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.

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", new EstoreOptions().useUnsafe(false));
   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},
}