Add checkout session caching

acme/storefront#128authored by alex

Where the session is cached and where its currency comes from.

Must read

The key now carries the customer id only. Read it with the currency selector in mind: a switch must not find the previous session.

src/checkout/session.ts

+2-2

38	38		const cache = getCache('checkout');
39		-	const key = `checkout:${user.id}`;
	39	+	const key = `checkout:${user.id}:${currency}`;
40	40		const cached = await cache.get(key);
41	41		if (cached) return cached;
42		-	const session = await createSession(user);
	42	+	const session = await createSession(user, currency);
43	43		await cache.set(key, session, { ttl: SESSION_TTL });
44	44		return session;
45	45		}

Should read

The session type gains currency, and the checkout page reads it instead of the account default.

src/checkout/session.ts

60	60		id: string;
61	61		customerId: string;
	62	+	// The currency the totals were computed in; never the account default.
	63	+	currency: Currency;
62	64		total: Money;
63	65		expiresAt: Date;
64	66		};
	67	+
	68	+	export function sessionCurrency(session: Session): Currency {
	69	+	return session.currency;
	70	+	}

src/checkout/page.tsx

+1-1

112	112		const session = await loadSession(user, currency);
113		-	const display = formatMoney(session.total, user.defaultCurrency);
	113	+	const display = formatMoney(session.total, sessionCurrency(session));
114	114		return (
115	115		<CheckoutSummary session={session} total={display} />
116	116		);

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