Implementing Prolog
Contents
1. 1 H Boley Knowledge Bases in the World Wide Web a Challenge for Logic Programming Techni cal Memo TM 96 02 Deutsches Forschungszentrum f r Kiinstliche Intelligenz October 1997 2 H Boley Beziehungen zwischen Logikprogrammierung und XML In Workshop Logische Programmierung W rzburg Germany Jan 2000 published as GMD Report 90 3 M Bryan SGML and HTML Explained Addison Wesley U S A 1997 4 D Cabeza and Hermenegildo The PiLLoW Program ming Library Reference Manual 2000 Available at http www clip dia fi upm es miscdocs pillow 5 D Cabeza M Hermenegildo and S Varma WWW Programming using Computational Logic Systems and the PiLLoW CIAO Library Technical report Computer Science Department Technical University of Madrid 1996 6 B Carpenter A Prolog CGI Interface http www colloquial com tlg cgi html 1999 7 W3 Committee HyperText Markup Language Home Page http www w3 org MarkUp February 2000 8 M Grobe and H Naseer An Instantaneous In troduction to CGI Scripts and HTML Forms http www cc ukans edu acs docs other forms intro shtml 1994 9 NCSA HTTPd The Common Gateway Interface http hoohoo ncsa uiuc edu cgi 1998 10 ISO ISO Prolog Standard Technical Report ISO IEC DIS 13211 1 1995 E International Organisation for Standardisation Geneva Switzerland 1995 11 Intelligent Systems Laboratory SICS tus Prolog User s Manual Swedish In2. button lt p gt lt form action http www yoursite com cgi bin script cgi method get gt lt textarea name query rows 10 cols 40 wrap physical gt lt textarea gt lt p gt lt input type submit name send value submit gt lt form gt lt html1 gt fo pe a e j a fet ce i aj tees Ete Te a s r e a Funa eee g ia Fie g ieee gF ag Pilate St H EE SS E 1 a E Figure 2 HTML Code Top for Form Bottom a service from the WWW site specified by the form A CGI script 6 8 9 explained in Section 3 below must be associated with an HTML form In our running ex ample we have made use of the get method and asso ciated our form with the CGI script script cgi stored in http www yoursite com cgi bin on the WWW server machine When the user clicks on the submit button the request query http www yoursite com cgi bin script cgi query Thististat simple tinput sentence amp send submit is sent over to the site www yoursite com The sub string of characters starting with and going un til the end comprises the information from the HTML form to be passed on to script cgi However if the post method had been used the input string would not have appeared in the request query as above rather it would have appeared in the body of the HTTP request We explain below some important technical details of scripts 3 The CGI Script A CGI script is a special kind of progr
3. the cgi bin sub folder of the WWW server machine otherwise it just does not work In some sites where security against invaders is a priority issue very strict policies are en forced concerning access to cgi bin folders and what can be made available in them For instance some sites may demand that the CGI script be thoroughly tested before they are made available to outside users We recommend that readers interact directly with the com puting support and WWW team of their sites to find out more about such restrictions If it turns out to be a delicate political issue to have a CGI script available via normal means one can always set up one s own inde pendent WWW server for instance using free software like Jigsaw see http www w3 org Protocols for more information Another important technical issue concerns the method employed in the HTML form Forms are used in various contexts as a standardised mechanism for bidirectional data exchange over the web 7 Methods are used to specify how the form is to be handled by the CGI script and they method and CGI script are closely related We have experimented with two meth ods post and get and their respective CGI scripts explained below If you use method get in your form specified by the HTML command lt form action http www yoursite com cgi bin script cgi method get gt as part of your HTML web page then your CGI program will receive the encoded form input in th
4. visualisation this command should be all in one line otherwise it won t work The command above invokes SICStus and pipes in a query The query is the ac tual bootstrapping program to start up the Linda server and to record the host name and port number in the server addr file The around the host name are essential if the site employs machine names containing special characters or dots Since they will not interfere when the names are strings of simple characters just leave them there After the above command is exe cuted a SICStus process will run in the background and the file server addr will be open with the Host Port information stored in it After the above command is executed and only after it otherwise it won t work we bootstrap the server it self It should run in the background hence the need to bootstrap it The command nohup echo main sicstus amp should be typed for that purpose It simply invokes SICStus and loads in program main p1 shown in Figure 6 Notice that line 16 will cause the execu tion of predicate start_main 0 upon the upload of the file and hence that start up command should be at the very end or any remaining clauses won t be loaded 5 Final Remarks and Summary In this document we have defined explained and jus tified an architecture for a WWW Server fully run by Prolog programs We have tried to provide sufficient implementational details so as to allow the reproduc
