There is a false challenge in testing a microservice. The application does not exist in isolation. It collaborates with other services in an interdependent web. How can one test a single strand of a web?
But test dependency management is not a new challenge. Using a microservice architecture increases the scale of the problem, and this forces a development team to address integration explicitly and strategically.
Before discussing a testing strategy for microservices, we need a simple model with explicitly defined layers. Examples are given for RESTful implementations, but this model could be adapted for any transport format.
Resources handle incoming requests. They validate request format, delegate to services, and then package responses. All handling of the transport format for incoming requests is managed in resources. For a RESTful service, this would include deserialization of requests, authentication, serialization of responses, and mapping exceptions to http status codes.
Services handle business logic for the application. They may collaborate with other services, adapters, or repositories to retrieve needed data to fulfill a request or to execute commands. Services only consume and produce domain objects. They do not interact with DTOs from the persistence layer or transport layer objects – requests and responses in a RESTful service, for example.
Adapters handle outgoing requests to external services. They marshal requests, unmarshal responses, and map them to domain objects that can be used by services. They are usually only called by services. All handling of the transport format for outgoing requests is managed in adapters.
Repositories handle transactions with the persistence layer (generally databases) in much the same way that adapters handle interactions with external services. All handling of persistent dependencies is managed in this layer.
A lightweight microservice might combine one or more of the above layers in a single component, but separation of concerns will make unit testing much simpler.
Planning for Speed and Endurance
A test strategy in general should prevent unwelcome surprises in production. We want to get as much valuable quality-related information as we can (coverage), in realistic conditions (verisimilitude), as fast as we can (speed), and with as little bother as possible (simplicity).
Every test method has trade-offs. Unit testing will provide fast results for many scenarios and are usually built into the build process – they have good coverage, speed, and simplicity, but they aren’t very realistic. Manual user testing has the most verisimilitude and can be very simple to execute, but has very poor speed and coverage.
To balance these trade-offs, we use a tiered testing strategy. Tests at the bottom of the pyramid are generally fast, numerous, and executed frequently, while tests at the top of the tier are generally slow, few in number, and executed less frequently. This article focuses on how these tiers are applied for microservices. Unit Testing
Unit tests cover individual components. In a microservice, unit tests are most useful in the service layer, where they can verify business logic under controlled circumstances against conditions provided by mock collaborators. They are also useful in resources, repositories, and adapters for testing exceptional conditions – service failures, marshaling errors, etc.
To get the most value from unit tests, they need to be executed frequently – every build should run the tests, and a failed test should fail the build. This is configured on a continuous integration server (Jenkins, TeamCity, Bamboo, e.g.) constantly monitoring for changes in the code.
Service testing encompasses all tests of the microservice as a whole, in isolation. Service testing is also often called “functional testing”, but this can be confusing since most tiers described here are technically functional. The purpose of service tests is to verify integration for all components is functionally correct for all components that do not require external dependencies. To enable testing in isolation, we typically use mock components in place of the adapters and in-memory data sources for the repositories, configured under a separate profile. Tests are executed using the same technology that incoming requests would use (http for a RESTful microservice, for example).
A team could avoid using mock implementations of adapters at this tier by testing against mock external services with recorded responses. This is more realistic, but in practice it adds a great deal of complexity – recorded responses must be maintained for each service and updated for all collaborators whenever a service changes. It also requires deploying these mock collaborators alongside the system under test during automated service testing, which adds complexity to the build process. It’s easier to rely on a quick, robust system integration testing process with automated deployments to reduce the lag between these two tiers.
Service tests can also be run as part of the build process using most build tools, ensuring that the application not only compiles but can also be deployed in an in-memory container without issue. System Integration Testing
System integration tests verify how the microservice behaves in a functionally realistic environment – real databases, collaborators, load-balancers, etc. For the sake of simplicity, these are often also end-to-end tests – rather than writing a suite of system integration tests for each micoservice, we develop a suite for the entire ecosystem. In this tier, we are focused on testing configuration and integration using “normal” user flows.
This test suite is also functionally critical because it is the first realistic test of the adapter/repository layer, since we rely on mocks or embedded databases in the lower layers. Because integration with other microservices is so critical, it’s important that this testing process be streamlined as much as possible. This is where an automated release, deployment, and testing process provides tremendous advantages.
User Acceptance Testing
System integration tests verify that the entire web of microservices behaves correctly when used in the fashion the development team assumes it will be used (against explicit requirements). User acceptance replaces assumptions with actual user behavior. Ideally, users are given a set of goals to accomplish and a few scenarios test rather than explicit scripts.
Because user acceptance tests are often manual, this process is generally not automated (though it is possible, with crowd-sourcing). As a result, this can happen informally as part of sprint demoes, formally only for major releases, or through live A/B testing with actual users.
Non-functional testing is a catchall term for tests that verify non-functional quality aspects: security, stability, and performance. While these tests are generally executed less frequently in a comprehensive manner, a sound goal is to try to infect the lower tiers with these aspects as well. For example, security can also be tested functionally (logging in with an invalid password, for example), but at some point it also needs to be tested as an end in itself (through security audits, penetration testing, port scanning, etc). As another example, performance testing can provide valuable information even during automated functional tests by setting thresholds for how long individual method calls may take, or during user acceptance testing by soliciting feedback on how the system responds to requests, but it also needs to be tested more rigorously against the system as a whole under realistic production load.
Ideally, these tests would be scheduled to run automatically following successful system integration testing, but this can be challenging if production-like environments are not always available or third-party dependencies are shared. Summation
The goal of the testing strategy, remember, is to be as fast, complete, realistic, and simple as possible. Each tier of testing adds complexity to development process. Complexity is a hidden cost that must be justified, and not just to project stakeholders – your future self will need to maintain them indefinitely.
This strategy can serve as a model for organizing your own tiered strategy for testing, modified as necessary for your context. If you’ve found new and interesting solutions to the problems discussed in this article, let me know at firstname.lastname@example.org.