PREvant is a web-based software tool that acts as a testing and review platform, simplifying the deployment and management of microservices for development teams. Operating as a Docker container, it connects continuous integration pipelines with container orchestration platforms, allowing developers to ensure that features align with domain expert requirements. PREvant's RESTful API helps to integrate microservices from different branches (in multi-repo development setups) into reviewable applications, creating preview environments for testing new features before they are finalized. This reduces the complexity and speeds up the development process, aligning with agile methodologies.
The name PREvant short for Preview servant, pronounced like "prevent"
(prɪˈvɛnt), reflects its role in preventing development errors by enabling
early reviews through its web interface, where stakeholders can assess and give
feedback on application developments efficiently.
Through PREvant's web interface domain experts, managers, developers, and sales experts can review and demonstrate the application development.
An application, that PREvant manages, is a composition of microservices based on an “architectural pattern that arranges an application as a collection of loosely coupled, fine-grained services, communicating through lightweight protocols.” (Wikipedia) Each application has a unique name which is the key to perform actions like creating, duplicating, modifying, or deleting these applications via REST API or Web UI.
Figure [above] illustrates the disjoint repositories and continuous delivery pipelines of the microservices order, shipping, and invoice. The build stage packages the microservices as a container image and pushes it to a container image registry (e.g. a Docker registry) to ensure that the services are ready for deployment in the acceptance and manual test stages. In a deployment phase, such as the manual test stage depicted in [above] Figure, the continuous delivery pipeline can utilize PREvant’s REST API. This REST request creates a software-defined network, initiates the container for the microservice, connects it to the network, and creates a reverse-proxy configuration, making the service accessible through PREvant’s web interface. Subsequent REST calls check whether the container image has a newer version, and if so, then the container is updated Joint Post-proceedings of the First and Second International Conference on Microservices (Microservices 2017/2019): PREvant (Preview Servant): Composing Microservices into Reviewable and Testable Applications.
In each application, PREvant manages the microservices as services which need to be available in the OCI Image Format (a.k.a. Docker images). At least one service needs to be available for an application. PREvant manages the following kind of services:
- Instance: a service labeled as instance is a service that has been configured explicitly when creating or updating an application.
- Replica: a service labeled as replica is a service that has been replicated from another application. By default if you create an application under any name PREvant will replicate all instances from the application master. Alternatively, any other application can be specified as a source of replication.
Additionally, PREvant provides a way of deploying services every time it creates an application. These services are called companions and there are two types of them.
- Application wide companion (short app companion): is a unique service for the entire application. For example, a Kafka instance can be started automatically every time an application is created, so that all services within the application can synchronize via events.
- Service companion: A companion can also be attached to a specific service a user wants to deploy. For example, a PostgreSQL container can be started for each service to provide it with a dedicated database.
Further instructions to configure Companions can be seen here.
In this section, you'll find examples of deploying PREvant in container environments:
- For Docker, refer to Docker example
- For Kubernetes (requires at least Kubernetes 1.15), refer to Kubernetes example
To customize the behavior of PREvant, you can mount a TOML file into the container at /app/config.toml. More details about the configuration can be found here.
PREvant is able to show the version of your service (build time, version string, and git commit hash) and also to integrate your API specification into the frontend through Swagger UI. In order to show the information, PREvant tries to resolve it by using the web-based protocol proposed by RFC 6415.
When you request the list of apps and services running through the frontend, PREvant makes a request for each service to the URL .well-known/host-meta.json and expects that the resource provides a host-meta document serialized as JSON:
{
"properties": {
"https://schema.org/softwareVersion": "0.9",
"https://schema.org/dateModified": "2019-04-09T15:31:01.363+0200",
"https://git-scm.com/docs/git-commit": "43de4c6edf3c7ed93cdf8983f1ea7d73115176cc"
},
"links": [
{
"rel": "https://github.com/OAI/OpenAPI-Specification",
"href": "https://example.com/master/service-name/swagger.json"
}
]
}This sample document contains the relevant information displayed in the frontend (each information is optional):
- The software version of the service (see
https://schema.org/softwareVersion) - The build time of the service (see
https://schema.org/dateModified) - The git commit id of the service (see
https://git-scm.com/docs/git-commit) - The link to the API specification (see
https://github.com/OAI/OpenAPI-Specification)
In order to generate the correct link to the API specification, PREvant adds following headers to each of these requests:
Forwardedheader withhostandproto.X-Forwarded-Prefix(used by some reverse proxies, cf. Traefik and Zuul).
In the Development section, you can view the detailed guide on, how to develop/run PREvant.
Developers looking to contribute to PREvant can engage through GitHub by addressing issues, enhancing documentation, and submitting pull requests. The project's open-source nature encourages collaboration and innovation from the developer community.
PREvant's concept has been published in the Joint Post-proceedings of the First and Second International Conference on Microservices (Microservices 2017/2019): PREvant (Preview Servant): Composing Microservices into Reviewable and Testable Applications. This paper is based on the abstract that has been published at the conference Microservices 2019 in Dortmund.
The talk delivered at the conference is available on YouTube. Click on the image to start the playback:
