Caching XML Web Services to Support Disconnected Operation

1
Caching XML Web Services to Support Disconnected
Operation

• Venugopalan Ramasubramanian
• Cornell University
• Doug Terry
• Microsoft Research, Silicon Valley

2
Web Services

• method of providing and accessing services on the
  Internet
• consumer services
• hotmail, orbitz, mapquest, ebay,
• B to B services
• supply chain management
• request-response paradigm
• RPCs on the internet

3
XML Web Services

• W3C (world wide web consortium) standards
• Microsoft, IBM, HP,
• Microsoft .Net web services (HailStorm)
• mycontacts, myprofile, myfavoritewebsites
• TerraServer, CoolRooster
• SOAP (simple object access protocol)
• standard representation of web service
  requests/responses (SOAP-RPC)
• WSDL (web services description language)
• description of web services

4
Availability of Web Services

• GOAL
• make web services available despite frequent
  disconnections and limited bandwidth!
• web service clients reside on all kinds of
  devices
• desktop, laptop, PDA, smart phone
• network outages (especially wireless)
• bandwidth restriction

5
Governing Principles

• cannot modify web services
• cannot modify access protocols
• can perhaps modify client
• must also comply with existing clients
• can interpose storage and computation

client-side caching is a solution to improve
availability!
6
XML Standards SOAP

• SOAP-RPC standard
• encoding definitions for data types
• success, failure definitions
• SOAP-Envelope
• outer-most element
• SOAP-Body
• obligatory
• request operation name, parameters
• response status return value, failure
• SOAP-Header
• optional, multiple header blocks.
• supplementary information kerberos ticket
• HTTP binding
• HTTP request and response messages

7
example soap request
ltsEnvelope xmlnsshttp//schemas.xmlsoap.org/so
ap/envelope/ xmlnsmhttp//schemas.micro
soft.com/hs/2001/10/myContacts
xmlnschttp//schemas.microsoft.com/hs/200
1/10/core xmlnsmp"http//schemas.microsoft.
com/hs/2001/10/myProfile" gt ltsHeadergt ltlicense
s xmlns"http//schemas.xmlsoap.org/soap/security/
2000-12"gt ltcidentitygt ltckerberosgt3240lt/ckerb
erosgt lt/cidentitygt lt/licensesgt ltpath
xmlns"http//schemas.xmlsoap.org/rp/"gt ltaction
gthttp//schemas.microsoft.com/hs/2001/10/corerequ
estlt/actiongt lttogthttp//terry.microsoft.comlt/to
gt ltfwdgtltvia /gtlt/fwdgtltrevgtltvia
/gtlt/revgt ltidgtb55528a4-5d63-49f1-87a2-5fab8d76f6
58lt/idgt lt/pathgt ltcrequest service"myContacts
" document"content" method"insert"
genResponse"always" gt ltkey puid"3240"
instance"1" cluster"1" /gt lt/crequestgt lt/sHe
adergt ltsBodygt ltcinsertRequest
select"/mmyContacts/mcontactmpname/mpgivenNa
me Terry'/mpemailAddress" gt ltmpemailgtterr
y_at_microsoft.comlt/mpemailgt lt/cinsertRequestgt lt
/sBodygt lt/sEnvelopegt
8
XML Standards WSDL

• concrete definition of the web service
• data structures
• interface offered by the web service
• operation names and parameters
• message formats (components of a message)
• protocol binding (SOAP)
• automatic generation of client-side stubs
• Visual Studio .Net

9
Experiments with Web Cache

• experiment with existing clients and services
  (Microsoft .Net web services)
• check feasibility by building a cache to store
  HTTP requests/responses

10
Issues in Caching

• web services are active
• default HTTP cache directive is No Cache!
• web services are diverse
• unlike files and databases, web services have
  custom interfaces
• fundamental questions
• which requests are cacheable?
• which operations have permanent side effects?
• how to understand requests/responses?
• services use different formats for
  requests/responses

