Data Flows

Data flows, or streams, are potentially unbound sequences of values. The flow API in CAF makes it easy to generate, transform, and consume observable sequences with a ReactiveX-style interface.

Data flows in CAF use backpressure to make sure that fast senders cannot overwhelm receivers.

We do not assume prior experience with ReactiveX for this chapter and there are some key differences to ReactiveX implementations that we point out in Key Differences to ReactiveX.

Introduction

The fundamental building blocks of the flow API are observable, observer and subscription.

observable

Emits a potentially unbound sequence of values to observers that have subscribed to the observable. Offers the single member function subscribe to add more observers.

observer

Subscribes to and consumes values from an observable. This interface bundles callbacks for the observable, namely on_subscribe, on_next, on_complete and on_error.

subscription

Manages the flow of items between an observable and an observer. An observer calls request to ask for more items or dispose to stop receiving data.

When working with data flows, these interfaces usually remain hidden and applications leverage high-level operators that either generate or transform an observable. For example, the code snippet below illustrates a trivial data flow for integers inside a single actor that only uses the high-level composition API without any manual setup for observers or subscriptions:

void caf_main(caf::actor_system& sys, const config& cfg) {
  auto n = get_or(cfg, "num-values", default_num_values);
  sys.spawn([n](caf::event_based_actor* self) {
    self
      // Get an observable factory.
      ->make_observable()
      // Produce an integer sequence starting at 1, i.e., 1, 2, 3, ...
      .iota(1)
      // Only take the requested number of items from the infinite sequence.
      .take(n)
      // Print each integer.
      .for_each([self](int x) { self->println("{}", x); });
  });
}

In the concurrency model of CAF, all of these building blocks describe processing steps that happen inside of an actor. The actor owns all of its observer, observable and subscription objects and they cannot outlive their parent actor. This means that an observable must not be shared with others.

To move data from one actor to another, CAF provides asynchronous buffers. The following figure depicts the general architecture when working with data flows across actor boundaries:

CAF flow running through two actors.

Here, actor A creates an observbale and then applies the flat_map and filter operators to it. The resulting items then flow into an asynchronous buffer that connects to actor B. Actor B then applies the map and filter operators to the incoming data before terminating the data flow, e.g., by printing each value.

Concurrent Processing

Flows that only run inside a single actors are of course quite useless outside of toy examples. For running different parts of a data flow on different actors, CAF offers two APIs: one for setting up a processing chain declaratively and one for setting up processing chains dynamically.

Declarative Setup: observe_on

If the entire processing chain is known at coding time, observe_on provides the easiest way to assign work to individual actors. The following example revisits our first example, but this time generates the numbers on one actor and then prints them on another.

void caf_main(caf::actor_system& sys, const config& cfg) {
  // Create two actors without actually running them yet.
  auto n = get_or(cfg, "num-values", default_num_values);
  auto [src, launch_src] = sys.spawn_inactive();
  auto [snk, launch_snk] = sys.spawn_inactive();
  // Define our data flow: generate data on `src` and print it on `snk`.
  src
    // Get an observable factory.
    ->make_observable()
    // Produce an integer sequence starting at 1, i.e., 1, 2, 3, ...
    .iota(1)
    // Only take the requested number of items from the infinite sequence.
    .take(n)
    // Switch to `snk` for further processing.
    .observe_on(snk)
    // Print each integer.
    .for_each([](int x) { std::cout << x << std::endl; });
  // Allow the actors to run. After this point, we may no longer dereference
  // the `src` and `snk` pointers! Calling these manually is optional. When
  // removing these two lines, CAF automatically launches the actors at scope
  // exit.
  launch_src();
  launch_snk();
}

Please note that calling observe_on requires that the target actor is inactive. Otherwise, this function call results in unsynchronized state access.

Dynamic Setup: Asynchronous Buffers

Our second option for spanning data flows across multiple actors is using SPSC (Single Producer Single Consumer) buffers. This option is more general. In fact, observe_on internally uses these buffers for connecting the actors. Further, the buffers allows bridging flows between actor and non-actor code.

