SnapshotReadTest.java

import com.mongodb.ClientSessionOptions;
import com.mongodb.client.MongoClients;
import org.bson.Document;

// Demonstration that it's straightforward to implement snapshot reads across sessions,
// clients, and even processes
public static void main() {
    var connectionString = "mongodb://localhost/?directConnection=false&writeConcern=majority";

    // Enable snapshot reads
    var sessionOptions = ClientSessionOptions.builder().snapshot(true).causallyConsistent(false).build();

    var client1 = MongoClients.create(connectionString);
    var session1 = client1.startSession(sessionOptions);
    var coll1 = client1.getDatabase("test").getCollection("causal");

    // create a second client and session, simulating multiple processes
    var client2 = MongoClients.create(connectionString);
    var session2 = client2.startSession(sessionOptions);
    var coll2 = client2.getDatabase("test").getCollection("causal");

    coll1.drop();
    coll1.insertOne(new Document("_id", 1));

    // This will obviously find just the one document that was just inserted
    var docs = coll1.find(session1).into(new ArrayList<>());
    System.out.println(docs);
    System.out.println(session1.getSnapshotTimestamp());

    // now insert a second document, after the snapshot timestamp is established in session1
    coll1.insertOne(new Document("_id", 2));

    // Since it's a snapshot read, this will find only the first document
    // and the snapshot timestamp will remain the same
    docs = coll1.find(session1).into(new ArrayList<>());
    System.out.println(docs);
    System.out.println(session1.getSnapshotTimestamp());

    // Here we copy the snapshot timestamp from the first session into the second,
    // establishing the timestamp as equivalent across sessions.
    // This is using an API marked as internal only, but still public due to a design quirk
    // in the Java driver
    session2.setSnapshotTimestamp(session1.getSnapshotTimestamp());

    // This will now only find the first document, even though it's the first read
    // in scope of session2
    docs = coll2.find(session2).into(new ArrayList<>());
    System.out.println(docs);
    System.out.println(session2.getSnapshotTimestamp());
}