11
example soap request
ltsEnvelope xmlnsshttp//schemas.xmlsoap.org/so
ap/envelope/ xmlnsmhttp//schemas.micro
soft.com/hs/2001/10/myContacts
xmlnschttp//schemas.microsoft.com/hs/200
1/10/core xmlnsmp"http//schemas.microsoft.
com/hs/2001/10/myProfile" gt ltsHeadergt ltlicense
s xmlns"http//schemas.xmlsoap.org/soap/security/
2000-12"gt ltcidentitygt ltckerberosgt3240lt/ckerb
erosgt lt/cidentitygt lt/licensesgt ltpath
xmlns"http//schemas.xmlsoap.org/rp/"gt ltaction
gthttp//schemas.microsoft.com/hs/2001/10/corerequ
estlt/actiongt lttogthttp//terry.microsoft.comlt/to
gt ltfwdgtltvia /gtlt/fwdgtltrevgtltvia
/gtlt/revgt ltidgtb55528a4-5d63-49f1-87a2-5fab8d76f6
58lt/idgt lt/pathgt ltcrequest service"myContacts
" document"content" method"insert"
genResponse"always" gt ltkey puid"3240"
instance"1" cluster"1" /gt lt/crequestgt lt/sHe
adergt ltsBodygt ltcinsertRequest
select"/mmyContacts/mcontactmpname/mpgivenNa
me Terry'/mpemailAddress" gt ltmpemailgtterr
y_at_microsoft.comlt/mpemailgt lt/cinsertRequestgt lt
/sBodygt lt/sEnvelopegt
12
Issues in Caching contd.
request 1 query request
ltqueryRequest select myContacts/contactnamet
erry /gt
request 2 delete request
ltdeleteRequest select myContacts/contactname
terry/phone_at_catcell /gt

• consistency
• later requests might invalidate responses cached
  earlier.
• read/write, write/write conflicts
• how to specify consistency requirements for
  generic web services?

13
More Issues

• user experience
• user unaware of web service cache
• operations reportedly successful could fail!
• hoarding
• keeping the cache hot
• user controlled hoard requests
• security
• enforce access control

14
Our Approach

• annotate WSDL description of web services to
  define cache properties
• published by service providers or third party
• no changes to server side code required ?
• transparent cache for web services
• acts as a web proxy on the client machine
• no modifications of the client program necessary
  ?
• custom cache managers for each web service
• generated automatically from the annotated WSDL
  description ?

15
Architecture
Proxy Server
Cache
WBQ
CCM1 Custom Cache Manager 1
WBQ Write Back Queue
16
WSDL Annotations for each Operation

• cacheable the operation can be cached
• lifetime the duration for which replies are
  cached
• play-back the operation has side effects and
  must be played back when connection is restored
• default-response a default response will be sent
  when connection is not available

17
WSDL Annotations for each Service

• identify the operation (operationName)
• xpath (xml query language) expression to extract
  the name of the operation
• extract the request message (identifier)
• portions of the request message should be ignored
  while caching (date)
• xpath expression to extract relevant parts of the
  message for identification

18
snippet from annotated myContacts.wsdl
ltbinding name"myContactsBinding"
type"tnsmyContactsPort" operationName
"substring-before(localname(/senvEnvelope/senvB
ody/1), 'Request')" Identifier
"/senvEnvelope/senvHeader/s0licenses
/senvEnvelope/senvHeader/s1request
/senvEnvelope/senvBody"gt ltsbinding
transport"http//schemas.xmls.org/s/http"
style"document" /gt ltoperation name"insert"
cacheable"false" playback"true"
defaultResponse"true" cacheHeader"true"gt
ltsoperation sAction"http//schemas.microsoft.com
/hs/2001/10/crequest" /gt
19
Annotations for Consistency

• when does request 2 invalidate the response of an
  earlier request 1 in the cache?
• an insert could invalidate an earlier query
  response
• consider requests to be functions with signatures
• req1 op1 (param1,1, param1,2, , param1,n)
• req2 op2 (param2,1, param2,2, , param2,m)
• invalidate condition is an expression of req1 and
  req2
• f(op1, op2, param1,1, , param2,1, )

20
Annotations for Consistency XSL Transformations

• extensible style sheet language (XSL)
• transforms XML documents in to html/text/xml
• Turing-complete language
• cache transform transforms a cached response
• input request1, reply1, request2, reply2
• output transformed reply1 (null if invalidated)
• powerful than just specifying invalidations
• can actually transform the old response

