - with Mutiny, you listen for events and handle them,
  • API Navigability - based on the event-driven nature, the API is built around the type of events and drive the navigation based on the kind of event you want to handle,
  • Simplicity - Mutiny provides only two types (Multi and Uni), which can handle any kind of asynchronous interactions.


When you use Mutiny, you design a pipeline in which the events flow. Your code observes these events and react.

Each processing stage is a new pipe you append to the pipeline. This pipe may change the events, create new ones, drops, buffers, whatever you need.

In general, events flow from upstream to downstream, from source to sinks. Some events can swim upstream from the sinks to the source.

Events going from upstream to downstream are published by Publishers and consumed by (downstream) Subscribers, which may also produce events for their own downstream, as illustrated by the following diagram:

   participant P as Publisher
   participant O1 as Processor 1 
   participant O2 as Processor 2 
   participant S as Subscriber

   P->>O1: onItem
   O1->>O2: onItem
   O2->>S: onItem

   P->>O1: onItem
   O1->>O2: onItem
   O2->>S: onItem

   P->>O1: onCompletion
   O1->>O2: onCompletion
   O2->>S: onCompletion

Four types of events can flow in this direction:

  • Subscribed - indicates that the upstream has taken into account the subscription - more on this later,
  • Items - events containing some (business) value,
  • Completion - event indicating that the source won’t emit any more items,
  • Failure - event telling that something terrible happened upstream and that the source cannot continue to emit items.

Failure and Completion are terminal events. Once they are sent, no more items will flow.

Three types of events flow in the opposite direction, i.e. from downstream to upstream:

  • Subscription - event sent by a subscriber to indicate its interest for the events (such as items) emitted by upstream
  • Requests - event sent by a subscriber indicating how many items event it can handle - this is related to back-pressure
  • Cancellation - event sent by a subscriber to stop the reception of events.

In a typical scenario:

  1. A subscriber subscribes to the upstream - the upstream receive the subscription request, and when initialized sends the subscribed event to the subscriber
  2. The subscriber gets the subscribed event with a subscription used to emit the requests and cancellation events
  3. The subscriber sends a request event indicating how many items it can handle at this moment; it can request 1, n, or infinite.
  4. The publisher receiving the request event starts emitting at most n item events to the subscriber
  5. The subscriber can decide at any time to request more events or to cancel the subscription
   participant P as Publisher
   participant O as Processor 
   participant S as Subscriber

   S->>O: subscribe
   O->>P: subscribe

   P->>O: subscription
   O->>S: subscription

   S->>O: request(5)
   O->>P: request(5)

   P->>O: onItem("a")
   O->>S: onItem("A")

   P->>O: onItem("b")
   O->>S: onItem("B")

   S->>O: cancel()
   O->>P: cancel()

The request event is the cornerstone of the back-pressure protocol. A subscriber should not request more than what it can handle, and a publisher should not emit more items than the amount of request received.


Mutiny uses the Reactive Streams protocol for back-pressure management.


Don’t forget to subscribe!

If no subscriber subscribes, no items will be emitted. More importantly, nothing will ever happen.

If your program does not do anything, check that it subscribes, it’s a very common error.

An event-driven API?#

Mutiny is an event-driven API.

For each type of event, there is an on associated method that lets you handle this specific event. For example:

Multi<String> source = Multi.createFrom().items("a", "b", "c");
  .onItem() // Called for every item
    .invoke(item -> log("Received item " + item))
  .onFailure() // Called on failure
    .invoke(failure -> log("Failed with " + failure))
  .onCompletion() // Called when the stream completes
    .invoke(() -> log("Completed"))
  .onSubscription() // Called when the upstream is ready
    .invoke(subscription -> log("We are subscribed!"))
  .onCancellation() // Called when the downstream cancels
    .invoke(() -> log("Cancelled :-("))
  .onRequest() // Called on downstream requests
    .invoke(n -> log("Downstream requested " + n + " items"))
    .with(item -> log("Subscriber received " + item));

Of course, the methods presented in this snippet are not very interesting, although they are quite useful to trace what’s going on.

You can see a common pattern emerging:

.onEvent().invoke(event -> ...);

invoke is just one of the methods available. Each group proposes methods specific to the type of event. For example, onFailure().recover, onCompletion().continueWith and so on.