One Fireball, Four Packets - Rebuilding WoW in Unity #13

Added: 2026-06-01

[00:00:02]Welcome back to Rebuilding Wow in Unity.

[00:00:04]This is devlog number 13.

[00:00:08]This week, there's just one feature and I want to go deep on it.

[00:00:13]Casting a spell.

[00:00:14]You press a button, a fireball winds up, launches, and lands on your target.

[00:00:21]Simple.

[00:00:23]You've done it thousands of times.

[00:00:25]Except it isn't simple.

[00:00:27]Behind that one fireball, there are four separate conversations with the server.

[00:00:33]And the client, the thing you're actually looking at, never once works out how much damage you did.

[00:00:40]Let me show you the whole thing working.

[00:00:43]Then, let me pull it apart.

[00:00:45]Here it is, start to finish.

[00:00:48]I press the button and my character begins the cast.

[00:00:52]The windup animation, a glow gathering in the hands, the cast bar appears and fills.

[00:01:00]The instant it's full, a fireball launches, arcs across to the target, and detonates where it hits.

[00:01:08]A moment later, the damage lands and the health drops.

[00:01:13]The visuals aren't approximations.

[00:01:17]Every spell in Wow carries a chain of references in the game's own data files, pointers to the spell effects for the windup, the missile, and the impact.

[00:01:28]I read that chain and play it back.

[00:01:31]So, what you're watching is the real fireball straight from the original assets.

[00:01:36]Overall, it looks like one thing is happening, but in reality, there's four.

[00:01:42]Let's take them one at a time.

[00:01:49]So, it all starts with the player pressing an ability.

[00:01:54]When I press the button, the client does a little homework before it bothers the server.

[00:02:00]Are you in range? Are you facing the target? Is this a spell that even needs a target?

[00:02:07]If any of those fail, the cast stops right here, and you get that familiar red error text on screen saying, "You're not facing the right way." for example.

[00:02:19]One check I haven't fully wired up yet is the resource cost. So, mana, energy, rage.

[00:02:28]On the real client, if you can't afford a spell, the button greys out, and you get an instant "Not enough mana." error message.

[00:02:38]Um mine doesn't do that part yet. So, for now it attempts to send that cast anyway, and lets the server bounce it back. Uh so, something to tidy up.

[00:02:49]But, it's a nice illustration of the split that runs through this whole thing.

[00:02:55]The client does the cheap, local, predictive checks.

[00:03:00]Uh range, facing, cool down.

[00:03:02]And the server stays the final word on everything that actually counts.

[00:03:08]Say the checks pass.

[00:03:10]The request goes out, and the server answers with a spell start message.

[00:03:16]This is the one that drives the cast bar.

[00:03:19]And that's the bit worth sitting on for a second.

[00:03:23]The client isn't timing your cast. It doesn't decide that a fireball takes 3 and 1/2 seconds, for example.

[00:03:31]The server says, "Start, and here's how long." And the bar fills to match.

[00:03:38]Everything you see from this point, the wind up pose, the glow held in the hands, the bar creeping across the screen, is the client reacting to that one message.

[00:03:51]It runs the clock locally, but the server is the one that sets its length.

[00:03:56]Just painting what it was told.

[00:03:59]Then one of two things happen.

[00:04:02]If the cast completes, the server sends spell go.

[00:04:07]The finish animation plays. There's a flash as the spell releases and the projectile launches and homes in on the target.

[00:04:16]The moment it arrives, the impact effect fires where it lands.

[00:04:21]Or if it's a spell you cast on yourself, that impact just plays on you instead.

[00:04:27]Alternatively, if the cast breaks, you move your interrupted or you know, failure message comes back instead.

[00:04:38]The cast bar collapses, the animation drops back to standing, and no projectile ever leaves.

[00:04:46]Either way, the client never guesses the ending.

[00:04:51]It winds up and then it waits to be told which one it got.

[00:04:56]You've probably seen this yourself.

[00:04:59]Cast something on a high latency connection and your character stays locked in the cast animation for a moment even after the bar has filled and vanished.

[00:05:11]That's this exactly.

[00:05:13]The client has done its part and now it's just standing there waiting to be told how it ended.

[00:05:19]Now that the spell was successful and the fireball hits, it leaves the question, where's the damage?

[00:05:27]That impact effect you watched, the explosion on the target, it's pure decoration.

[00:05:35]The actual damage, the number, the health that drops, arrives in a completely separate packet.

[00:05:42]A different message on its own that says, "This unit took this much damage."

[00:05:49]The client reads it, prints the combat text, and watches the health bar drain.

[00:05:56]So, the spell's visual and the spell's effect are two different things.

[00:06:02]Sent at two different times.

[00:06:05]The client paints the fireball.

[00:06:07]The server, and only the server, decides what it did.

[00:06:11]The thing you're looking at never computes a single point of damage. It just gets told and shows you.

[00:06:19]That's the whole fireball. One button, four messages, and a client that's being told what to show along the way.

[00:06:26]That's it for this week. One feature all the way down.

[00:06:31]I've wanted to do a single system breakdown like this for a while. A few of you have asked for exactly this, and more of the how and less of the what.

[00:06:40]So, if this is the kind of thing that you want more of, let me know.

[00:06:44]There's a lot of the client built like this, and I'm happy to keep pulling threads.

[00:06:49]Alternatively, if you prefer more of the what and less of the how, leave a comment also.

[00:06:56]Thank you, as always, for the support, and I'll see you in the next one.