21
Cache Transform Example

• request 1 query request

ltqueryRequest select myContacts/contactnamet
erry /gt
request 2 delete request
ltdeleteRequest select myContacts/contactname
terry/phone_at_catcell /gt
smart cache transform would delete the cell
phone number from the cached query response
22

ltxsltemplate match"/"gt ltxslvariable
name"service1" select"req1/sHeader/crequest/_at_
service"/gt ltxslvariable name"service2"
select"req2/sHeader/crequest/_at_service"/gt
ltxslvariable name"opName1" select"substring-bef
ore(local-name(req1/sBody/1), 'Request')"/gt
ltxslvariable name"opName2"
select"substring-before(local-name(req2/sBody/
1), 'Request')"/gt ltxslchoosegt
ltxslwhen test"service1 service2"gt
ltxslchoosegt ltxslwhen
test"opName2 'query' and (opName1 'insert'
or opName1 'delete' or opName1
'replace')"gt ltxslvariable
name"cleanQuery1"gt ltxslcall-template
name"StripSegment"gt ltxslwith-param
name"xpQuery" select"substring-after(req1/sBod
y/c/_at_select, '/')"/gt lt/xslcall-templategt
lt/xslvariablegt
ltxslvariable name"cleanQuery2"gt ltxslcall
-template name"StripSegment"gt
ltxslwith-param name"xpQuery" select"substring
after(req2/sBody/cqueryRequest/cxpQuery/_at_selec
t, '/')"/gt lt/xslcall-templategt
lt/xslvariablegt
ltxslcall-template name"CheckIntersection"gt ltxsl
with-param name"xpQuery1" select"cleanQuery1"/
gt ltxslwith-param name"xpQuery2"
select"cleanQuery2"/gt
lt/xslcall-templategt lt/xslwhengt
ltxslotherwisegt
ltxslvalue-of select"rep2"/gt
lt/xslotherwisegt lt/xslchoosegt
lt/xslwhengt ltxslotherwisegt
ltxslvalue-of select"rep2"/gt
lt/xslotherwisegt lt/xslchoosegt lt/xsltemplategt

23
Picking Level of Consistency

• user-freedom in choosing consistency guarantees
• multiple consistency transforms
• strong consistency
• less availability ?
• better user experience ?
• weak consistency
• user experience could deteriorate ?
• operations reportedly successful could fail!
• optional cache header
• better availability ?

24
More Transforms

• response transform
• response from the cache may have to be changed
  before returning to the client.
• adding time-stamp, unique identifiers etc.
• default response transform
• generates a default response for a request.
• default responses are returned when disconnected
  but request is queued for play-back

25
Optional Cache Header

• cache provides information to the client using
  cache header
• response from cache or server
• age of cached response
• request will be played back in the future
• no changes to the definition of WSDL
• would not affect existing clients in any way.
• cache aware clients can provide additional
  information to the user

26
example default response and cache header
ltsEnvelope xmlnsshttp//schemas.xmlsoap.org/so
ap/envelope/ xmlnshs"http//schemas.microsoft.c
om/hs/2001/10/core"gt ltsHeadergt ltpath
xmlns"http//schemas.xmlsoap.org/rp/"gt ltaction
gthttp//schemas.microsoft.com/hs/2001/10/coreresp
onselt/actiongt lt/revgt ltfromgthttp//terry.micr
osoft.comlt/fromgt ltrelatesTo gt
d978b559-aceb-4e9e-9747-b8a306234bc8
ltrelatesTogt lt/pathgt lt response xmlns
"http//schemas.microsoft.com/hs/2001/10/core"
/gt ltcacheHeader defaultResponse"true"
toPlayback"true" xmlns"http//localhost/wsdlanno
tation" /gt lt/sHeadergt ltsBodygt lthsinsertResp
onse status"success" selectedNodeCount"1"
newChangeNumber"0" /gt lt/sBodygt lt/sEnvelopegt
27
Conclusion

• built a prototype web services cache
• experimented with Hailstorm web services and
  clients
• annotated Hailstorm WSDL files
• the prototype demonstrates custom cache managers
  in action for Hailstorm
• couldnt give a demo ?

28
Work for the Future

• WSDL annotations for more web services
• hard to find interesting web services with WSDL
  descriptions yet!
• hoarding to enhance availability
• specify user controlled hoard queries
• hoard transform to obtain response from cached
  hoard requests
• incorporate security constraints
• tune cache performance