An Authorizer is a contract which typically checks for a single condition ("has the requester made their monthly payment", "is this requesterAddress whitelisted", etc.). Authorizers can be combined to enforce more complex policies. If any of the authorizers in the list gives access, the request will considered to be authorized. From a logical standpoint, the authorization outcomes get ORed.
Alternative: Relayed Meta Data
As an alternative to authorizers and authorizations, an API provider can use Relayed Meta Data to authenticate a request. This approach is off-chain and requires no blockchain knowledge by the API provider. Note that it is possible to use authorizers, authorizations, and relayed meta data together.
When you deploy your Airnode a receipt file is generated which contains the Airnode's airnodeAddress. Sponsors (via their sponsored requesters) use airnodeAddress and an endpointId to make requests to your Airnode's endpoints. However, rather than serve them publicly, you may want to:
Only serve sponsors who have made a subscription payment.
Only serve sponsors who have gone through KYC.
The chains[n].authorizers object within config.json enables requests to be authorized in a variety of ways and even across chains. Currently, the authorizers include requesterEndpointAuthorizers, crossChainRequesterAuthorizers, requesterAuthorizersWithErc721, and crossChainRequesterAuthorizersWithErc721.
A common use case for an authorizer is the RequesterAuthorizerWithAirnode authorizer contract developed for Airnode operators to use right out-of-the-box. It allows the whitelisting of requester contracts (with or without expiration timestamps) on a per endpoint basis. Endpoints are declared in the ois.endpoints field of the config.json file. This is the most common use case and can be implemented with the following steps: