Gunnar Morling

Gunnar Morling

Random Musings on All Things Software Engineering

Gunnar Morling

Gunnar Morling

Random Musings on All Things Software Engineering

The Emitter Parameter Pattern for Flexible SPI Contracts

Posted at May 4, 2020

For libraries and frameworks it’s a common requirement to make specific aspects customizeable via service provider interfaces (SPIs): contracts to be implemented by the application developer, which then are invoked by framework code, adding new or replacing existing functionality.

Often times, the method implementations of such an SPI need to return value(s) to the framework. An alternative to return values are "emitter parameters": passed by the framework to the SPI method, they offer an API for receiving value(s) via method calls. Certainly not revolutionary or even a new idea, I find myself using emitter parameters more and more in libraries and frameworks I work on. Hence I’d like to discuss some advantages I perceive about the emitter parameter pattern.

An Example

As an example, let’s consider a blogging platform which provides an SPI for extracting categories and tags from given blog posts. Application developers can plug in custom implementations of that SPI, e.g. based on the latest and greatest algorithms in information retrieval and machine learning. Here’s how a basic SPI contract for this use case could look like, using regular method return values:

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public interface BlogPostDataExtractor {

  Set<String> extractCategories(String contents);
  Set<String> extractTags(String contents);
}

This probably would get the job done, but there are a few problems: any implementation will have to do two passes on the given blog post contents, once in each method — not ideal. Also let’s assume that most blog posts only belong to exactly one category. Implementations still would have to allocate a set for the single returned category.

While there’s not much we can do about the second issue with a return value based design, the former problem could be addressed by combining the two methods:

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public interface BlogPostDataExtractor {

  CategoriesAndTags extractCategoriesAndTags(String contents);
}

Now an implementation can retrieve both categories and tags at once. But it’s worth thinking about how an SPI implementation would instantiate the return type.

Exposing a concrete class to be instantiated by implementors poses a challenge for future evolution of the SPI: following the best practice and making the return object type immutable, all its properties must be passed to its constructor. Now if an additional attribute should be extracted from blog posts, such as a teaser, the existing constructor cannot be modified, so to not break existing user code. Instead, we’d have to introduce new constructors whenever adding further attributes. Dealing with all these constructors could become quite inconvenient, in particular if a specific SPI implementation is only interested in producing some of the attributes.

All in all, for SPIs it’s often a good idea to only expose interfaces, but no concrete classes. So we could make the return type an interface and leave it to SPI implementors to create an implementation class, but that’d be rather tedious.

The Emitter Parameter Pattern

Or, we could provide some sort of builder object which can be used to construct CategoriesAndTags objects. But then why even return an object at all, instead of simply mutating the state of a builder that is provided through a method parameter? And that’s essentially what the emitter parameter pattern is about: passing in an object which can be used to emit the values which should be "returned" by the method.

I’m not aware of any specific name for this pattern, so I came up with "emitter parameter pattern" (the notion of callback parameters is related, yet different). And hey, perhaps I’ll become famous for coining a design pattern name ;) Please let me know in the comments below if you know this pattern under a different name.

Here’s how the extractor SPI could look like when designed with an emitter parameter:

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public interface BlogPostDataExtractor {

  void extractData(String contents, BlogPostDataReceiver data); (1)

  interface BlogPostDataReceiver { (2)

    void addCategory(String category);
    void addTag(String tag);
  }
}
1 SPI method with input parameter and emitter parameter
2 Emitter parameter type

An implementation would emit the retrieved information by invoking the methods on the data parameter:

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public class MyBlogPostDataExtractor implements BlogPostDataExtractor {

  public void extractData(String contents, BlogPostDataReceiver data) {
    String category = ...;
    Stream<String> tags = ...;

    data.addCatgory(category);
    tags.forEach(data::addTag);
  }
}

This approach nicely avoids all the issues with the return value based design:

  • Single and multiple value case handled uniformly: an implementation can call addCategory() just once, or multiple times; either way, it doesn’t have to deal with the creation of a set, list, or other container for the produced value(s)

  • Flexible evolution of the SPI contract: new methods such as addTeaser(), or addTags(String…​ tags) can be added to the emitter parameter type, avoiding the creation of more and more return type constructors; as the passed BlogPostDataReceiver instance is controlled by the framework itself, we also could add methods which provide more context required for the task at hand

  • No need for exposing concrete types on the SPI surface: as no return value needs to be instantiated by SPI implementations, the solution works solely with interfaces on the SPI surface; this provides more control to the framework, e.g. the emitter object could be re-used etc.

  • Flexible implementation choices: by not requiring SPI implementations to allocate any return objects, the platform gains a lot of flexibility for how it’s processing the emitted values: while it could collect the values in a set or list, it also has the option to not allocate any intermediary collections, but process and pass on values one-by-one in a streaming-based way, without any of this impacting SPI implementors

Now, are there some downsides to this approach, too? I can see two: if a method only ever should yield a single value, the emitter API might be misleading. We could raise an exception though if an emitter method is called more than once. Also an implementation might hold on to the emitter object and invoke its methods after the call flow has returned from the SPI method, which typically isn’t desirable. Again that’s something that can be prevented by invalidating the emitter object after the SPI method returned, raising an exception in case of further method invocations.

Overall, I think the emitter parameter pattern is a valuable tool in the box of library and framework authors; it provides flexibility for implementation choices and future evolution when designing SPIs. Real-world examples include the ValueExtractor SPI in Bean Validation 2.0 (where it was chosen to provide a uniform value of extracting single and multiple values from container objects) and the ChangeRecordEmitter contract in Debezium’s SPI.

Many thanks to Hans-Peter Grahsl and Nils Hartmann for reviewing an early version of this blog post.

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