2 Stateful Servlets

▼ |  | Web Applications in Racket

1 | Running Web Servlets
2 | Stateful Servlets
3 | Stateless Servlets
4 | HTTP: Hypertext Transfer Protocol
5 | URL-Based Dispatch
6 | Formlets: Functional Form Abstraction
7 | Templates: Separation of View
8 | Page: Short-hand for Common Patterns
9 | Testing Servlets
10 | Troubleshooting and Tips
| Index

► | 2 | Stateful Servlets

2.1 | Example
2.2 | Responses
2.3 | Web Interaction
2.4 | Web Cells
2.5 | Continuation Managers

On this page:

2.1 Example
interface-version
manager
start
2.2 Responses
can-be-response?
any->response
set-any->response!
2.3 Web Interaction
send/ back
send/ suspend
send/ suspend/ url
send/ suspend/ dispatch
send/ suspend/ url/ dispatch
send/ forward
send/ finish
redirect/ get
redirect/ get/ forget
current-servlet-continuation-expiration-handler
clear-continuation-table!
with-errors-to-browser
adjust-timeout!
continuation-url?
servlet-prompt
2.4 Web Cells
web-cell?
make-web-cell
web-cell-ref
web-cell-shadow
2.5 Continuation Managers
2.5.1 General
manager
exn: fail: servlet-manager: no-instance
exn: fail: servlet-manager: no-continuation
2.5.2 No Continuations
create-none-manager
2.5.3 Timeouts
create-timeout-manager
2.5.4 LRU
create-LRU-manager
make-threshold-LRU-manager

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## 2 Stateful Servlets🔗ℹ

(require web-server/servlet) | package: web-server-lib

A stateful servlet should provide the following exports:

> value interface-version : (one-of/c 'v2)

This indicates that the servlet is a version two servlet.

> value manager : manager?

The manager for the continuations of this servlet. See Continuation Managers for options.

> procedure (start initial-request) → can-be-response?
> initial-request : request?

This function is called when an instance of this servlet is started. The argument is the HTTP request that initiated the instance.

An example version 2 module:

> #lang racket
> (require web-server/http
> web-server/managers/none)
> (provide interface-version manager start)
> (define interface-version 'v2)
> (define manager
> (create-none-manager
> (lambda (req)
> (response/xexpr
> `(html (head (title "No Continuations Here!"))
> (body (h1 "No Continuations Here!")))))))
> (define (start req)
> (response/xexpr
> `(html (head (title "Hello World!"))
> (body (h1 "Hi Mom!")))))

These servlets have an extensive API available to them: net/url, web-server/http, web-server/http/bindings, web-server/servlet/servlet-structs, web-server/servlet/web, web-server/servlet/web-cells, and web-server/dispatch. Some of these are documented in the subsections that follow.

### 2.2 Responses🔗ℹ

(require web-server/servlet/servlet-structs)
| package: web-server-lib

Servlets communicate to the Web Server by returning HTTP responses. In order to accommodate lightweight programs (and backwards compatibility), the Web Server provides an indirection from application-specific response formats and the internal HTTP response format, response.

> procedure (can-be-response? x) → boolean?
> x : any/c
> procedure (any->response x) → (or/c false/c response?)
> x : any/c
> procedure (set-any->response! new-any->response) → void
> new-any->response : (-> any/c (or/c false/c response?))

any->response coerces any value into a response or returns #f if coercion is not possible. any->response guarantees that any response? input must always be returned exactly (i.e. eq?.) The default always returns #f, signifying that no coercion is possible.
can-be-response? returns #t if x is a response or can be turned into a response by calling any->response.
Users of any->response should protect themselves by using can-be-response? as a contract. If they do so, they can safely ignore the #f return case of any->response.
set-any->response! replaces the global any->response with the supplied argument. This function should return the same value for eq? inputs to ensure that can-be-response? is any accurate predicate. Similarly, this function should be cheap to call multiple times on the same input, since it will be used in contract checking as well as coercion before transmission. You may want to use a weak eq?-based hash-table to cache the results for this purpose. (See make-weak-hasheq.)

