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XML-RPC

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XML-RPC Remote Procedure Call (RPC) is a model that specifies how cooperating processes on different nodes in a heterogeneous computing environment can communicate ... – PowerPoint PPT presentation

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Title: XML-RPC


1
XML-RPC
Remote Procedure Call (RPC) is a model that
specifies how cooperating processes on different
nodes in a heterogeneous computing environment
can communicate and coordinate activities. The
paradigm of RPC is based on the concept of a
procedure call in a higher level programming
language. The semantics of RPC are almost
identical to the semantics of the traditional
procedure call. The major difference is that
while a normal procedure call takes place between
procedures of a single process in the same memory
space on a single system, RPC takes place between
processes on clients and servers in a
heterogeneous computing environment.
2
Ordinary procedure call
  • Value parameters
  • the actual value of the parameter is placed on
    the stack. This can then be used and modified by
    the procedure without any change to any original
    variable.
  • Reference parameters
  • The address of the parameter is passed into the
    procedure. Any use of the parameter within the
    procedure uses the address to access/change the
    value.

3
Remote procedure call
  • The remote procedure call act like a procedure
    call, but act across the network.
  • The process makes a remote procedure call by
    pushing its parameters and a return address onto
    the stack, and jumping to the start of the
    procedure. The procedure itself is responsible
    for accessing and using the network.
  • After the remote execution is over, the procedure
    jumps back to the return address. The calling
    process then continues.

4
Without RPC
Consider how you would implement a procedure to
find the time on a remote machine as a string,
using the IP socket calls int remote_time(char
machine, char time_buf) struct sockaddr_in
serv_addr int sockfd int nread if (sockfd
socket(AF_INET, SOCK_STREAM, 0)) lt 0) return 1
serv_addr.sin_family AF_INET
serv_addr.sin_addr.s_addr inet_addr(machine)
serv_addr.sin_port htons(13) if
(connect(sockfd, serv_addr, sizeof(serv_addr)) lt
0) return 2 nread read(sockfd, time_buf,
sizeof(time_buf)) time_bufnread '\0'
close(sockfd) return 0
5
model
6
  • The client calls the local stub procedure. The
    stub packages up the parameters into a network
    message. This is called marshalling.
  • Networking functions in the O/S kernel are called
    by the stub to send the message.
  • The kernel sends the message(s) to the remote
    system. This may be connection-oriented or
    connectionless.
  • A server stub unmarshals the arguments from the
    network message.
  • The server stub executes a local procedure call.
  • The procedure completes, returning execution to
    the server stub.
  • The server stub marshals the return values into a
    network message.
  • The return messages are sent back.
  • The client stub reads the messages using the
    network functions.
  • The message is unmarshalled. and the return
    values are set on the stack for the local
    process.

7
  • When the calling process calls a procedure, the
    action performed by that procedure will not be
    the actual code as written, but code that begins
    network communication. It has to connect to the
    remote machine, send all the parameters down to
    it, wait for replies, do the right thing to the
    stack and return. This is the client side stub.
  • The server side stub has to wait for messages
    asking for a procedure to run. It has to read the
    parameters, and present them in a suitable form
    to execute the procedure locally. After
    execution,it has to send the results back to the
    calling process.

8
Basic process for building server
  • Server program defines the servers interface
    using an interface definition language(IDL)
  • The IDL specifies the names, params, and type
    for all server procedures
  • A stub compiler reads the IDL and produces two
    stub procedures for each server procedure a
    client-side stub and a server-side stub
  • The server writer writes the server and links it
    with the server-side stubs the client writes her
    program and links it with the client-side stubs.

9
RPC Binding
  • The server, when starts up, exports its
    interface, identifying itself to a network name
    server and telling the local runtime its
    dispatcher address
  • The client, before issuing any calls, imports the
    server, which causes the RPC runtime to lookup
    the server through the name service and contact
    the requested server to setup a connection

10
RPC Stubs
  • A client-side stub looks to the client as if it
    were a callable server procedure
  • A server-side stub looks to the server as if it
    were a calling client
  • The client program thinks it is calling the
    server, in fact it calling the client stub
  • The server program thinks it is called by client,
    in fact, it is called by the server stub
  • The stubs send messages to each other to make the
    RPC happen

11
RPC Marshalling
  • Marshalling is the packing of procedure
    parameters into a message packet.
  • The RPC stubs call procedures to marshal(or
    unmarshal)all parameters.
  • On the client side, the client stub marshals the
    parameters into the call packet on the server
    side the server stub unmarshals the parameters in
    order to call the servers procedure
  • On the return, the server stub marshals return
    parameters into the return packet the client
    stub unmarshals return parameters and returns to
    the client.

12
XML-RPC
  • XML-RPC is remote procedure call using HTTP as
    the transport and XML as the marshalling format.
    XML is as important as c or java.
  • With XML, it is easy to marshal
  • With XML, it is to see what its doing
  • XML is broad support

13
XML-RPC
  • An XML-RPC message is an HTTP-POST request.
  • The body of the request is in XML, and the value
    returned is also XML.
  • Procedure parameters can be scalars, numbers,
    strings, dates and structures.

