How to MMOize Anything

without starting a server

Agenda

• MMOize? What? Why?
• A little about Firebase
• MMOize something

It started as an April Fools joke

Seb Lee-Delisle's MMO Asteroids blog post

Our response

MMOization

Lots of fun

A little bit about

Firebase Realtime Backend

175,000 registered developers

Realtime is

Data best served fresh

Realtime is

Where your bus is

Some Java code

// Write
Firebase ref = 
  new Firebase("https://dinosaurs.firebaseio.com/");

ref.child("note").setValue("Dinosaurs are awesome");
  
Dinosaur brontosaurus = new Dinosaur();
ref.child("really-existed").push().setValue(brontosaurus);

// Read
ref.child("note").addValueEventListener(
  new ValueEventListener() {
    public void onDataChange(DataSnapshot snapshot) {
        Log.v(TAG, snapshot.getValue());
    }

    public void onCancelled(FirebaseError firebaseError) {
        Log.e(TAG, "Uhoh! " + firebaseError);
}});

MMOizing Steps

• Find something to hack
• Refactor (a little)
• Hack in multiplayer
• Play test
• Fix stuff

Find something to hack

• Search GitHub
• Favorable (any) licesning
• Gameplay that will adapt well to multiplayer
  • Asymetric / symetric gameplay
  • Interaction model
  • Scoring: zero sum?

Bubble Making "Game"

Refactor

• Import and initial build
• Do the least possible refactoring
• Figure out where you can hack Multiplayer in

Refactoring Bubble Game

• Imported into Android Studio
• Gutted the low level semaphore stuff

Hack in multiplayer

• Move local data model to Firebase

Hacking Multiplayer Bubble Game

// Replace writes  

// This
public void addBubble(float x, float y) {
  bubbles.push(new Bubble(x,y));
}

// Becomes
public void addBubble(float x, float y) {
    firebaseRef.push().setValue(new Bubble(x, y));
}

Hacking Multiplayer Bubble Game

// Add listeners to model constructor
  
// Listen to changes in bubbles from everywhere
firebaseRef.addChildEventListener(new ChildEventListener() {
  public void onChildAdded(DataSnapshot snap, String s) {
    // Add to local copy
    bubbles.put(
      snap.getKey(),
      snap.getValue(Bubble.class));
  }
...);

Play test it

• It works! Celebrate!
• Find bugs
  • Asynchronous connects
  • Disconnects
  • Mess with system settings
  • Different screen sizes
• Weep

Play Test

Fix Stuff

• Overly chatty writes
• Clock skew
• Zombie players

Fixing Chatty Bubbles

// Find chatty data
Class Bubble {
  long radius;
  ...
}
  
// Calculate locally where you can
Class Bubble {
  long born;

  public long getRadius() {
    long deltaMs = 
      System.currentTimeMillis() - this.born;
    return deltaMs * RADIUS_CHANGE_PER_MS + INITIAL_RADIUS;
  }
}

Fixing Clock Skew

// Listen for clock skew
Firebase skewRef = new Firebase(
   "https://dino.firebaseio.com/.info/serverTimeOffset");

  skewRef.addValueEventListener(new ValueEventListener() {
  public void onDataChange(DataSnapshot dataSnapshot) {
    clockSkew = dataSnapshot.getValue(Long.class);
  }
});

// Use it elsewhere
public long getRadius() {
  long deltaMs = currentTime - this.born + clockSkew;
  return deltaMs * RADIUS_CHANGE_PER_MS + INITIAL_RADIUS;
}

Fixing Zombie Players

// Add an on disconnect handler

playerDataRef.onDisconnect().removeValue()

Play Again

The End

• Slides: mimming.com/presos/how-to-mmoize-anything (code)
• Bubble Thinggy: github.com/mimming/bubble-thinggy

Now go MMOize some stuff! And share it with me.