Understanding the Dumper Program Google Application Engine University of Michigan Informatics

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UnderstandingtheDumperProgram GoogleApplicationEngine UniversityofMichigan Informatics Thishandoutdescribesaverysimpleandlow levelgoogleapplicationengine applicationcalled Dumper

1 UnderstandingtheDumperProgram GoogleApplicationEngine UniversityofMichigan Informatics Thishandoutdescribesaverysimpleandlow levelgoogleapplicationengine applicationcalled Dumper thatjustdumpsoutthedatafromahttprequest. Thisapplicationdoesnotdemonstratethepatternwewillfollowforthereal applications butitdoesservetoshowhowthingsactuallyworkatalowlevel. Request/ResponseCycle Thebasicinteractionbetweenawebbrowserandthewebserveristhattheuser clicksonalink(getrequest)orsubmitsaformwithdata(postrequest).the browseropensatcp/ipconnectiontotheserverandsendstherequest(getor POST)totheserver.Ontheserver,theURLisroutedtotheapplicationandallof theinputdata(path,parameters,etc)ispassedtotheapplication. Theapplicationthenruns,possiblyaccessingadatabaseorsomeothersourceof dataandthenreturnsthehttpresponsetothebrowserfordisplay. Reference: Inthisapplication,wesimplydumpouttheinputdatafromtheGETorPOST requestandreturnahmtlresponsethatincludesaformandthedumpeddata fromthepreviousrequest. TheDumperProgram Note:Thesamplecodeforthisapplicationcanbedownloadedfrom Thedumperprogramconsistsofaverysimpleapp.yamlfileandasingleindex.py PythonfilewhichcontainsthelogicofourAppEngineprogram.

2 app.yaml: application: ae-02-dumper version: 1 runtime: python api_version: 1 handlers: - url: /.* script: index.py Thissimplynamesoutapplication(ae 02 dumper)androutesallincomingrequests totheindex.pyscript. index.py: import os import sys print 'Content-Type: text/html' print '<form method="post" action="/" >' print 'Zap Data: <input type="text" name="zap"><br>' print 'Zot Data: <input type="text" name="zot"><br>' print '<input type="submit">' print '</form>' print '<pre>' print 'Environment keys:' for param in os.environ.keys(): print param, ':', os.environ[param] print 'Data' count = 0 for line in sys.stdin: count = count + 1 print line if count > 100: break print '</pre>' Note:Youwilllikelyendupwithproblemscuttingandpastingsourcecodefrom PDFhandoutsintotextfiles.Itisbettertoeitherdownloadthesourcecodeortype itin. Theindex.pyprogramisbrokenintothreeparts:

3 ThefirstsetofprintstatementssimplyproducetheHTMLforaformwhichcanbe usedtodoatestpostofsoedatatooutprogram. print 'Content-Type: text/html' print '<form method="post" action="/" >' print 'Zap Data: <input type="text" name="zap"><br>' print 'Zot Data: <input type="text" name="zot"><br>' print '<input type="submit">' print '</form>' ThefirstlineisaheaderlinethatisnotartoftheHTMLdataresponse.Sinceweare notusingaframework,whichwouldsendheadersforus,wemanuallyproducethe headerlineandthenablanklinetoindicatetheendoftheheadersandstartofthe actualhtmldata. Theformisquitebasicwithtwotextfieldsandasubmitbuttonwhichwillrender likethis: ThenextlinesoftheprogramreadinasetofvariablespassedinasaPython dictionary.thesearethe environment variables.theyareacombinationofthe serverconfigurationaswellasinformationabouttheparticularrequestitself. Wesimplyiteratethroughthedictionaryandthenprinttheitemsout: print '<pre>' print 'Environment keys:' for param in os.environ.keys(): print param, ':', os.environ[param] Theoutputfromthissectionisasfollows:

4 Theenvironmentvariablesfallintothreecategories: Variablesdescribingtheserverenvironment(SERVER_SOFTWARE, SERVER_NAME) VariablesdescribingtheRequestdata(REQUET_METHOD, HTTP_USER_AGENT,orCONTENT_TYPE) Variablesdescribingthebrowserenvironmentvariables (HTTP_USER_AGENT,HTTP_ACCEPT,etc.) Interestinglythedocumentationabouttheseparametersisdescribedhere: Thisisavery old websitethatdescribesthecommongatewayinterface(cgi) whichwasthewaythattheveryfirstwebserverspassedinputdatafromanhttp requestintoapplicationcoderunningontheserver. Whenweareprogrammingatthislevel(whichwewillnotdoforlong) weare usingtheoldmysticalwaysoftheearlyworld wide web.wewon tusethis

programpatternformuchlonger butitisgoodtostartbyunderstandingthelowleveldetailsandthendelegatethehandlingofthosedetailstoawebframework. Thelastpartoftheindex.

