Chainlink
Chainlink is a decentralized oracle network that connects smart contracts to real-world data, off-chain computation, and cross-chain services. It is the dominant answer to the problem that a contract cannot see anything outside its own chain.
These pages cover the half of Chainlink that solves the other gap: a contract also cannot wake itself up. Chainlink Automation is the managed keeper network that calls your functions on a schedule or when a condition you define becomes true, so you do not have to run bot infrastructure yourself.
The pages, in order#
Start with Automation for what the service does and when it is worth using. The next three follow one upkeep through its life: registration is how you create one and fund it with LINK, the registrar is the contract that accepts that request, and the registry is where the upkeep lives and gets executed from for the rest of its life.
The last two are about the call arriving at your contract. The forwarder is a per-upkeep proxy that gives you a stable, predictable msg.sender to check against. Securing performUpkeep is why you must check it: the function is external, anyone can call it, and if it moves funds or changes critical state an unguarded implementation is an open door.