Creating a Persistent Collection

Creating a Persistent Collection
Abstracting from database implementations

Creating a Persistent Collection

In HexArc, a PersistentCollection is a collection (much in the same sense as java.util.Collection) whose content persist over multiple runs of an application.

The process of creating a PersistentCollection may vary depending on which tools the specific implementation of PersistentCollection relies on. Still, every single PersistentCollection is bound to a specific QueryLanguage (e.g. MongoDB, SQL…), which is either inferred from the implementation or specified from the user.

A QueryLanguage defines how a service can query the data of a PersistentCollection. For example, the MongoDBPersistentCollection is a PersistentCollection based on the MongoDBQueryLanguage, where queries are defined in terms of BSON documents.

Example

import io.github.jahrim.hexarc.persistence.PersistentCollection
import io.github.jahrim.hexarc.persistence.mongodb.MongoDBPersistentCollection
import io.github.jahrim.hexarc.persistence.mongodb.language.MongoDBQueryLanguage

val persistentCollection: PersistentCollection with MongoDBQueryLanguage =
  MongoDBPersistentCollection(
    connection = "mongodb://mongodb0.example.com:27017",
    database = "example-authentication",
    collection = "example-users"
  ).get

Abstracting from database implementations

Separating the concept of a PersistentCollection from its QueryLanguage lets a service declare what language it will be using for communicating with the PersistentCollection, without ever specifying the specific technology it requires for storing its data.

Example

A service may declare to require a PersistentCollection based on the MongoDBQueryLanguage, which the service will use to formulate its queries.

import io.github.jahrim.hexarc.architecture.vertx.core.dsl.VertxDSL.*
import io.github.jahrim.hexarc.persistence.PersistentCollection
import io.github.jahrim.hexarc.persistence.mongodb.MongoDBPersistentCollection
import io.github.jahrim.hexarc.persistence.mongodb.language.MongoDBQueryLanguage

/** This service must use MongoDB as a `PersistentCollection`. */
def strictAuthenticationService(
  users: MongoDBPersistentCollection
): DeploymentGroup ?=> Service = ???

/**
 * This service could use any `PersistentCollection` capable of understanding MongoDB queries.
 * In other words, it is technology-agnostic.
 */
def flexibleAuthenticationService(
  users: PersistentCollection with MongoDBQueryLanguage
): DeploymentGroup ?=> Service = ???

Here the second service is not declaring a dependency on a MongoDB database. Instead, it’s just declaring that it will use the MongoDBQueryLanguage to formulate its queries.

In this way, any PersistentCollection based on any database (e.g. MongoDB, MySQL, SQLServer…) can be served to the service, as long as it is capable of interpreting a query written in the MongoDBQueryLanguage that will be used by the service (e.g. translating BSON queries into SQL queries…).

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