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Transforming items asynchronously#

The previous examples illustrated how to transform each item from a stream into another item. Yet, there are cases where we need to go beyond this, for example:

  • Transforming an item into a Uni – any asynchronous processing such as calling a remote service, interacting with a database, etc
  • Transforming an item into a Multi – producing a multi-items stream based on the incoming item, filtering out items, etc

Having the possibility to transform an item into a stream gives us many opportunities. To implement such transformations, we use onItem().transformToUni(Function<T, Uni<O>> and onItem().transformToMulti(Function<T, Multi<O>>)

Uni - Transforming an item into a Uni#

sequenceDiagram
    autonumber
    participant M as Uni
    participant O as Transformer
    participant U as Uni(item)
    participant D as Subscriber

    M->>O: onItem(item)
    O--)U: subscribe(...) 
    U--)O: onSubscribe(sub)
    U--)O: onItem(result) 
    O->>D: onItem(result)

Imagine that you have a Uni<String>, and you want to call a remote service.

Calling a remote service is an asynchronous action represented by a Uni, as in:

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Uni<String> invokeRemoteGreetingService(String name);

To call this service, you need to transform the item received from the first Uni into the Uni returned by the service:

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Uni<String> result = uni
    .onItem().transformToUni(name -> invokeRemoteGreetingService(name));

This snippet chains the first Uni with another one. The returned Uni (result) emits the result from the remote service or a failure if anything wrong happened:

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Uni<String> uni = Uni.createFrom().item("Cameron");
uni
    .onItem().transformToUni(name -> invokeRemoteGreetingService(name))
    .subscribe().with(
            item -> System.out.println(item), // Print "Hello Cameron",
            fail -> fail.printStackTrace()); // Print the failure stack trace

Uni - Transforming an item into a Multi#

The previous example produced a single item. You may want to transform the received item into a stream which is… a Multi!

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Multi<String> result = uni
    .onItem().transformToMulti(item -> Multi.createFrom().items(item, item));

This code creates a stream of two elements, duplicating the received item.

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uni
    .onItem().transformToMulti(item -> Multi.createFrom().items(item, item))
    .subscribe().with(
            item -> System.out.println(item)); // Called twice

sequenceDiagram
    autonumber
    participant M as Uni
    participant O as Transformer
    participant U as Multi(item)
    participant D as Subscriber

    M->>O: onItem(item)
    O--)U: subscribe(...) 
    U--)O: onSubscribe(sub)
    U--)O: onItem(item)
    O->>D: onItem(item) 
    U--)O: onItem(item)
    O->>D: onItem(item)

The produced Multi objects can of course be more complicated than that and emit items in an asynchronous fashion.

Transforming items from Multi - the merge vs concatenate dilemma#

When transforming items emitted by an upstream Multi, we need to answer the following question: how are we going to merge the produced items back?

Let’s take an example. Imagine a Multi emitting the Cameron and Donna items (in order), and you want to call the invokeRemoteGreetingService from above. It thus calls invokeRemoteGreetingService("Cameron") then invokeRemoteGreetingService("Donna").

The service does not have a constant response time (because of network latency or the load), which means that responses can be interleaved. Indeed, you may receive "Hello Donna" before "Hello Cameron".

Now, how do you want to handle this case? Do you need to preserve the order and ensure that the downstream subscriber will always get "Hello Cameron" first, or do you accept interleaved responses?

When transforming items from Multi into streams, you need to decide in which order the items emitted by the produced stream are going to be received by the downstream subscriber. Mutiny offers two possibilities:

  1. Merging – it does not preserve the order and emits the items from the produced streams as they come, or
  2. Concatenating – it maintains and concatenates the streams produced for each item.

Multi - Transforming an item into a Uni#

To implement the scenario from the last section, you will use onItem().transformToUniAndMerge or onItem().transformToUniAndConcatenate() depending on your ordering choice:

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Multi<String> merged = multi
    .onItem().transformToUniAndMerge(name -> invokeRemoteGreetingService(name));

Multi<String> concat = multi
    .onItem().transformToUniAndConcatenate(name -> invokeRemoteGreetingService(name));

Multi - Transforming an item into a Multi#

onItem().transformToMultiAndMerge and onItem().transformToMultiAndConcatenate transform incoming items into Multi streams. The produced Multi objects are either merged or concatenated:

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Multi<String> merged = multi
    .onItem().transformToMultiAndMerge(item -> someMulti(item));

Multi<String> concat = multi
    .onItem().transformToMultiAndConcatenate(item -> someMulti(item));