### 2.3 Web Interaction🔗ℹ

(require web-server/servlet/web) | package: web-server-lib

The web-server/servlet/web library provides the primary functions of interest for the servlet developer.

> procedure (send/back response) → void?
> response : can-be-response?

Sends response to the client. No continuation is captured, so the servlet is done.

Example:

> (send/back
> (response/xexpr
> `(html
> (body
> (h1 "The sum is: "
> ,(+ first-number
> second-number))))))

> procedure (send/suspend make-response) → request?
> make-response : (string? . -> . can-be-response?)

Captures the current continuation, stores it with (current-servlet-continuation-expiration-handler) as the expiration handler, and binds it to a URL. make-response is called with this URL and is expected to generate a can-be-response?, which is sent to the client. If the continuation URL is invoked, the captured continuation is invoked and the request is returned from this call to send/suspend.

Example:

> (send/suspend
> (lambda (k-url)
> (response/xexpr
> `(html (head (title "Enter a number"))
> (body
> (form ([action ,k-url])
> "Enter a number: "
> (input ([name "number"]))
> (input ([type "submit"]))))))))

When this form is submitted by the browser, the request will be sent to the URL generated by send/suspend. Thus, the request will be “returned” from send/suspend to the continuation of this call.

> procedure (send/suspend/url make-response) → request?
> make-response : (url? . -> . can-be-response?)

Like send/suspend but with a URL struct.

> procedure (send/suspend/dispatch make-response) → any
> make-response : (((request? . -> . any) . -> . string?) . -> . can-be-response?)

Calls make-response with a function (often named embed/url) that, when called with a procedure from request? to any/c will generate a URL, that when invoked will call the function with the request? object and return the result to the caller of send/suspend/dispatch. Therefore, if you pass embed/url the identity function, send/suspend/dispatch devolves into send/suspend:

> (define (send/suspend response-generator)
> (send/suspend/dispatch
> (lambda (embed/url)
> (response-generator (embed/url (lambda (x) x))))))

Use send/suspend/dispatch when there are multiple ‘logical’ continuations of a page. For example, we could either add to a number or subtract from it:

> (define (count-dot-com i)
> (count-dot-com
> (send/suspend/dispatch
> (lambda (embed/url)
> (response/xexpr
> `(html
> (head (title "Count!"))
> (body
> (h2 (a ([href
> ,(embed/url
> (lambda (req)
> (sub1 i)))])
> "-"))
> (h1 ,(number->string i))
> (h2 (a ([href
> ,(embed/url
> (lambda (req)
> (add1 i)))])
> "+")))))))))

Notice that in this example the result of the handlers are returned to the continuation of send/suspend/dispatch. However, it is very common that the return value of send/suspend/dispatch is irrelevant in your application and you may think of it as “embedding” value-less callbacks. Here is the same example in this style:

> (define (count-dot-com i)
> (send/suspend/dispatch
> (lambda (embed/url)
> (response/xexpr
> `(html
> (head (title "Count!"))
> (body
> (h2 (a ([href
> ,(embed/url
> (lambda (req)
> (count-dot-com (sub1 i))))])
> "-"))
> (h1 ,(number->string i))
> (h2 (a ([href
> ,(embed/url
> (lambda (req)
> (count-dot-com (add1 i))))])
> "+"))))))))

> procedure (send/suspend/url/dispatch make-response) → any
> make-response : (((request? . -> . any) . -> . url?) . -> . can-be-response?)

Like send/suspend/dispatch, but with a URL struct.

> procedure (send/forward make-response) → request?
> make-response : (string? . -> . can-be-response?)

Calls clear-continuation-table!, then send/suspend.
Use this if the user can logically go ‘forward’ in your application, but cannot go backward.

