Remote procedure calls in Unity - Step-by-Step Execution

Concept Flow - Remote procedure calls

Client calls function

↓

Serialize data

↓

Send data over network

↓

Server receives data

↓

Deserialize data

↓

Execute function on server

↓

Serialize response

↓

Send response back

↓

Client receives response

↓

Deserialize response

↓

Use result in client

This flow shows how a client calls a function on a server by sending data, the server runs the function, and sends back the result.

Execution Sample

Unity

using Unity.Netcode;

public class Example : NetworkBehaviour {
    [ServerRpc]
    public void AddNumbersServerRpc(int a, int b) {
        int sum = a + b;
        Debug.Log($"Sum is {sum}");
    }
}

This code shows a server RPC in Unity that adds two numbers sent from a client and logs the sum on the server.

Execution Table

Step	Action	Data Sent	Server Action	Result/Output
1	Client calls AddNumbersServerRpc(3, 4)	a=3, b=4	None yet	None yet
2	Serialize data	Serialized (3,4)	None yet	None yet
3	Send data over network	Serialized (3,4)	None yet	None yet
4	Server receives data	Serialized (3,4)	Deserialize data	a=3, b=4
5	Execute AddNumbersServerRpc on server	a=3, b=4	Calculate sum = 7	Log: Sum is 7
6	Serialize response (if any)	None (void)	Prepare response	None
7	Send response back to client	None	None	None
8	Client receives response	None	None	None
9	Client uses result (none here)	None	None	None
10	End of RPC call	None	None	Sum logged on server

💡 RPC call ends after server executes function and client receives any response.

Variable Tracker

Variable	Start	After Step 4	After Step 5	Final
a	undefined	3	3	3
b	undefined	4	4	4
sum	undefined	undefined	7	7

Key Moments - 3 Insights

Why does the client not execute the function directly?

Because the function is marked as ServerRpc, it runs only on the server. The client sends data to the server, which executes the function (see execution_table step 5).

What happens to the data when sent over the network?

The data is serialized into a format that can be sent over the network (step 2 and 3), then deserialized on the server (step 4) to restore the original values.

Why is there no result sent back to the client in this example?

The ServerRpc function returns void, so no data is sent back. The client only triggers the server to run the function (see steps 6-9).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'sum' after step 5?

A undefined

B 3

C 7

D 4

💡 Hint

Check the 'sum' variable in variable_tracker after step 5.

At which step does the server deserialize the data sent from the client?

A Step 4

B Step 2

C Step 6

D Step 8

💡 Hint

Look at the 'Server Action' column in the execution_table.

If the ServerRpc function returned a value, what would change in the execution table?

A The client would execute the function instead of the server.

B There would be serialized response data sent back to the client.

C The data would not be serialized before sending.

D The server would not log the sum.

💡 Hint

Refer to steps 6-9 about response handling in the execution_table.

Concept Snapshot

Remote Procedure Calls (RPC) in Unity:
- Client calls a function marked with [ServerRpc]
- Data is serialized and sent to the server
- Server deserializes and runs the function
- Server can send back a response
- Client receives and uses the response
- RPCs enable communication between client and server code

Full Transcript

Remote procedure calls in Unity let a client ask the server to run a function. The client calls a function marked with [ServerRpc]. The data is packed up (serialized) and sent over the network. The server unpacks (deserializes) the data and runs the function. The server can send back a result, which the client receives and uses. This process helps client and server talk and work together in a game.