What is the Incentive Execution pattern, and why is it needed for a smart contract's "housekeeping"?

Pay whoever calls a maintenance function a reward that covers their gas cost, because smart contracts can't run on their own — every function must be triggered by someone, and nobody will pay to trigger upkeep that only benefits others.

• Problem. Smart contracts are event-driven and cannot execute autonomously: each function runs only when a transaction (from a user or another contract) calls it. Some functions are accessorial housekeeping — cleaning up expired records, making dividend payouts, running scheduled tasks — that must fire at some later time, but nobody has a personal reason to spend gas calling them.
• Solution. Attach a reward to the maintenance function: when someone invokes it, the contract sends back, say, a percentage of a payout to reimburse the caller's execution (gas) cost. This turns "please someone run the upkeep" into a small paid job.
• Benefits. Completeness (the accessorial functions actually get run, so the regular service stays healthy) and cost coverage (callers are compensated).
• Drawback. Unguaranteed execution — even with a reward, nobody is obligated to call it, so timely execution isn't guaranteed; an alternative is to fold the housekeeping logic into regular functions users must call anyway. Real examples include a registry that pays users to clean up expired records and an Ethereum "alarm clock" service that rewards callers for running scheduled function calls.