5. main program stop and return an error This can be handled by catching all the exceptions that occur while processing a request Long delays on the other hand may happen if a request takes too long to process all the other posterior requests will have to wait for the long one to finish Timeout and exception handling can be implemented with SICStus module timeout and SICStus exception handling mechanisms 11 In Fig ure 6 timeout and exception handling routines would have to be part of predicate services 2 in line 11 4 2 Bootstrapping the WWW Server We explain here the two step procedure for boot strapping our WWW server implemented as a set of Linda client server processes The two programs parser_token pl and main pl will be run as Linda clients A third server process is necessary to estab lish the communication channels i e the handle between them The set comprising these three pro cesses comprises our WWW server plus of course the Prolog CGI Script see Figure 1 There is no need to write yet another Prolog program for this purpose The following Unix bootstrapping command to be run in the WWW server machine is enough to start up our Linda server process echo use_module library linda server linda Host Port tell server addr write write Host write write Port write told sicstus amp Although we have prettyprinted it above to improve
6. paths The names of the files for the CGI script and the parser tokeniser can of course be changed If however you use method post in your HTML form including the line lt form action http www yoursite com cgi bin script cgi method post gt as part of your HTML form then your CGI program will receive the encoded form input on STDIN that is the standard input The server will not send you an EOF end of file at the end of the data Instead you should use the environment variable CONTENT_LENGTH to determine how much data you should read from STDIN The CGI script corresponding to the post method is presented in Figure 4 1 bin perl 2 print Content type text html n n 3 read STDIN buffer ENV CONTENT LENGTH 4 open TMP bin sicstus r www cgi bin parser_token a buffer 5 while in lt TMP gt 6 all all in T gt 8 close TMP 9 print all Figure 4 CGI Script script cgi post Method Basically the differences between the get method script and the post one are lines 3 and 4 In line 3 a sequence of bytes defined by CONTENT LENGTH is read in from STDIN and stored in variable buffer This vari able is passed on to our SICStus Program via command line 4 One should notice that the variable buffer is used in place of the environment variable QUERY STRING of our previous script for the get method An essential issue concerns the format of the out put of our programs Si
7. string of characters assigned to variable Arg all performed by prolog flag 2 Lines 4 8 comprise two nested if then else commands which tokenise and parse the input string into a list of keys handling the potential errors by appropriate messages predicate displayMessage 2 pretty prints messages and prepares the HTML page taking care of all its details We have used the code provided in 6 for the actual parsing and tokenis ing predicates tokenizeatom 2 and parse_cgi 2 If the input string has successfully been tokenised and parsed then we proceed to predicate process 1 carry ing with us a list of keys each of which is of the form key KeyWName KeyContent Again more details can be found in 6 The predicate process 1 handles a specific list of keys as shown in the pattern of its arguments This predicate simply establishes a communication link be tween the parser tokeniser and the main Prolog pro gram that actual handles the HTML forms In this sense the parser tokeniser is like a receptionist that receives the input from the CGI script makes sure this is OK then passes it on to the main program Lines 11 18 comprise the usual sequence of commands for achiev ing Linda client server communication 11 initially line 11 the file server addr previously recorded see below for more details is opened and its contents of the form Host Port is read line 12 and then the file is closed line 13 A communication cha
8. Implementing Prolog Run WWW Sites Wamberto Vasconcelos Rolf Schwitter Diego Molla Department of Information Technology University of Zurich Winterthurerstr 190 CH 8057 Zurich Switzerland wvasconcelos acm org schwitt ifi unizh ch molla ifi unizh ch Jo o Cavalcanti Division of Informatics University of Edinburgh 80 South Bridge EH1 1HN Edinburgh Scotland UK joaoc dai ed ac uk Abstract We describe a modular and customisable architecture for a WWW server run by Prolog programs and show how each of its components can be implemented Our proposal employs standard Prolog CGI technology but to improve efficiency we also use client server modules to perform the actual services of the WWW site 1 Introduction The purpose of this document is to describe a mod ular and customisable architecture for a WWW server which employs Prolog programs to handle requests from HTML forms It is our intention to provide as much detail as necessary for the appropriate adaptation and customisation of our architecture to other similar con texts Our proposed architecture consists of the following components e an HTML form a web page is offered as an inter face to remote users who want to pose requests to the WWW server e aProlog CGI interface the request posed by users to the WWW site via the HTML form is handled by a CGI Common Gateway Interface applica tion Our CGI application is a simple Perl pro gram that
