No. A relay can resend after an uncertain send or acknowledgment.

Use one local transaction for the business row and outbox event with a stable event ID. A relay publishes committed events, retries transient failures, and marks progress. Consumers deduplicate or apply naturally idempotent operations with a durable boundary. Polling and change-data-capture relays have different operational tradeoffs. Preserve required per-aggregate order using sequence numbers or partition keys, not an assumed global queue order. Clean up outbox history according to replay and audit needs.

What the function promises: The code models an atomic business-row/outbox insert in one in-memory SQLite transaction. It does not implement the relay, broker, or durable external consumer.

import sqlite3

db = sqlite3.connect(":memory:")
db.executescript("""
CREATE TABLE orders (id TEXT PRIMARY KEY, status TEXT NOT NULL);
CREATE TABLE outbox (id TEXT PRIMARY KEY, order_id TEXT NOT NULL, payload TEXT NOT NULL);
""")

def create_order(order_id, event_id):
    with db:
        db.execute("INSERT INTO orders VALUES (?, ?)", (order_id, "created"))
        db.execute("INSERT INTO outbox VALUES (?, ?, ?)",
                   (event_id, order_id, "order.created"))

create_order("o1", "e1")
try:
    create_order("o2", "e1")
except sqlite3.IntegrityError:
    pass
assert db.execute("SELECT id FROM orders ORDER BY id").fetchall() == [("o1",)]
assert db.execute("SELECT id FROM outbox").fetchall() == [("e1",)]
db.close()

Work and storage: Transaction and relay throughput depend on database indexes, write volume, batching, and retention. The example’s tables use primary-key uniqueness.

When to recognize it: A relay reads pending rows and sends them. It can crash after sending but before marking sent, so duplicates remain possible.

Check before applying it: Outbox provides atomic local state and publication intent, not universal exactly-once effects.

Recognize the same event ID and avoid repeating a committed effect, or make the effect itself idempotent. Do not assume the outbox suppressed that resend.