While one could use an SPSC buffer directly, they usually remain hidden behind another abstraction: asynchronous resources. The resources in CAF usually come in pairs and users may create new ones by calling make_spsc_buffer_resource. This function returns a producer resource and a consumer resource. With these two resources, we can then spawn actors that open the resources for either reading or writing.

To illustrate how the API pieces fit together, we revisit our example a third time. This time, we spawn the actors individually and connect them via the buffer resources:

void caf_main(caf::actor_system& sys, const config& cfg) {
  auto [pull, push] = caf::async::make_spsc_buffer_resource<int>();
  auto n = get_or(cfg, "num-values", default_num_values);
  sys.spawn(sink, std::move(pull));
  sys.spawn(source, std::move(push), n);
}

In this iteration of our example, we have moved the implementation for the source and sink actors to their own functions. The source once again creates the data, only this time we subscribe the buffer to the generated sequence:

// Simple source for generating a stream of integers from 1 to n.
void source(caf::event_based_actor* self,
            caf::async::producer_resource<int> out, size_t n) {
  self
    // Get an observable factory.
    ->make_observable()
    // Produce an integer sequence starting at 1, i.e., 1, 2, 3, ...
    .iota(1)
    // Only take the requested number of items from the infinite sequence.
    .take(n)
    // Subscribe the resource to the sequence, thereby starting the stream.
    .subscribe(out);
}

For the sink, we generate an observable from the consumer resource and then once more call for_each:

// Simple sink for consuming a stream of integers, printing it to stdout.
void sink(caf::event_based_actor* self, caf::async::consumer_resource<int> in) {
  self
    // Get an observable factory.
    ->make_observable()
    // Lift the input to an observable flow.
    .from_resource(std::move(in))
    // Print each integer.
    .for_each([self](int x) { self->println("{}", x); });
}

Building and Transforming Observables

When building processing pipelines, CAF fuses as many processing steps as possible into a single C++ object. In our examples, we composed the source part like this: self->make_observable().from_callable(...).take(...)....

The first bit, self->make_observable(), returns an observable_builder. This class implements factory functions for creating observable sequences from containers, repeated values, and so on. However, most functions do not actually return an observable. Instead, they return a generation<...> object.

The generation class is a variadic template that allows CAF to incrementally define consecutive processing steps. In our example, we call from_callable on the builder object, which returns a generation<callable_source<...>>. The generation is meant as temporary object only. Hence, most member functions may only get called on an rvalue.

After calling .take(...) on the returned generation, we get a new temporary object of type generation<callable_source<...>, limit_step<...>>.

The generation class also mimics the interface of observable. When calling a member function that requires an actual observable, CAF uses the blueprint stored in the generation to create an actual observable object and then forward the member function call. For example, calling for_each on a generation internally constructs the observable and then calls for_each on that new object.

Users can also call as_observable on a generation explicitly to turn the blueprint into an actual observable sequence.

By delaying the construction of actual observable instances, CAF can fuse consecutive steps into single objects. This reduces the number of heap allocations and also accelerates processing, since the fused processing steps result in simple function call chains without subscriptions and backpressure between them.

Analogues to the generation class for creating new observables from inputs, CAF uses a template called transformation that represents a blueprint for applying operators to existing observables.

Operators

Most operators transform an observable by applying one or more processing steps on all observed values and then emit the result as a new observable. Since the result of a transformation usually is new observable, these operators compose into complex data stream operations.

The operators presented here are available on the template classes observable, generation and transformation.

Concat

The concat operator takes multiple input observables and re-emits the observed items as a single sequence of items without interleaving them.

_images/concat.svg

Concat Map

The concat_map operator takes a function object converts a given input to an observable and then applies concat to all of them.

_images/concat_map.svg

Distinct

The distinct operator makes all items unique by filtering all items have been emitted in the past.