9. Page Main Program PAN ras fon QO g eet ey Web Browser WWW Server Figure 1 Architecture for Prolog Run WWW Sites a chronology of the events numbered circles involved in an interaction between a user client and the main program server in a circular fashion Initially step 1 a user fills out an HTML form and sends a HTTP request to the WWW Server via a browser A CGI Common Gateway Interface script is activated step 2 and the request is passed on to a Prolog program to parse and tokenise the request Parser Tokeniser box After this the Parser Tokeniser step 3 commu nicates with a Linda Server Prolog process which on its turn step 4 communicates with the main Prolog program Main Program box that actually handles the request Either the parser tokeniser or the main program or both can generate output step 5 which is piped to the CGI script The main Prolog program handles the request and upon its completion the CGI script returns step 6 whatever output is produced The CGI script then directs any output to the user s browser who then gets the response to the request step 7 In the next sections we describe each of the items above giving their details and explaining how they can be adapted for different contexts Throughout this pa per we make intensive use of material taken from 6 8 However we expand and improve on these references by recording the experiences and d
10. am started via an HTML form and run on the WWW site s machines Scripts are sequences of commands like ordinary pro grams however their commands consist of instructions at the operating system level such as handling files deleting or printing them for instance or running other scripts and programs in programming languages like C or in our case Prolog We suggest 9 for a more thorough explanation on the Common Gateway Interface Any programming language which can read envi ronment variables and which provides executable code like C C or Java can be used as a CGI appli cation Prolog being a high level symbolic language is an excellent tool for programming WWW applica tions 1 2 12 but some implementations do not allow the creation of executable code Furthermore in the standard Prolog definition 10 there is no way to re fer directly to environment variables That is why if we want to use Prolog as a programming language for WWW applications we have to provide an executable program i e a binary or script file that can invoke the Prolog interpreter load a given program and run it For the sake of convenience this executable will also deal with the CGI particularities We have used a CGI script 6 8 9 for this purpose and we proceed below to describe this approach A first piece of technical information is that a CGI script in order to handle requests of HTML forms has to be stored in a special folder viz
11. e Another alternative would be to write a C or Java program to deal with CGI and interface this program with an executable Prolog program Although we know that in principle this ap proach is feasible we haven t investigated it thoroughly environment variable QUERY_STRING Since the size of environment variables content is limited to 1024 characters you might not be able to use the get method if the query is too long Supposing we have access to the cgi bin folder of our WWW site we shall store our script cgi file with the contents depicted in Figure 3 the numbering is not part of the actual con tents and have been used to make references to specific portions of the script easier The first line informs the 1 bin perl 2 print Content type text html n n 3 open TMP bin sicstus r www cgi bin parser_token a QUERY_STRING 4 while in lt TMP gt 5 all all in 6 7 close TMP 8 9 print all Figure 3 CGI Script script cgi get Method operating system that the CGI script makes uses of the scripting language Perl We shall not go into details as to what precisely the components of the syntax of each line mean we refer readers to 13 14 for a more comprehensive study on Perl s form and capabilities We provide here though an overall explanation for each command The second line prints the preamble to the output which is essential since it will be displayed on a browser as HTML te
