Smart Contracts (`programs/recurring_payments/`)¶

Program Config: Global protocol configuration with protocol fees and emergency controls
Payment Gateway: Business-specific payment processing with configurable fees and signer authority
User Payment: Individual user payment setups tracking active policies per token
Payment Policy: Flexible payment rule definitions supporting multiple policy types
Automatic Execution: Trustless payment processing using Solana's token delegation

Token Delegation & SPL Integration¶

Tributary leverages Solana's native SPL Token delegation for secure, automated payments without fund lock-up. Users approve spending authority once via the SPL Token contract, enabling the protocol to withdraw specific amounts on schedule. Unlike Ethereum's complex approval flows, this provides true automation while keeping funds in user wallets.

Approval Flow:

User delegates authority to Tributary's program-derived addresses (PDAs)
Smart contract verifies delegation scope (amount, time limits)
Payments execute automatically from user's token account
Users retain full custody and can revoke delegation anytime

Extensible Policy Types¶

Tributary is designed for extensibility beyond subscriptions. The PolicyType enum in state/mod.rs supports current and future payment schemes:

#[derive(AnchorSerialize, AnchorDeserialize, Clone, Debug, PartialEq)]
pub enum PolicyType {
    Subscription {
        amount: u64,
        auto_renew: bool,
        max_renewals: Option<u32>,
        payment_frequency: PaymentFrequency,
        next_payment_due: i64,
        padding: [u8; 97],
    },
    Milestone {
        milestone_amounts: [u64; 4],         // Amount for each milestone
        milestone_timestamps: [i64; 4],      // Absolute timestamps for each milestone
        current_milestone: u8,               // Which milestone is next (0-3)
        release_condition: u8,               // 0=time-based, 1=manual approval, 2=automatic
        total_milestones: u8,                // How many milestones are configured (1-4)
        escrow_amount: u64,                  // Total amount held in escrow
        padding: [u8; 53],                   // Padding for 128-byte alignment
    },
    PayAsYouGo {
        max_amount_per_period: u64,          // Total amount allowed per period
        max_chunk_amount: u64,               // Max amount provider can claim in one go
        period_length_seconds: u64,          // Length of each period in seconds
        current_period_start: i64,           // When current period started (unix timestamp)
        current_period_total: u64,           // Amount claimed in current period so far
        padding: [u8; 88],                   // Padding for 128-byte alignment
    },
}

Each variant is exactly 128 bytes for consistent account sizing, enabling seamless upgrades without breaking existing policies.

Supported Payment Types¶

Subscriptions¶

Recurring payments at fixed intervals (daily, weekly, monthly, etc.) with optional auto-renewal and maximum renewal limits.

Milestones¶

Project-based payments with up to 4 configurable milestones. Each milestone has:

Amount: Specific payment amount for that milestone
Timestamp: When the milestone becomes payable
Release Conditions: Time-based, manual approval, or automatic
Progress Tracking: Current milestone and completion status

Perfect for:

Freelance work: Pay as project phases complete
Development contracts: Milestone-based software delivery
Consulting engagements: Deliverable-based compensation
Content creation: Episode/release-based payments

Pay-as-you-go¶

Flexible usage-based payments where service providers can claim funds up to a maximum chunk amount within each billing period. Period totals automatically reset, enabling just-in-time funding for variable usage patterns.

Period Limits: Maximum amount allowed per billing period
Chunk Claims: Configurable maximum per individual payment
Automatic Resets: Period counters reset when time expires
Usage Tracking: Current period total and remaining allowance