> procedure (send/finish response) → void?
> response : can-be-response?

Calls clear-continuation-table!, then send/back.

Use this if the user is truly ‘done’ with your application. For example, it may be used to display the post-logout page:

> (send/finish
> (response/xexpr
> `(html (head (title "Logged out"))
> (body (p "Thank you for using the services "
> "of the Add Two Numbers, Inc.")))))

> procedure (redirect/get [#:headers hs]) → request?
> hs : (listof header?) = empty

Calls send/suspend with redirect-to, passing hs as the headers and see-other as the redirection status.
This implements the Post-Redirect-Get pattern. Use this to prevent the Refresh button from duplicating effects, such as adding items to a database.

Changed in version 1.3 of package web-server-lib: Use see-other instead of temporarily.

> procedure (redirect/get/forget [#:headers hs]) → request?
> hs : (listof header?) = empty

Like redirect/get, but using send/forward instead of send/suspend.

Changed in version 1.3 of package web-server-lib: Use see-other instead of temporarily, as with redirect/get.

> value current-servlet-continuation-expiration-handler
> : |
> (parameter/c (or/c false/c
> (request? . -> . can-be-response?)))

Holds the expiration handler to be used when a continuation captured in this context is expired, then looked up.

Example:

> (parameterize
> ([current-servlet-continuation-expiration-handler
> (lambda (req)
> (response/xexpr
> `(html (head (title "Custom Expiration!")))))])
> (send/suspend
> ....))

> procedure (clear-continuation-table!) → void?

Calls the servlet’s manager’s clear-continuation-table! function. Normally, this deletes all the previously captured continuations.

> > procedure
> > (with-errors-to-browser |  | send/finish-or-back |  |  |  |
> > |  | thunk) |  | → |  | any
> send/finish-or-back : (can-be-response? . -> . request?)
> thunk : (-> any)

Calls thunk with an exception handler that generates an HTML error page and calls send/finish-or-back.

Example:

> (with-errors-to-browser
> send/back
> (lambda ()
> (/ 1 (get-number (request-number)))))

> procedure (adjust-timeout! t) → void?
> t : number?

Calls the servlet’s manager’s adjust-timeout! function.
Warning: This is deprecated and will be removed in a future release.

> procedure (continuation-url? u)
> → (or/c false/c (list/c number? number? number?))
> u : url?

Checks if u is a URL that refers to a continuation, if so returns the instance id, continuation id, and nonce.

> value servlet-prompt : continuation-prompt-tag?

The tag used for Web interaction continuation capture.

### 2.4 Web Cells🔗ℹ

(require web-server/servlet/web-cells)
| package: web-server-lib

The web-server/servlet/web-cells library provides the interface to Web cells.
A Web cell is a kind of state defined relative to the frame tree. The frame-tree is a mirror of the user’s browsing session. Every time a continuation is invoked, a new frame (called the current frame) is created as a child of the current frame when the continuation was captured.
You should use Web cells if you want an effect to be encapsulated in all interactions linked from (in a transitive sense) the HTTP response being generated. For more information on their semantics, consult the paper "Interaction-Safe State for the Web".

> procedure (web-cell? v) → boolean?
> v : any/c

Determines if v is a web-cell.

> procedure (make-web-cell v) → web-cell?
> v : any/c

Creates a web-cell with a default value of v.

> procedure (web-cell-ref wc) → any/c
> wc : web-cell?

Looks up the value of wc found in the nearest frame.

> procedure (web-cell-shadow wc v) → void
> wc : web-cell?
> v : any/c

Binds wc to v in the current frame, shadowing any other bindings to wc in the current frame.

Below is an extended example that demonstrates how Web cells allow the creation of reusable Web abstractions without requiring global transformations of the program into continuation or store passing style.