5 programpatternformuchlonger butitisgoodtostartbyunderstandingthelowleveldetailsandthendelegatethehandlingofthosedetailstoawebframework. Thelastpartoftheindex.pyprogramdumpsoutuptothefirst100linesofPOST dataifthedataexists: print 'Data' count = 0 for line in sys.stdin: count = count + 1 print line if count > 100: break AccordingtotherulesofCommonGatewayInterface thepostdataissimply presentedtotheapplicationonits StandardInput.InPythonwecanreadthrough thepredefinedfilehandlesys.stdintoaccessourpostdata. Ifyoulookatthebottomoftheinitialoutputoftheprogram youwillseethatthere isnopostdata becausewhenyounavigatetohttp://localhost:8080thebrowser issuesagetrequestfortheinitialpage. TotestPOSTdatadumpingcode,wemustentersomedataintotheZapandZotdata fieldsandpresssubmit: Whenwepress Submit,ourbrowsersendsaPOSTrequest whichyoucan immediatelyseeintherequet_methodvariablechangefromgettopost: Andifwescrolldowntothebottomoftheoutput,youcanseetheactualPOSTdata:

ThePOSTdataisencodedbyescapingspacesandspecialcharacters.Wewould havetoparsetheinputdatausingstringparsingandthenun escapethedatatoget backtotheactualdatathatwastypedintotheform.

Advanced:UploadingFiles Ifyouwanttoplayabit youcanexperimentwithhowfilesareuploadedinthe HTTPrequest/responsecycle.

6 ThePOSTdataisencodedbyescapingspacesandspecialcharacters.Wewould havetoparsetheinputdatausingstringparsingandthenun escapethedatatoget backtotheactualdatathatwastypedintotheform. Thankfully inournextapplication,awebframeworkwillsoondoallthatparsing andescapingforus. Advanced:UploadingFiles Ifyouwanttoplayabit youcanexperimentwithhowfilesareuploadedinthe HTTPrequest/responsecycle.Makethefollowingchangestotheformcode: print '<form method="post" action="/" enctype="multipart/form-data">' print 'Zap Data: <input type="text" name="zap"><br>' print 'Zot Data: <input type="text" name="zot"><br>' print 'File Data: <input type="file" name="filedat"><br>' print '<input type="submit">' Thenruntheprogramagain,selectingafile,typingsomedataandthenpressing submit: Ifyoulookattheoutputfromtheprogramitlooksasfollows:

7 Thecontenttypechangechangedto multipart/form data andnowtheform valuesandthefiledataarespreadoutintheinputstreamwiththiscomplexlooking separatorthatdividesalltheparts. Thisisnecessarybecausethefilewillbealotofdata anditwillcomeinasmany lines.alsothereneedstobealotofdescriptionaboutthenatureofthedata. Reference: Alsoifyouarelookingatthelogfromyourapplicationyouwillseeentriesas follows: INFO :09:21,151 dev_appserver.py] "GET / HTTP/1.1" 200 INFO :09:23,374 dev_appserver.py] "GET / HTTP/1.1" 200 INFO :09:30,463 dev_appserver.py] "POST / HTTP/1.1" YoucanseethedifferentGETandPOSTrequestsbeginprocessedintheAppEngine log. Summary

8 ThissimpleprogramallowsustolookathowtheHTTPRequest/Responsestyleis supportedinthegoogleapplicationengine.theapplicationengineframework givesusaveryprimitivecommongateway(gci)compliantinterfacewith environmentvariables,standardinput,andstandardoutput.wecanexamineall thedatathattheappenginemakesavailabletoourscripts. Ultimatelythisisonlyofpassinginterestbecausewewilldelegatemuchofthe detailofhandlingtherequestandresponsetothebuiltinwebapplication frameworkinthegoogleapplicationengine. Whenweusetheframework,ourcodemayseemalittlemorecomplexbutthe frameworktakescareofamyriadofsmalldetailsofparameterpassing,parsing, headers,andconversion. Reference: ThismaterialsisCopyrightCreativeCommonsAttribution2.5 CharlesSeverance chuck.com

Chapter 4 Sending Data to Your Application

Chapter 4 Sending Data to Your Application Charles Severance and Jim Eng csev@umich.edu jimeng@umich.edu Textbook: Using Google App Engine, Charles Severance Unless otherwise noted, the content of this