12. enables the requests to be processed by a Prolog program e a set of client server modules in order to pro cess our requests a Prolog program is initially run to parse and tokenise the actual input and only then the requests can be handled Since the pro cessing of requests can be quite complex we sug gest splitting these two independent stages pars ing tokenising and handling the request into two programs However we notice that the program which processes requests can become very large and that loading large programs can take consider able time and degrade the server performance due to simultaneous executions triggered by different requests We describe how we can use client server modules to circumvent these problems A graphic representation of our architecture is shown in Figure 1 below Besides the components of our architec ture and their relationships the diagram also conveys On a Post Doctoral leave of absence from Departamento de Estat stica e Computa o Universidade Estadual do Ceara Cear Brazil sponsored by the Brazilian Research Council CNPq grant no 201340 91 7 tOn leave of absence from Departamento de Ci ncia da Com putacao Universidade do Amazonas Amazonas Brazil spon sored by the Brazilian Research Council CAPES grant no 1991 97 3 Linda Server aoa HTTP Request Prolog CGI Script Parser Tokeniser HTML
13. ifficulties we had im plementing our complete WWW server as well as how problems were solved Although our discussion here deals specifically with SICStus Prolog 11 running on a Unix platform we believe readers will find enough details here so as to enable them to adapt our tech niques to their pet version of Prolog interpreter or even to another language 2 The HTML Form HTML forms are interfaces between users and WWW sites They come in many different formats ranging from simple click on buttons to scroll down menus and lIn Section 4 we justify the need for this set of client server modules to implement the WWW server At this point we simply say that it is concerned with efficiency issues areas to type in actual text etc on their own and combined together It is not our goal to provide here an extensive compilation of HTML forms and we suggest 3 7 for that purpose There are two methods to submit forms Depending on which method is used the encoded results of the form is passed to the CGI program in a different way This issue is explained in Section 3 We shall employ a sim ple form comprising an area and a button whose HTML code is shown in Figure 2 The user is to type in a piece of text e g This is a simple input text and click on the submit button By doing so the user requests lt htmi gt lt titlesiil iForm lt title gt SY lt p gt Please type your sentence below and click on the submit
14. l re ceives the HTML form checks its format and issues appropriate messages if contents are not correct This program makes use of SICStus Linda Client Server technology There is no need to change it unless an altogether different architecture is envis aged Once the program is stable it should be saved to be later restored by the CGI script 3 The actual WWW server handling the queries is programmed as main p1 which communicates with parser_token p1 via Linda Client Server technol ogy This program should embed all the processing of the site and the predicate services 2 should be adequately programmed for that purpose 4 The bootstrapping procedure should be carried out in the sequence above first the Linda Server pro cess should be triggered off to establish the handle through which parser_token pl and main pl can communicate then the server should be started off There s no need to change any of the boot strapping steps The SICStus process running the server should never stop when that happens the WWW server will be knocked out This will hap pen if the machine running the processes crashes in which case the bootstrapping procedure will have to be performed again the bootstrapping commands could be included in the batch file used to re start the machine The proposal described in this paper has been used to implement the WWW site http www ifi unizh ch cgi bin schwitter client name 27 27 References
15. nce the output is to be viewed through a web browser its format should reflect this with suitable HTML typesetting commands and tags It is important that the very first string to be output should be Content type text html followed by at least one blank line which works as a preamble to an HTML document This has been ensured in our scripts above by their second line Without this command the output will be shown as a string without any HTML formatting even though the HTML tags are in the ap propriate places 3 1 The Parser Tokeniser The above CGI script interfaces the HTML form with a Prolog program The restored Prolog program the parser tokeniser is run with the submitted contents of the HTML form Each user who submits a request via our HTML form above will trigger an execution of the CGI script on the WWW server machine In order to provide a quick and smooth upload of the Prolog program that initially handles the HTML form we restore a program whose state has been previously saved This is quicker than loading the actual program Besides when we save a program we can specify the predicate that is to be invoked whenever the program is restored More details on saving and restoring SICStus Prolog programs can be found in 11 We show in Figure 5 a simplified edited version of our parser tokeniser with its overall structure again num bers have been used to make references to specific lines easier The first line inform
