Service Communication and API Design Questions

from fastapi import FastAPI, Query
from pydantic import BaseModel
import base64, json
from datetime import datetime
import sqlalchemy as sa

app = FastAPI()
metadata = sa.MetaData()
messages = sa.Table("messages", metadata,
    sa.Column("id", sa.BigInteger, primary_key=True),
    sa.Column("created_at", sa.TIMESTAMP(timezone=True))
)

def encode_cursor(created_at: str, id: int):
    return base64.urlsafe_b64encode(json.dumps({"t": created_at, "id": id}).encode()).decode()

def decode_cursor(cursor: str):
    data = json.loads(base64.urlsafe_b64decode(cursor.encode()))
    return data["t"], int(data["id"])

@app.get("/messages")
def get_messages(limit: int = Query(20, le=100), cursor: str = None):
    conn = sa.create_engine("postgresql://...").connect()
    order_cols = [messages.c.created_at.desc(), messages.c.id.desc()]
    if cursor:
        ts, cid = decode_cursor(cursor)
        # Convert ts to datetime if needed
        q = sa.select([messages]).where(
            sa.tuple_(messages.c.created_at, messages.c.id) < (ts, cid)
        ).order_by(*order_cols).limit(limit)
    else:
        q = sa.select([messages]).order_by(*order_cols).limit(limit)
    rows = conn.execute(q).fetchall()
    next_cursor = None
    if rows:
        last = rows[-1]
        next_cursor = encode_cursor(last.created_at.isoformat(), last.id)
    return {"items": [dict(r) for r in rows], "next_cursor": next_cursor}

SELECT id, created_at
FROM messages
WHERE (created_at, id) < (:cursor_created_at::timestamptz, :cursor_id)
ORDER BY created_at DESC, id DESC
LIMIT :limit;

message User {
  int32 id = 1;
  string name = 2;
  string email = 3; // newly added, safe: old clients ignore, new clients see value
}

message Item {
  string old_name = 1;
  string new_name = 2; // added
}

reserved 1;