_images/distinct.svg

Element At

The element_at operator re-emits the item at a given index while ignoring all other items.

_images/element_at.svg

Filter

The filter operator re-emits items from its input observable that pass a predicate test.

_images/filter.svg

First

The first operator re-emits only the first item of the input observable.

_images/first.svg

Flat Map

The flat_map operator takes a function object converts a given input to an observable and then applies merge to all of them.

_images/flat_map.svg

Head and Tail

The head_and_tail operator splits an observable into its first item and an observable for the remainder.

_images/head_and_tail.svg

Ignore Elements

The ignore_elements operator ignores all items of the input observable and only emits the completion (or error) signal.

_images/ignore_elements.svg

Last

The last operator re-emits only the last item of the input observable.

_images/last.svg

Map

The map operator applies a unary operation to all items of the input observable and re-emits the resulting items. Similar to std::transform.

_images/map.svg

Merge

The merge operator takes multiple input observables and re-emits the observed items as a single sequence of items as soon as they appear.

_images/merge.svg

Observe On

The observe_on operator pipes data from one actor to another through an asynchronous buffer. The target actor must not run at the point of calling this operator. In the image below, alice (red) and bob (blue) are two actors.

_images/observe_on.svg

Prefix and Tail

The head_and_tail operator splits an observable into its first n items (stores in a caf::cow_vector) and an observable for the remainder.

_images/prefix_and_tail.svg

Reduce

The reduce operator is similar to std::accumulate, only that it operates on an observable instead of an iterator range.

_images/reduce.svg

Ref Count

The ref_count operator turns a connectable back to a regular observable by automatically calling connect as soon as there is an initial “reference” (subscription). After the last “reference” goes away (no more subscription), the ref_count operators unsubscribes from its source.

_images/ref_count.svg

Skip Last

The skip_last operator re-emits all but the last n items from its input observable.

_images/skip_last.svg

Skip

The skip operator re-emits all but the first n items from its input observable.

_images/skip.svg

Sum

The sum operator accumulates all items and emits the result after the input observable has completed.

_images/sum.svg

Take Last

The take_last operator re-emits the last n items from its input observable.

_images/take_last.svg

Take

The take operator re-emits the first n items from its input observable.

_images/take.svg

Take While

The take_while operator re-emits items from its input observable until its predicate returns false.

_images/take_while.svg

To Vector

The to_vector operator collects all items and emits a single vector containing all observed items after the source observable has completed.

_images/to_vector.svg

Notes on Performance

When working with data flows in CAF, it is important to remember that values are frequently copied and buffered. In languages with call-by-reference semantics such as Java, this is not an issue since the flows basically pass pointers down the chain. In C++ however, programmers must choose wisely what data types are used in a flow.

Passing down types such as std::string or std::vector will invariably slow down your application to a crawl. CAF offers three classes that help mitigate this problem: caf::cow_string, caf::cow_vector and caf::cow_tuple. These type are thin wrappers around their standard library counterpart that add copy-on-write (COW) semantics. Internally, all COW-types have a pointer to the actual data with a reference count. This makes them cheap to copy and save to use in a data flow. Most of the time, data in a flow does not need to change after creating and is de-facto immutable. However, the COW-optimization still gives you a mutable reference if you really need it and you make a deep copy only if you must, i.e., if there are multiple references to the data.

Key Differences to ReactiveX

Observables are not thread-safe. They describe a flow of data within an actor and are thus considered private to an actor.

CAF is more “opinionated” than ReactiveX when it comes to concurrency and ownership. The intended way for connecting concurrent parts of the system is by creating buffer resources and turning them into observables at the observing actor.

Furthermore, CAF does not support the scheduler interface from ReactiveX. Data flows are usually managed by an actor. Hence, there is no analog for operators such as SubscribeOn. That being said, the flow API does not tie observables or observers to actor types. The interface caf::flow::coordinator manages scheduling of flow-related work and can be implemented to run CAF flows without actors, e.g., to integrate them into a custom event loop.