Datomic Peer Server, in-mem

Minimal project setup to test using Molecule with a Datomic Peer Server in-memory database (protocol: mem).

import sbt.Keys._

lazy val demo = project.in(file("."))
  .settings(name := "molecule-datomic-peerserver-mem")

lazy val app = project.in(file("app"))
    scalaVersion := "2.13.5",
    resolvers ++= Seq(
      "clojars" at "https://clojars.org/repo",
      "my.datomic.com" at "https://my.datomic.com/repo"
      Downloading Datomic Starter/Pro requires authentication of your license:
      Create a ~/.sbt/.credentials file with the following content:

        realm=Datomic Maven Repo

      Then let sbt provide your secret credentials:
    credentials += Credentials(Path.userHome / ".sbt" / ".credentials"),

    libraryDependencies ++= Seq(
      "org.scalamolecule" %% "molecule" % "0.25.1",
      "com.datomic" % "datomic-pro" % "1.0.6269"

    // Important to exclude fee version when using pro to avoid clashes with pro version
    excludeDependencies += ExclusionRule("com.datomic", "datomic-free"),

    // path to domain model directory
    moleculeDataModelPaths := Seq("app"),

    // Generate Molecule boilerplate code with `sbt clean compile -Dmolecule=true`
    moleculePluginActive := sys.props.get("molecule") == Some("true"),

    // Let IDE detect created jars in unmanaged lib directory
    exportJars := true

1. Start Peer Server

First, you need to start the Peer Server with an automatically created in-memory database:

cd <your-datomic-starter/pro-distribution>
bin/run -m datomic.peer-server -h localhost -p 8998 -a k,s -d sampledb,datomic:mem://sampledb

In this setup we use a database connection URI with the “mem” protocol which is intended for in-memory databases.

If you need persisting data, please see other storage options or have a look at one of the other sample projects.

The other connection options explained:

-m datomic.peer-server              // the peer-server command
-h localhost                        // host name
-p 8998                             // port number
-a k,s                              // access-key,secret
-d sampledb,datomic:mem://sampledb  // dbName-alias,URI

There can be no space after comma in the pairs of options!

For simplicity, we just chose to write “k,s” for access-key,secret. The important thing is that you need to supply the same pair when you connect to the Peer Server in your code (as with host/port names).

If successful, it will show something like “Serving datomic:mem://sampledb as sampledb”.

2. Connect to Peer Server

Presuming the transactor is running and the Peer Server is serving sampledb we can connect to it:

implicit val conn = 
  Datomic_PeerServer("k", "s", "localhost:8998")

We use the same coordinates here as when we started the Peer Server.

For the purpose of testing, we want to make sure that our schema is up-to-date and therefore transact it on every run:

implicit val conn = 
  Datomic_PeerServer("k", "s", "localhost:8998")
    .transactSchema(SampleSchema, "sampledb")

3. Make molecules

From here on, we can start transacting and querying sampledb with molecules:

// Transact

// Query
assert(Person.name.age.get.head == ("John", 24))

Add/change definitions in SampleDataModel and run sbt clean compile -Dmolecule=true in your project root to have Molecule re-generate boilerplate code. Then you can try out using your new attributes in new molecules in SampleApp.