16. ng carried out limitations and or idiosyncrasies of the li brary may be found 4 A WWW Server as Client Server Modules Our WWW server architecture is based on client server modules Our design decision was based on perfor mance concerns One could use a single program to deal with both the reception and the actual dealing of the HTML form in such case it would not be necessary to use any of this client server technology Our Prolog program of Figure 5 could be altered in a straightfor ward fashion the body of process 1 would contain the actions related to the HTML form However we envis age a not so simple setting in which our assumptions and constraints are 1 We suppose that our site will need a large Prolog program to handle all its services 2 We have adopted CGI scripts as a means to inter face the network protocol with Prolog 3 CGI scripts can only do as good as the operating system allows them to the only way to run a Pro log program is by initially invoking SICStus then loading the program However rather than load ing the program code and interpreting it we can do better by restoring a saved state thus making the uploading a lot more efficient We have profitably made use of this trick in our small receptionist program parser_token above 4 Notwithstanding our main program is large enough to make even the saved state restore trick not much help Saved states can get very large if the source prog
17. nnel called a handle is then opened line 14 between the pro gram parser_token and the Linda server a message is passed line 15 to the server via out 1 and the con trol of execution waits for the server response which is received in line 16 The Result is processed in line 17 an adequate definition for this predicate should be supplied as part of your Parser Tokeniser program Fi nally line 18 the communication channel is closed For the sake of completion we have also included a my_save 0 predicate which saves the program inform ing the predicate that should be initially invoked that is main 0 in order for it to be restored by the CGI script Once the program parser_token p1 is loaded in SICStus the command my_save should be typed in and a large ish file will be created as parser_token This is the file that should be stored in the cgi bin folder because the CGI script will need it An altogether different alternative to this approach is to use existing Prolog libraries such as PiLLoW CIAO 5 4 as ready to use Prolog code to handle CGI and long HTML typesetting commands Such ready made macros subroutines can save a great deal of time pro gramming and debugging and allow for a cleaner pro gramming style However as is the case with all ready to use libraries some time must be invested in learning to correctly use their constructs Moreover when more sophisticated or special purpose programming is bei
18. ram is large loading big files can take considerable amounts of time 5 Furthermore for each user that submits an HTML form a copy of parser_token is run If there are too many requests being dealt at the same time and each request requires a huge file to be loaded then this may seriously affect the performance of the server Although in remote applications the overhead of data transfer across narrow bandwidth slow transfer rate can overshadow any other de lays the degradation of the server performance is not irrelevant With this rationale we have split our WWW server into two parts a first simple part that receives the input from the CGI script and checks its correctness explained in Section 3 and a second more sophisticated part that handles the form request explained in Sec tion 4 1 The former will have as many copies running as there are requests the latter will have only one copy permanently running on the server and handling the requests from our simple programs The simple pro grams are started run interact with the main program of our server and then disappear our main program will persist all the way through 4 1 The WWW Server Main Program In this section we show and explain the actual contents of our main program In Figure 6 we can see the Prolog source code for the main program main pl The first 1 use_module library linda client 2 start_main 3 see server addr 4 read Host Por
19. s SICStus that two of its 1 use_module library linda client use_module library system 2 main 3 prolog_flag argv Arg 4 tokenizeatom Arg TokenList gt 5 parse_cgi TokenList KeyVals gt 6 process KeyVals displayMessage Arg Could not be parsed displayMessage Arg Could not be tokenised halt 10 process key query Query key send submit 11 see server addr wooo N 12 read Host Port 13 seen 14 linda_client Host Port 15 out query Query 16 in result Result 17 process_result Result 18 close_client 19 my _save save_program parser_token main Figure 5 Parser Tokeniser parser_token p1 library modules linda client and system are to be loaded for the program to be run As mentioned above our architecture employs Prolog client server technol ogy to improve efficiency of the WWW site hence the need to load linda client The predicate main 0 de fined from line 2 to 9 is to be invoked initially when the program is restored this is specified by one of the argu ments of SICStus save_program 2 built in explained below Predicate main 0 starts off line 3 by importing into the program the contents of the CGI script s variable QUERY_STRING that is the contents of the HTML form built in prolog_flag 2 predicate is part of the library system hence the need to load it in line 1 The contents of the HTML form is made available as a
