In-memory Object Graph Stores

Implementation of an in-memory object graph store, dubbed ϵStore. Our key innovation is a storage model -- epsilon store -- that equates an object on the heap to a node in a graph store. Thus any object on the heap (without changes) can be a part of one, or multiple, graph stores, and vice versa, any node in a graph store can be accessed like any other object on the heap. ϵStore uses a subset of the Cypher query language to query the graph store. By design, the result of any query is a table of references to objects on the heap, which users can manipulate the same way as any other object on the heap in their programs.

Docker

If you want to run in an isolated container, skip installing Java, Maven, and the other tools in Prerequisites. Docker installs those dependencies and ϵStore during the build. Start a shell with:

docker build -t estore .
docker run --rm -it -p 1234:1234 estore

Port 1234 is mapped to the host. Inside the container, run ./s exec_estore and query the server as described under Running the Application.

Examples

1. Capturing a Java object graph and querying it with Cypher-like syntax.

   Person charlie = new Person("Charlie", 25);
   Person bob = new Person("Bob", 30, charlie);
   Person alice = new Person("Alice", 28, bob);
   
   Estore db = new Estore("exampleDb");
   db.captureAll(alice);
   
   // MATCH finds Person objects; RETURN puts them in column p
   Table result = db.query("MATCH (p:`org.estore.example.Person`) RETURN p");
   
   result.print();
   Person p = (Person) result.get("p").get(0);
   System.out.println(p.name + ", " + p.age);
   System.out.println(p.friend.name);

   alice is an ordinary Java Person object (name "Alice", age 28). Its friend field points to Bob, and Bob's friend field points to Charlie, so the in-memory graph is Alice → Bob → Charlie. captureAll(alice) walks that graph from Alice and stores every reachable object. The query finds those Person objects (MATCH) and returns them as a table column named p (RETURN). Cells in that table are the same heap objects, so they can be printed, cast to Person, and used like any other Java object — including following friend in ordinary Java.

2. Querying object relationships.

   Table friends =
       db.query("MATCH (a:`org.estore.example.Person`)-[:friend]->(b:`org.estore.example.Person`) RETURN a, b");
   
   for (int i = 0; i < friends.getSize(); i++) {
       Person a = (Person) friends.get("a").get(i);
       Person b = (Person) friends.get("b").get(i);
       System.out.println(a.name + " → " + b.name);
   }

   -[:friend]-> follows the friend field between captured Person objects and returns each matched pair. The loop casts those cells back to Person and prints the names.

Using ϵStore in a Maven Project

After packaging (see the next section), ϵStore can be used in a Maven project.

The client jar can be added as a dependency to a third-party project by adding the following to its pom.

  <dependency>
    <groupId>org.estore</groupId>
    <artifactId>estore</artifactId>
    <version>1.0.0</version>
    <scope>system</scope>
    <systemPath><!-- ENTER full path to client jar including jar name --></systemPath>
  </dependency>

Getting Started (with Development)

Prerequisites

The project requires the following dependencies:

• Java 8
• Maven
• wget
• zstd
• tar
• gzip
• nc for the quick query test

Installation

1. Install Dependencies

Run the installation script to automatically install all required dependencies:

./s install_deps

This will check for and install any missing dependencies on your system.

Alternatively, verify your dependencies are correctly installed:

./s check_deps

On Debian/Ubuntu, run install_deps with sudo. On macOS, use Homebrew instead (brew install openjdk@8 maven wget zstd).

2. Build the Project

Compile the estore project:

./s compile_estore

3. Full Installation

To compile and install the complete project:

./s install_estore

Running the Project

Run Tests

Execute the test suite:

mvn -pl estore test verify

The JaCoCo code coverage report is generated at estore/target/site/jacoco/index.html.

Run the Application

Start the estore server and query it over the network:

./s exec_estore

In another terminal, send a Cypher-like query over TCP (default port 1234):

echo 'MATCH (n) RETURN n' | nc localhost 1234

Example output:

╔═════════╗
║ n       ║
╠═════════╣
║ (empty) ║
╚═════════╝

Send q to stop the server.

Format Code

Auto-format Java code according to project standards:

mvn spotless:apply
mvn verify

End-to-End Setup

Perform a complete setup with dependency checks and full installation:

./s end_to_end

Citation

This repository contains code related to the following publication:

@inproceedings{ThimmaiahETAL25eStore,
  author = {Thimmaiah, Aditya and Yi, Zijian and Kenis, Joseph and Rossbach, Christopher J. and Gligoric, Milos},
  title = {In-memory Object Graph Stores},
  booktitle = {European Conference on Object-Oriented Programming},
  pages = {30:1--30:30},
  year = {2025},
}