> #lang web-server/insta
> (define (start initial-request)
> (define counter1 (make-counter))
> (define counter2 (make-counter))
> (define include1 (include-counter counter1))
> (define include2 (include-counter counter2))
> (send/suspend/dispatch
> (lambda (embed/url)
> (response/xexpr
> `(html
> (body (h2 "Double Counters")
> (div (h3 "First")
> ,(include1 embed/url))
> (div (h3 "Second")
> ,(include2 embed/url))))))))
> (define (make-counter)
> (make-web-cell 0))
> (define (include-counter a-counter)
> (call-with-current-continuation
> (λ (k)
> (let loop ()
> (k
> (lambda (embed/url)
> `(div (h3 ,(number->string (web-cell-ref a-counter)))
> (a ([href
> ,(embed/url
> (lambda _
> (define last (web-cell-ref a-counter))
> (web-cell-shadow a-counter (add1 last))
> (loop)))])
> "+"))))))
> servlet-prompt))

### 2.5 Continuation Managers🔗ℹ

Since Racket servlets store their continuations on the server, they take up memory on the server. Furthermore, garbage collection can not be used to free this memory, because there are roots outside the system: users’ browsers, bookmarks, brains, and notebooks. Therefore, some other strategy must be used if memory usage is to be controlled. This functionality is pluggable through the manager interface.

#### 2.5.1 General🔗ℹ

(require web-server/managers/manager)
| package: web-server-lib

This module defines the manager interface. It is required by the users and implementors of managers.

> > struct
> > (struct |  | manager | ( | create-instance
> > |  |  |  | adjust-timeout!
> > |  |  |  | clear-continuations!
> > |  |  |  | continuation-store!
> > |  |  |  | continuation-lookup
> > |  |  |  | continuation-peek)
> #:extra-constructor-name make-manager)
> create-instance : ((-> void) . -> . number?)
> adjust-timeout! : (number? number? . -> . void)
> clear-continuations! : (number? . -> . void)
> | continuation-store! |  | : |  |
> (number? any/c
> (or/c false/c
> (request? . -> . can-be-response?))
> . -> . (list/c number? number?))
> continuation-lookup : (number? number? number? . -> . any/c)
> continuation-peek : (number? number? number? . -> . any/c)

create-instance is called to initialize a instance, to hold the continuations of one servlet session. It is passed a function to call when the instance is expired. It runs the id of the instance.
adjust-timeout! is a to-be-deprecated function that takes an instance-id and a number. It is specific to the timeout-based manager and will be removed.
clear-continuations! expires all the continuations of an instance.
continuation-store! is given an instance-id, a continuation value, and a function to include in the exception thrown if the continuation is looked up and has been expired. The two numbers returned are a continuation-id and a nonce.
continuation-lookup finds the continuation value associated with the instance-id, continuation-id, and nonce triple it is given.
continuation-peek is identical to continuation-lookup except that its use must not affect the resource management policy decisions on the instance or continuation accessed. It is intended to be used by debuggers and benchmarks.

> > struct
> > (struct |  | exn:fail:servlet-manager:no-instance exn:fail
> > |  | (expiration-handler)
> #:extra-constructor-name
> make-exn:fail:servlet-manager:no-instance)
> | expiration-handler |  | : |  |
> (or/c false/c
> (request? . -> . can-be-response?))

This exception should be thrown by a manager when an instance is looked up that does not exist.

> > struct
> > (struct |  | exn:fail:servlet-manager:no-continuation exn:fail
> > |  | (expiration-handler)
> #:extra-constructor-name
> make-exn:fail:servlet-manager:no-continuation)
> | expiration-handler |  | : |  |
> (or/c false/c
> (request? . -> . can-be-response?))

This exception should be thrown by a manager when a continuation is looked up that does not exist.

#### 2.5.2 No Continuations🔗ℹ

(require web-server/managers/none) | package: web-server-lib

This module defines a manager constructor:

> procedure (create-none-manager instance-expiration-handler) → manager?
> | instance-expiration-handler |  | : |  |
> (or/c false/c
> (request? . -> . can-be-response?))

