Simple Code: Contracts
Code works around contracts and contracts should be carefully thought and crafted.
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What Are Contracts?
A high abstraction level of contracts for code is APIs. They define an interface that is basically a contract that the producer and consumer of the API agree to use to communicate with each other. Two common forms of APIs are libraries that are used in code and external APIs that are used via HTTP, RPC, etc.
When thinking a bit deeper, contracts consist firstly of functions, methods, or external endpoints and secondly of data, more precisely on data models and data types within the models.
Contracts should always be defined with careful thought. A few times, I've come across someone saying that "this is for internal use only, so it doesn't need to be defined and/or documented as thoughtfully as a public API would," but I disagree with that. The same care should be given to internal and external contracts because the contracts are defined to be used by other code and most likely by other developers. Contracts are a key ingredient in the developer experience.
Tests Love Contracts
Contracts also work as a base for testing whether it's unit, integration, or acceptance testing, they are all implemented against some sort of contract. Without properly defined contracts, testing would be nearly impossible because of all the possible variations a badly designed API would enable.
Formats of Contracts
One incarnation of data model contracts is schemas. Those who have worked long enough in the industry can remember SOAP, XML Schemas, and WSDL, after XML came JSON and *drum roll* JSON schemas that are probably most often seen in Open API specifications. Some of the newer formats for defining contracts for code are e.g. Apache Avro and Protobuf.
They're all basically the same thing just defined in a bit different format and they all address the same issue of defining a contract of what the input and output data must look like and some of the definitions also define transport protocols.
Contracts appear in many ways and the trick is to understand that they are actually contracts. Not just some random bits of code that can be whipped together without a second thought but something that deserves attention.
In the next part, I'll be writing about a special thing called immutability.
Published at DZone with permission of Jori Lytter, DZone MVB. See the original article here.
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