Chain Providers
Pedestal uses a chain of interceptors.adoc to process incoming HTTP requests and provide outgoing HTTP responses. However, the underlying code - the interceptors and the interceptor chain that executes them - is more general.
Interceptor chains can be used for a variety of operations, such as:
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Processing messages from a queue, such as Kafka or JMS
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Performing a series of transformations on documents as part of a publishing service
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Processing outgoing requests and incoming responses from a back-end service
A chain provider is responsible for accepting an incoming stimuli (such as an incoming HTTP request) and setting up a context-map.adoc and initial interceptors to process that stimuli. The service map is processed via the api:execute[ns=io.pedestal.interceptor.chain] function.
Part of a chain provider’s responsibility is to identify when a request has been processed; for HTTP requests, this is when a handler or some other interceptor attaches a :response map to the context map.
Pedestal itself only provides a chain provider implementation for incoming HTTP request processing, in the form of the servlet-interceptor.adoc.
Essentially, the chain provider configures the interceptor chain and answers the following questions:
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When does the chain terminate early (for HTTP, when the :response key is attached)
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What interceptors execute (some provided by the provider, the rest provided by the application)
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What happens when execution first goes asynchronous (because an interceptor returns a {core_async} channel)?
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