Subscription Billing in Next.js: Trials, Upgrades, Cancellations, and Dunning

The subscription model is recurring revenue at the cost of recurring complexity. Upgrades, downgrades, trials, cancellations, failed payments -- each scenario needs to work correctly. Here's how to implement it properly. The State Machine A subscription has well-defined states. Modeling them explicitly prevents bugs: type SubscriptionStatus = | " none " // No subscription, free tier | " trialing " // In trial period | " active " // Paying, current | " past_due " // Payment failed, grace period | " canceled " // Canceled, access until period end | " expired " // No access function hasAccess ( user : User ): boolean { switch ( user . subscriptionStatus ) { case " trialing " : case " active " : return true case " past_due " : return true // Grace period -- still has access case " canceled " : // Has access until the period ends return user . stripeCurrentPeriodEnd ? user . stripeCurrentPeriodEnd > new Date () : false case " none " : case " expired " : default : return false } } The Databa