diff --git a/api/server/pom.xml b/api/server/pom.xml
index 137e9bb..d68bb29 100644
--- a/api/server/pom.xml
+++ b/api/server/pom.xml
@@ -1,116 +1,121 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
 <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
     <modelVersion>4.0.0</modelVersion>
 
     <groupId>org.softwareheritage.graph</groupId>
     <artifactId>graph</artifactId>
     <version>1.0-ALPHA</version>
 
     <name>graph</name>
     <url>https://www.softwareheritage.org/</url>
 
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <maven.compiler.source>1.8</maven.compiler.source>
         <maven.compiler.target>1.8</maven.compiler.target>
     </properties>
 
     <dependencies>
         <dependency>
             <groupId>junit</groupId>
             <artifactId>junit</artifactId>
             <version>4.11</version>
             <scope>test</scope>
         </dependency>
         <dependency>
             <groupId>io.javalin</groupId>
             <artifactId>javalin</artifactId>
             <version>2.8.0</version>
         </dependency>
         <dependency>
             <groupId>org.slf4j</groupId>
             <artifactId>slf4j-simple</artifactId>
             <version>1.7.26</version>
         </dependency>
         <dependency>
             <groupId>com.fasterxml.jackson.core</groupId>
             <artifactId>jackson-databind</artifactId>
             <version>2.9.8</version>
         </dependency>
         <dependency>
             <groupId>it.unimi.dsi</groupId>
             <artifactId>webgraph-big</artifactId>
             <version>3.5.0</version>
         </dependency>
+        <dependency>
+            <groupId>it.unimi.dsi</groupId>
+            <artifactId>fastutil</artifactId>
+            <version>8.2.2</version>
+        </dependency>
     </dependencies>
 
     <build>
         <pluginManagement><!-- lock down plugins versions to avoid using Maven defaults (may be moved to parent pom) -->
             <plugins>
                 <!-- clean lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#clean_Lifecycle -->
                 <plugin>
                     <artifactId>maven-clean-plugin</artifactId>
                     <version>3.1.0</version>
                 </plugin>
                 <!-- default lifecycle, jar packaging: see https://maven.apache.org/ref/current/maven-core/default-bindings.html#Plugin_bindings_for_jar_packaging -->
                 <plugin>
                     <artifactId>maven-resources-plugin</artifactId>
                     <version>3.0.2</version>
                 </plugin>
                 <plugin>
                     <artifactId>maven-compiler-plugin</artifactId>
                     <version>3.8.0</version>
                 </plugin>
                 <plugin>
                     <artifactId>maven-surefire-plugin</artifactId>
                     <version>2.22.1</version>
                 </plugin>
                 <plugin>
                     <artifactId>maven-jar-plugin</artifactId>
                     <version>3.0.2</version>
                 </plugin>
                 <plugin>
                     <artifactId>maven-install-plugin</artifactId>
                     <version>2.5.2</version>
                 </plugin>
                 <plugin>
                     <artifactId>maven-deploy-plugin</artifactId>
                     <version>2.8.2</version>
                 </plugin>
                 <!-- site lifecycle, see https://maven.apache.org/ref/current/maven-core/lifecycles.html#site_Lifecycle -->
                 <plugin>
                     <artifactId>maven-site-plugin</artifactId>
                     <version>3.7.1</version>
                 </plugin>
                 <plugin>
                     <artifactId>maven-project-info-reports-plugin</artifactId>
                     <version>3.0.0</version>
                 </plugin>
                 <plugin>
                     <artifactId>maven-assembly-plugin</artifactId>
                     <configuration>
                         <archive>
                             <manifest>
                                 <mainClass>org.softwareheritage.graph.App</mainClass>
                             </manifest>
                         </archive>
                         <descriptorRefs>
                             <descriptorRef>jar-with-dependencies</descriptorRef>
                         </descriptorRefs>
                     </configuration>
                     <executions>
                         <execution>
                             <id>make-assembly</id> <!-- this is used for inheritance merges -->
                             <phase>package</phase> <!-- bind to the packaging phase -->
                             <goals>
                                 <goal>single</goal>
                             </goals>
                         </execution>
                     </executions>
                 </plugin>
             </plugins>
         </pluginManagement>
     </build>
 </project>
diff --git a/api/server/src/main/java/org/softwareheritage/graph/App.java b/api/server/src/main/java/org/softwareheritage/graph/App.java
index ffad3ad..f93e671 100644
--- a/api/server/src/main/java/org/softwareheritage/graph/App.java
+++ b/api/server/src/main/java/org/softwareheritage/graph/App.java
@@ -1,18 +1,23 @@
 package org.softwareheritage.graph;
 
+import java.nio.file.Path;
+import java.nio.file.Paths;
+
 import io.javalin.Javalin;
 
-import org.softwareheritage.graph.GraphAPI;
+import org.softwareheritage.graph.Dataset;
+import org.softwareheritage.graph.Graph;
 
 public class App
 {
     public static void main(String[] args)
     {
-        GraphAPI graph = new GraphAPI("/path/to/graph");
+        Path path = Paths.get(args[0]);
+        Graph graph = new Graph(path.toString());
 