This manager does not actually store any continuation or instance data. You could use it if you know your servlet does not use the continuation capturing functions and want the server to not allocate meta-data structures for each instance.
If you do use a continuation capturing function, the continuation is simply not stored. If the URL is visited, the instance-expiration-handler is called with the request.
If you are considering using this manager, also consider using the Web Language. (See Stateless Servlets.)

#### 2.5.3 Timeouts🔗ℹ

(require web-server/managers/timeouts)
| package: web-server-lib

This module defines a manager constructor:

> > procedure
> > (create-timeout-manager |  | instance-exp-handler |  |  |  |
> > |  | instance-timeout |  |  |  |
> > |  | continuation-timeout) |  | → |  | manager?
> | instance-exp-handler |  | : |  |
> (or/c false/c
> (request? . -> . can-be-response?))
> instance-timeout : number?
> continuation-timeout : number?

Instances managed by this manager will be expired instance-timeout seconds after the last time it is accessed. If an expired instance is looked up, the exn:fail:servlet-manager:no-instance exception is thrown with instance-exp-handler as the expiration handler.
Continuations managed by this manager will be expired continuation-timeout seconds after the last time it is accessed. If an expired continuation is looked up, the exn:fail:servlet-manager:no-continuation exception is thrown with instance-exp-handler as the expiration handler, if no expiration-handler was passed to continuation-store!.
adjust-timeout! corresponds to reset-timer! on the timer responsible for the servlet instance.
This manager has been found to be... problematic... in large-scale deployments of the Web Server .

#### 2.5.4 LRU🔗ℹ

(require web-server/managers/lru) | package: web-server-lib

This module defines a manager constructor:

> > procedure
> > (create-LRU-manager |  | instance-expiration-handler |  |  |  |
> > |  | check-interval |  |  |  |
> > |  | collect-interval |  |  |  |
> > |  | collect? |  |  |  |
> > | [ | #:initial-count initial-count |  |  |  |
> > |  | #:inform-p inform-p]) |  | → |  | manager?
> | instance-expiration-handler |  | : |  |
> (or/c false/c
> (request? . -> . can-be-response?))
> check-interval : integer?
> collect-interval : integer?
> collect? : (-> boolean?)
> initial-count : integer? = 1
> inform-p : (integer? . -> . void) = (lambda _ (void))

Instances managed by this manager will be expired if there are no continuations associated with them, after the instance is unlocked. If an expired instance is looked up, the exn:fail:servlet-manager:no-instance exception is thrown with instance-exp-handler as the expiration handler.
Continuations managed by this manager are given a "Life Count" of initial-count initially. If an expired continuation is looked up, the exn:fail:servlet-manager:no-continuation exception is thrown with instance-exp-handler as the expiration handler, if no expiration-handler was passed to continuation-store!.
Every check-interval seconds collect? is called to determine if the collection routine should be run. Every collect-interval seconds the collection routine is run.
Every time the collection routine runs, the "Life Count" of every continuation is decremented by 1. If a continuation’s count reaches 0, it is expired. The inform-p function is called if any continuations are expired, with the number of continuations expired.
The recommended usage of this manager is codified as the following function:

> > procedure
> > (make-threshold-LRU-manager |  | instance-expiration-handler |
> > |  | memory-threshold) |
> → manager?
> | instance-expiration-handler |  | : |  |
> (or/c false/c
> (request? . -> . can-be-response?))
> memory-threshold : number?

This creates an LRU manager with the following behavior: The memory limit is set to memory-threshold bytes. Continuations start with 24 life points. Life points are deducted at the rate of one every 10 minutes, or one every 5 seconds when the memory limit is exceeded. Hence the maximum life time for a continuation is 4 hours, and the minimum is 2 minutes.
If the load on the server spikes—as indicated by memory usage—the server will quickly expire continuations, until the memory is back under control. If the load stays low, it will still efficiently expire old continuations.
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