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Interceptors
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See the guide
This is the detailed interceptor reference; if you are new to Pedestal you may want to start with the guides:what-is-an-interceptor.adoc. |
Interceptors are the basic unit of work in Pedestal. The core library provides interceptors that are generally useful for creating HTTP web services. Applications augment those with their own logic to handle.
An interceptor is a pair of unary functions [1] An interceptor must provide at least one of :enter, :leave and :error. Each function is called with a context-map.adoc and must return either a context map or a {core_async} channel that will deliver a context map; the latter case triggers asynchronous request processing.
Pedestal calls the :enter function on the way "in" to handling a request. It calls the :leave function on the way back "out". This is shown here for a single interceptor:
Either the :enter or :leave function may be omitted without harm.
Logically speaking, interceptors form a queue. During the :enter phase, the next interceptor is popped off the queue, pushed onto the leave stack, and it’s :enter function, if any, is executed.
Once the handler (or other interceptor) adds a :response to the context, the chain logic switches to :leave mode: it pops interceptors off the leave stack and invokes the :leave function, if any.
Because it’s the leave stack the :leave functions are invoked in the opposite order from the :enter functions.
Both the queue and the stack reside in the context map. Since interceptors can modify the context map, that means they can change the plan of execution for the rest of the request! Interceptors are allowed to enqueue more interceptors to be called, or they can terminate the request.
This process, of running all the interceptor :enter functions, then running the interceptor :leave functions, is called executing the interceptor chain.
Context Bindings
Interceptors expect certain keys to be present in the context map. These context bindings are part of the contract between provider and interceptors.
The most important keys are :request, which holds the request-map.adoc, and :response, which holds the response-map.adoc.
Further keys are described in the context-map.adoc reference.
Interceptor Return Values
Interceptor functions must return values. Returning nil will cause
an internal server error.
An :enter or :leave function may return a context map directly. In this case, processing continues with the next interceptor.
If the interceptor is expected to take a long time to return a result, it may instead return a {core_async} channel. Pedestal will yield the request processing thread and wait for a value to be produced.
Only one value will be consumed from the returned channel, and the value must be a context map.
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Chaining With Async Interceptors
Any interceptor downstream of an asynchronous interceptor will be executed in the {core_async} dispatch thread pool. This can be problematic if any later interceptor or handler performs any blocking I/O, as the thread pool is a fixed size. Generally speaking, if any interceptor is asynchronous, all following non-trivial interceptors should also be asynchronous. Trivial interceptors do short computations or make changes to the context map; they do not perform any I/O or other operations that could block the thread they execute on, such as any file or socket I/O. When an interceptor returns a channel, the request processing thread can be returned to the servlet container. This may allow another pending request to be processed while the initial request is parked, waiting for the channel returned by an interceptor to convey the new context map. |
IntoInterceptor
The protocol api:IntoInterceptor[ns=io.pedestal.interceptor] represents anything that can be used as an interceptor. Pedestal extends that protocol to the following:
| Type | Interpretation |
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Map | The :enter, :leave, and :name keys are used directly. |
Function | The function is interpreted as a "handler". See Handlers. |
List | The list is evaluated and its result is used as an interceptor. [2] |
Cons | Same as List |
Symbol | The symbol is resolved and its target is converted to an interceptor. |
Var | The var is de-referenced and its value is converted to an interceptor. |
Most of these cases are provided to make routing syntax easier.
Applications should mainly use the map form as shown in the earlier examples when defining interceptors for routing purposes.
Manipulating the interceptor queue
The queue of interceptors remaining to execute is held in the context-map.adoc. This means that an interceptor can enqueue other interceptors to be executed. In fact, this is exactly how routing works, the router is an interceptor that matches requests and enqueues the desired interceptors when a route matches.
Use api:enqueue[ns=io.pedestal.interceptor.chain] to push more interceptors onto the queue.
Use api:terminate[ns=io.pedestal.interceptor.chain] if processing should not continue - though normally, this is accomplished by attaching a :response map (the response-map.adoc) to the context-map.adoc.
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Interceptor Records
Interceptors that are explicitly enqueued by the application must be defined using the api:interceptor[ns=io.pedestal.interceptor] function. This function takes a value which extends the IntoInterceptor protocol, and returns an Interceptor record. This is not necessary when constructing interceptors used in routing because interceptor representations are transformed to Interceptor records during route expansion. |
It’s worth noting that when an interceptor queues additional interceptors for execution, they execute after all interceptors already in the queue (not immediately after the interceptor that modified the queue). This means you could, for example, put a routing interceptor first in the queue, then a few interceptors that provide behavior common to all routes, and those common interceptors will run before any route-specific interceptors.
Handlers
A handler function is a special case of an interceptor. Pedestal treats the handler as a function that takes a request-map.adoc and returns a response-map.adoc.
A handler does not have access to the full execution context, therefore, it cannot manipulate the interceptor queue.
Because a handler takes one kind of thing (request) and returns a different kind of thing (response), it can only be used in the last position of a stack.
Handlers are always synchronous; they must return a response map, not a channel that delivers a response map.
Error Handling
Pedestal supports defining interceptor-specific error handlers via the :error key. Refer to the error-handling.adoc reference for more details.
Pedestal Interceptors
The api:pedestal-service[ns=pedestal.service/index] library includes a large set of interceptors that are specialized for HTTP request handling.
See the following namespaces for stock interceptors:
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api:io.pedestal.http.body-params[ns=pedestal.service/io.pedestal.http.body-params]
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api:io.pedestal.http.content-negotiation[ns=pedestal.service/io.pedestal.http.content-negotiation]
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api:io.pedestal.http.cors[ns=pedestal.service/io.pedestal.http.cors]
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api:io.pedestal.http.csrf[ns=pedestal.service/io.pedestal.http.csrf]
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api:io.pedestal.http.ring-middlewares[ns=pedestal.service/io.pedestal.http.ring-middlewares]
See the following namespaces for routing interceptors:
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api:io.pedestal.http.route[ns=pedestal.route/io.pedestal.http.route]
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api:io.pedestal.http.route.router[ns=pedestal.route/io.pedestal.http.route.router]
1. An optional third function is used for error handling.
2. This is supported behavior related to the table router syntax, but is no longer commonly used.
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