         Javalin app = Javalin.create().start(5010);
         app.get("/nb_nodes", ctx -> {
             ctx.json(graph.nbNodes());
         });
     }
 }
diff --git a/api/server/src/main/java/org/softwareheritage/graph/Dataset.java b/api/server/src/main/java/org/softwareheritage/graph/Dataset.java
new file mode 100644
index 0000000..dda4917
--- /dev/null
+++ b/api/server/src/main/java/org/softwareheritage/graph/Dataset.java
@@ -0,0 +1,115 @@
+package org.softwareheritage.graph;
+
+import java.io.FileInputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.zip.GZIPInputStream;
+
+import it.unimi.dsi.big.webgraph.BVGraph;
+import it.unimi.dsi.big.webgraph.LazyLongIterator;
+import it.unimi.dsi.fastutil.io.BinIO;
+import it.unimi.dsi.fastutil.longs.LongBigArrays;
+import it.unimi.dsi.fastutil.objects.Object2LongFunction;
+import it.unimi.dsi.io.FastBufferedReader;
+import it.unimi.dsi.io.LineIterator;
+import it.unimi.dsi.lang.MutableString;
+
+public class Dataset
+{
+    public enum Name {
+        DIR_TO_DIR,
+        DIR_TO_FILE,
+        DIR_TO_REV,
+        ORIGIN_TO_SNAPSHOT,
+        RELEASE_TO_OBJ,
+        REV_TO_DIR,
+        REV_TO_REV,
+        SNAPSHOT_TO_OBJ
+    }
+
+    BVGraph graph;
+    String path;
+    HashMap<String, Long> hashToNode;
+    HashMap<Long, String> nodeToHash;
+
+    public Dataset(String datasetPath)
+    {
+        try {
+            this.graph = BVGraph.load(datasetPath);
+            this.path = datasetPath;
+            setupNodesMapping();
+        } catch (Exception e) {
+            System.out.println("[WARNING] Could not load dataset " + datasetPath + ": " + e);
+        }
+    }
+
+    void setupNodesMapping() throws IOException, ClassNotFoundException
+    {
+        this.hashToNode = new HashMap<String, Long>();
+        this.nodeToHash = new HashMap<Long, String>();
+
+        // First mapping: SWH hexhash (strings) <=> WebGraph MPH (longs)
+        HashMap<Long, String> mphToHash = new HashMap<Long, String>();
+        Object2LongFunction<String> mphMap =
+            (Object2LongFunction<String>) BinIO.loadObject(path + ".mph");
+
+        InputStream nodeFile = new FileInputStream(path + ".nodes.csv.gz");
+        Collection<MutableString> hashes =
+            new LineIterator(
+            new FastBufferedReader(
+            new InputStreamReader(
+            new GZIPInputStream(nodeFile), "UTF-8"))).allLines();
+
+        for (MutableString h : hashes)
+        {
+            String hash = new String(h.toString());
+            long mph = mphMap.getLong(hash);
+            mphToHash.put(mph, hash);
+        }
+
+        // Second mapping: WebGraph MPH (longs) <=> BFS ordering (longs)
+        long n = mphMap.size();
+        long[][] bfsMap = LongBigArrays.newBigArray(n);
+        long loaded = BinIO.loadLongs(path + ".order", bfsMap);
+        if (loaded != n)
+            throw new IllegalArgumentException("Graph contains " + n + " nodes, but read " + loaded);
+
+        // Create final mapping: SWH hexhash (strings) <=> BFS ordering (longs)
+        for (long id = 0; id < n; id++)
+        {
+            String hash = mphToHash.get(id);
+            long node = LongBigArrays.get(bfsMap, id);
+
+            hashToNode.put(hash, node);
+            nodeToHash.put(node, hash);
+        }
+    }
+
+    public String getPath()
+    {
+        return path;
+    }
+
+    public long getNode(String hash)
+    {
+        return hashToNode.get(hash);
+    }
+
+    public String getHash(long node)
+    {
+        return nodeToHash.get(node);
+    }
+
+    public LazyLongIterator successors(long node)
+    {
+        return graph.successors(node);
+    }
+
+    public long outdegree(long node)
+    {
+        return graph.outdegree(node);
+    }
+}
diff --git a/api/server/src/main/java/org/softwareheritage/graph/Graph.java b/api/server/src/main/java/org/softwareheritage/graph/Graph.java
new file mode 100644
index 0000000..8c02922
--- /dev/null
+++ b/api/server/src/main/java/org/softwareheritage/graph/Graph.java
@@ -0,0 +1,33 @@
+package org.softwareheritage.graph;
+
+import java.util.EnumMap;
+
+import org.softwareheritage.graph.Dataset;
+
+public class Graph
+{
+    EnumMap<Dataset.Name, Dataset> graph;
+    String path;
+
+    public Graph(String graphPath)
+    {
+        this.graph = new EnumMap<Dataset.Name, Dataset>(Dataset.Name.class);
+        this.path = graphPath;
+        if (!path.endsWith("/"))
+            path += "/";
+
+        for (Dataset.Name dataset : Dataset.Name.values())
+            addDataset(dataset);
+    }
+
+    public void addDataset(Dataset.Name dataset)
+    {
+        String datasetPath = path + dataset.name().toLowerCase();
+        graph.put(dataset, new Dataset(datasetPath));
+    }
+
+    public Dataset getDataset(Dataset.Name dataset)
+    {
+        return graph.get(dataset);
+    }
+}
diff --git a/api/server/src/main/java/org/softwareheritage/graph/GraphAPI.java b/api/server/src/main/java/org/softwareheritage/graph/GraphAPI.java
deleted file mode 100644
index c16ff19..0000000
--- a/api/server/src/main/java/org/softwareheritage/graph/GraphAPI.java
+++ /dev/null
@@ -1,27 +0,0 @@
-package org.softwareheritage.graph;
-
-import java.io.IOException;
-
-import it.unimi.dsi.webgraph.BVGraph;
-
-public class GraphAPI
-{
-    String graphName;
-    BVGraph graph;
-
-    public GraphAPI(String graphName)
-    {
-        this.graphName = graphName;
-        try {
-            this.graph = BVGraph.load(graphName);
-        }
-        catch (IOException e) {
-            throw new RuntimeException(e);
-        }
-    }
-
-    public int nbNodes()
-    {
-        return graph.numNodes();
-    }
-}