20. stitute of Computer Science available at http www sics se isl sicstus2 html Manuals February 2000 12 S W Loke and A Davison Logic Programming with the World Wide Web In Proc Hypertext 96 pages 235 245 Washington DC 1996 ACM 13 R L Schwartz T Christiansen and L Wall Learning Perl O Reilly amp Associates U S A 2nd edition 1997 14 L Wall T Christiansen R L Schwartz and S Potter Programming Perl O Reilly amp Associates U S A 2nd edition 1996
21. t 5 seen 6 linda_client Host Port 7 main_loop 8 main_loop 9 repeat 10 in query Query 11 services Query Answer 12 out result Answer 13 fail 14 15 services Query Answer 16 start _main Figure 6 Main Program main pl line tells SICStus that the Linda client library is used in the program our main program is another Linda client and not a server process but contrary to the parser_token program it will run permanently The start_main 0 predicate similar to what happens in the process 1 predicate of parser_token p1 program reads in the Host Port address through which the com munication between parser_token pl and main pl will take place In line 7 we have a call to main_loop 0 defined in lines 8 13 that predicate is an endless loop receiving requests line 10 processing them line 11 and outputting the result line 12 One should notice the reverse order in the sequence of in 1 and out 1 subgoals in the programs The predicate services 2 is the core of the server where the processing of the query actually takes place Line 16 starts up the server and should be the very last line of the file this com mand is necessary when bootstrapping the server as we shall see below Blockages or long delays may happen during the main loop lines 8 13 if too many copies of the CGI script simultaneously try to communicate with main pl Blockages may happen if a request makes the
22. tion and adaptation of our experiments The actual code of the programs has been edited for the sake of saving space but all the essential parts have been kept In order to deal with efficiency issues we have split our server into two specialised programs parser_token pl and main pl The former is respon sible for the reception and checking of the HTTP re quest provided by the HTML form and the latter is concerned with providing the actual services of the WWW site We have integrated them by means of client server modules By splitting our WWW server into these two programs we are able to circumvent the overhead of uploading huge files to process each request It may be argued that this division is subjective and that the actual parsing tokenising could be performed by the server or conversely that some simpler services could be handled by the initial program However one should try and keep the initial program as small as possible because each time a request is posed via an HTML form a new process will be started in the WWW server machine thus taking up time and computational resources A summary of this paper with suggestions on what one should do to adapt our ideas is as follows 1 A CGI script interfaces the HTML form with a simple Prolog program parser_token pl The contents of the CGI script should not be changed with the exception of the paths for SICStus and the cgi bin directory 2 A simple Prolog program parser_token p
23. xt if such a preamble is not printed then the output will be displayed in the browser not as hypertext but simply as a string of characters without the formatting typesetting graphics hyperlinks etc The third line consists of a command to load SICStus bin sicstus and restore flag r the saved program parser_token stored in www cgi bin the cgi bin folder otherwise it won t work using as an input argument flag a the substring from the HTML form starting after the 2 referred in Perl simply as QUERY_STRING This corresponds to step 2 in our diagram of Figure 1 Any output resulting from running SICStus with the restored program parser_token should be piped that is directed to temporary area TMP as indicated by the symbol and the TMP as the first argument of open The fourth fifth and sixth lines define a simple loop transferring the contents of TMP to variable all and line 9 prints the contents of all that is any output of the computations arising after running the command of line 3 onto the browser of the user who submitted the HTML form This output process corresponds to step 6 of our diagram shown in Figure 1 The contents of the CGI script shown above should remain unchanged with the exception of the path to both SICStus and the saved program which should be adapted accordingly The full path to these components should be provided within Perl scripts even though the installation has default
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