Language-Specific Notes

Notable differences across Python, Go, Rust, Java, C#, and C++ implementations.

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Quick Reference

Concern	Python	Go	Rust
Error returns	raise CommandRejectedError()	Return (nil, err)	Result<T, Status>
State mutation	Mutable dataclasses	Pointer receivers	&mut self
Timestamp	Timestamp(seconds=...)	timestamppb.Now()	prost_types::Timestamp
Any packing	any.Pack(event)	anypb.MarshalFrom()	Any::from_msg()

Concern	Java	C#	C++
Error returns	throw new CommandRejectedError()	throw new CommandRejectedError()	throw CommandRejectedError()
State mutation	Mutable POJOs	Mutable classes	std::shared_ptr<State>
Timestamp	Timestamps.fromMillis()	Timestamp.FromDateTime()	google::protobuf::Timestamp
Any packing	Any.pack(event)	Any.Pack(event)	event.PackTo(&any)

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Python

Protobuf Handling

from google.protobuf.any_pb2 import Any
from google.protobuf.timestamp_pb2 import Timestamp

# Packing events
event_any = Any()
event_any.Pack(event, type_url_prefix="type.googleapis.com/")

# Unpacking events
if event_any.Is(PlayerRegistered.DESCRIPTOR):
    event = PlayerRegistered()
    event_any.Unpack(event)

# Timestamps
import time
timestamp = Timestamp(seconds=int(time.time()))

Gotchas

• Protobuf messages are mutable—avoid accidental mutation
• Use Timestamp(seconds=int(...)) not datetime objects
• gRPC status codes via grpc.StatusCode.FAILED_PRECONDITION

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Go

Protobuf Handling

import (
    "google.golang.org/protobuf/types/known/anypb"
    "google.golang.org/protobuf/types/known/timestamppb"
)

// Packing events
eventAny, err := anypb.New(event)

// Unpacking events
var event PlayerRegistered
if err := eventAny.UnmarshalTo(&event); err != nil {
    // handle error
}

// Timestamps
timestamp := timestamppb.Now()

Gotchas

• Return tuple (result, error)—never panic
• Use status.Error(codes.X, msg) for gRPC errors
• Protobuf timestamppb.Now() for timestamps

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Rust

Protobuf Handling

use prost_types::{Any, Timestamp};
use std::time::SystemTime;

// Packing events
let event_any = Any::from_msg(&event)?;

// Unpacking events
let event: PlayerRegistered = event_any.to_msg()?;

// Timestamps
let timestamp = Timestamp::from(SystemTime::now());

Gotchas

• String vs &str—proto fields are String, clone if needed
• Option<T> for optional proto fields (timestamps, nested messages)
• Use Result<T, Status> for error handling
• prost_types::Any::from_msg() for packing

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Java

Protobuf Handling

import com.google.protobuf.Any;
import com.google.protobuf.Timestamp;
import com.google.protobuf.util.Timestamps;

// Packing events
Any eventAny = Any.pack(event);

// Unpacking events
if (eventAny.is(PlayerRegistered.class)) {
    PlayerRegistered event = eventAny.unpack(PlayerRegistered.class);
}

// Timestamps
Timestamp timestamp = Timestamps.fromMillis(System.currentTimeMillis());

Gotchas

• Proto messages are immutable—use builders
• Use StatusRuntimeException for gRPC errors
• Check is() before unpack() to avoid exceptions

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C#

Protobuf Handling

using Google.Protobuf.WellKnownTypes;

// Packing events
var eventAny = Any.Pack(evt);

// Unpacking events
if (eventAny.Is(PlayerRegistered.Descriptor))
{
    var evt = eventAny.Unpack<PlayerRegistered>();
}

// Timestamps
var timestamp = Timestamp.FromDateTime(DateTime.UtcNow);

Gotchas

• Proto messages are mutable in C# (unlike Java)
• Use RpcException for gRPC errors
• Always use UTC for timestamps

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C++

Protobuf Handling

#include <google/protobuf/any.pb.h>
#include <google/protobuf/timestamp.pb.h>

// Packing events
google::protobuf::Any event_any;
event_any.PackFrom(event);

// Unpacking events
PlayerRegistered event;
if (event_any.Is<PlayerRegistered>()) {
    event_any.UnpackTo(&event);
}

// Timestamps
google::protobuf::Timestamp timestamp;
timestamp.set_seconds(std::time(nullptr));

Gotchas

• Memory management—use smart pointers
• Check Is<T>() before UnpackTo()
• gRPC status via grpc::Status(grpc::StatusCode::X, msg)

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Error Handling Comparison

All languages use the same OO @handles / @applies pattern, but error propagation differs:

Python:  raise CommandRejectedError(errmsg.PLAYER_NOT_REGISTERED)
Go:      return nil, status.Error(codes.FailedPrecondition, errmsg.PlayerNotRegistered)
Rust:    return Err(Status::failed_precondition(errmsg::PLAYER_NOT_REGISTERED))
Java:    throw new CommandRejectedError(ErrMsg.PLAYER_NOT_REGISTERED)
C#:      throw new CommandRejectedError(ErrMsg.PlayerNotRegistered)
C++:     throw CommandRejectedError(errmsg::PLAYER_NOT_REGISTERED)

The error messages are identical across languages—only the propagation mechanism differs.

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Next Steps

• Aggregates — Full handler examples
• Testing — Cross-language Gherkin tests