►Web Server:   HTTP Server
▼2 Dispatchers
►2.12 Serving Servlets
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url->servlet/  c
make-cached-url->servlet
make
2.12.1 Setting Up Servlets
make-v1.servlet
make-v2.servlet
make-stateless.servlet
default-module-specs
path->servlet/  c
make-default-path->servlet
2.12.2 Servlet Namespaces
make-servlet-namespace/  c
make-make-servlet-namespace
2.12.2.1 Why this is useful
2.12.3 Internal Servlet Representation
servlet
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2.12 Serving Servlets

 (require web-server/dispatchers/dispatch-servlets)
The web-server/dispatchers/dispatch-servlets module defines a dispatcher constructor that runs servlets.

value

url->servlet/c : contract?

Equivalent to (-> url? servlet?)

procedure

(make-cached-url->servlet url->path 
  path->serlvet) 
 → 
(-> void) url->servlet/c
  url->path : url->path/c
  path->serlvet : path->servlet/c
The first return value flushes the cache. The second is a procedure that uses url->path to resolve the URL to a path, then uses path->servlet to resolve that path to a servlet, caching the results in an internal table.

procedure

(make url->servlet 
  [#:responders-servlet-loading responders-servlet-loading 
  #:responders-servlet responders-servlet]) 
 → dispatcher/c
  url->servlet : url->servlet/c
  responders-servlet-loading : (url? exn? . -> . can-be-response?)
   = servlet-loading-responder
  responders-servlet : (url? exn? . -> . can-be-response?)
   = servlet-error-responder
This dispatcher runs racket servlets, using url->servlet to resolve URLs to the underlying servlets. If servlets have errors loading, then responders-servlet-loading is used. Other errors are handled with responders-servlet. If a servlet raises calls next-dispatcher, then the signal is propagated by this dispatcher.

2.12.1 Setting Up Servlets

 (require web-server/servlet/setup)
This module is used internally to build and load servlets. It may be useful to those who are trying to extend the server.

procedure

(make-v1.servlet directory timeout start) → servlet?

  directory : path-string?
  timeout : integer?
  start : (request? . -> . can-be-response?)
Creates a version 1 servlet that uses directory as its current directory, a timeout manager with a timeout timeout, and start as the request handler.

procedure

(make-v2.servlet directory manager start) → servlet?

  directory : path-string?
  manager : manager?
  start : (request? . -> . can-be-response?)
Creates a version 2 servlet that uses directory as its current directory, a manager as the continuation manager, and start as the request handler.

procedure

(make-stateless.servlet directory    
  stuffer    
  manager    
  start) → servlet?
  directory : path-string?
  stuffer : (stuffer/c serializable? bytes?)
  manager : manager?
  start : (request? . -> . can-be-response?)
Creates a stateless web-server servlet that uses directory as its current directory, stuffer as its stuffer, and manager as the continuation manager, and start as the request handler.

value

default-module-specs : (listof module-path?)

The modules that the Web Server needs to share with all servlets.

value

path->servlet/c : contract?

Equivalent to (-> path? servlet?).

procedure

(make-default-path->servlet 
  [#:make-servlet-namespace make-servlet-namespace 
  #:timeouts-default-servlet timeouts-default-servlet]) 
 → path->servlet/c
  make-servlet-namespace : make-servlet-namespace/c
   = (make-make-servlet-namespace)
  timeouts-default-servlet : integer? = 30
Constructs a procedure that loads a servlet from the path in a namespace created with make-servlet-namespace, using a timeout manager with timeouts-default-servlet as the default timeout (if no manager is given.)

2.12.2 Servlet Namespaces

 (require web-server/configuration/namespace)
This module provides a function to help create the make-servlet-namespace procedure needed by the make function of web-server/dispatchers/dispatch-servlets.

value

make-servlet-namespace/c : contract?

Equivalent to
(->* ()
     (#:additional-specs (listof module-path?))
     namespace?)

procedure

(make-make-servlet-namespace #:to-be-copied-module-specs to-be-copied-module-specs)

 → make-servlet-namespace/c
  to-be-copied-module-specs : (listof module-path?)
This function creates a function that when called will construct a new namespace that has all the modules from to-be-copied-module-specs and additional-specs, as well as racket and mred, provided they are already attached to the (current-namespace) of the call-site.

Example:
(make-make-servlet-namespace
 #:to-be-copied-module-specs `((lib "database.rkt" "my-module")))

2.12.2.1 Why this is useful

A different namespace is needed for each servlet, so that if servlet A and servlet B both use a stateful module C, they will be isolated from one another. We see the Web Server as an operating system for servlets, so we inherit the isolation requirement on operating systems.

However, there are some modules which must be shared. If they were not, then structures cannot be passed from the Web Server to the servlets, because Racket’s structures are generative.

Since, on occasion, a user will actually wanted servlets A and B to interact through module C. A custom make-servlet-namespace can be created, through this procedure, that attaches module C to all servlet namespaces. Through other means (see Dispatchers) different sets of servlets can share different sets of modules.

2.12.3 Internal Servlet Representation

 (require web-server/private/servlet)

struct

(struct servlet (custodian namespace manager directory handler)
  #:constructor-name make-servlet
  #:mutable)
  custodian : custodian?
  namespace : namespace?
  manager : manager?
  directory : path-string?
  handler : (request? . -> . can-be-response?)
Instances of this structure hold the necessary parts of a servlet: the custodian responsible for the servlet’s resources, the namespace the servlet is executed within, the manager responsible for the servlet’s continuations, the current directory of the servlet, and the handler for all requests to the servlet.

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