14
Request example
  • POST /RPC2 HTTP/1.0
  • User-Agent Frontier/5.1.2 (WinNT)
  • Host betty.userland.com
  • Content-Type text/xml
  • Content-length 181
  • lt?xml version"1.0"?gt
  • ltmethodCallgt ltmethodNamegtexamples.getStateNamelt/
    methodNamegt ltparamsgt
  • ltparamgt ltvaluegtlti4gt41lt/i4gtlt/valuegt lt/paramgt
    lt/paramsgt
  • lt/methodCallgt

15
header requirements
  • The format of the URI is not specified. For
    example, it could be empty, a single slash, if
    the server is only handling XML-RPC calls.
    However, if the server is handling a mix of
    incoming HTTP requests, we allow the URI to help
    route the request to the code that handles
    XML-RPC requests.
  • (the URI is /RPC2, telling the server to route
    the request to the "RPC2" responder.)
  • A User-Agent and Host must be specified.
  • The Content-Type is text/xml.
  • The Content-Length must be specified and must be
    correct.

16
Payload format
  • The payload is in XML, a single ltmethodCallgt
    structure.
  • The ltmethodCallgt must contain a ltmethodNamegt
    sub-item, a string, containing the name of the
    method to be called. It's entirely up to the
    server to decide how to interpret the characters
    in a methodName.
  • The methodName could be
  • the name of a file containing a script.
  • the name of a cell in a database table.
  • A path to a file
  • If the procedure call has parameters, the
    ltmethodCallgt must contain a ltparamsgt sub-item.
    The ltparamsgt sub-item can contain any number of
    ltparamgts, each of which has a ltvaluegt.

17
  • Scalar ltvaluegts
  • ltvaluegts can be scalars
  • lti4gt or ltintgt
  • ltbooleangt
  • ltstringgt
  • ltdoublegt
  • ltdateTime.iso8601gt
  • ltbase64gt

18
ltstructgts A value can also be of type ltstructgt.
A ltstructgt contains ltmembergts and each ltmembergt
contains a ltnamegt and a ltvaluegt. Here's an
example of a two-element ltstructgt ltstructgt
ltmembergt ltnamegtlowerBoundlt/namegt
ltvaluegtlti4gt18lt/i4gtlt/valuegt lt/membergt
ltmembergt ltnamegtupperBoundlt/namegt
ltvaluegtlti4gt139lt/i4gtlt/valuegt lt/membergt
lt/structgt ltstructgts can be recursive, any ltvaluegt
may contain a ltstructgt or any other type,
including an ltarraygt.
19
  • ltarraygts
  • A value can also be of type ltarraygt.
  • An ltarraygt contains a single ltdatagt element,
    which can contain any number of ltvaluegts.
  • Here's an example of a four-element array
  • ltarraygt
  • ltdatagt
  • ltvaluegtlti4gt12lt/i4gtlt/valuegt ltvaluegtltstringgtEgyp
    tlt/stringgtlt/valuegt ltvaluegtltbooleangt0lt/booleangtlt/v
    aluegt
  • ltvaluegtlti4gt-31lt/i4gtlt/valuegt
  • lt/datagt
  • lt/arraygt
  • ltarraygt elements do not have names.
  • You can mix types as the example above
    illustrates.
  • ltarraysgts can be recursive, any value may contain
    an ltarraygt or any other type, including a
    ltstructgt

20
  • Response example
  • HTTP/1.1 200 OK
  • Connection close
  • Content-Length 158
  • Content-Type text/xml
  • Date Fri, 17 Jul 1998 195508 GMT
  • Server UserLand Frontier/5.1.2-WinNT
  • lt?xml version"1.0"?gt
  • ltmethodResponsegt
  • ltparamsgt
  • ltparamgt
  • ltvaluegtltstringgtSouth Dakotalt/stringgtlt/valuegt
  • lt/paramgt
  • lt/paramsgt
  • lt/methodResponsegt

21
Response format
  • Unless there's a lower-level error, always return
    200 OK.
  • The Content-Type is text/xml. Content-Length must
    be present and correct.
  • The body of the response is a single XML
    structure, a ltmethodResponsegt, which can contain
    a single ltparamsgt which contains a single ltparamgt
    which contains a single ltvaluegt.
  • The ltmethodResponsegt could also contain a ltfaultgt
    which contains a ltvaluegt which is a ltstructgt
    containing two elements, one named ltfaultCodegt,
    an ltintgt and one named ltfaultStringgt, a ltstringgt.
  • A ltmethodResponsegt can not contain both a ltfaultgt
    and a ltparamsgt.

22
  • Fault example
  • HTTP/1.1 200 OK
  • Connection close
  • Content-Length 426
  • Content-Type text/xml
  • Date Fri, 17 Jul 1998 195502 GMT
  • Server UserLand Frontier/5.1.2-WinNT
  • lt?xml version"1.0"?gt
  • ltmethodResponsegt
  • ltfaultgt
  • ltvaluegt
  • ltstructgt
  • ltmembergt
  • ltnamegtfaultCodelt/namegt
  • ltvaluegtltintgt4lt/intgtlt/valuegt
  • lt/membergt
  • ltmembergt
  • ltnamegtfaultStringlt/namegt

23
  • Firewalls. The goal of this protocol is to lay a
    compatible foundation across different
    environments, no new power is provided beyond the
    capabilities of the CGI interface. Firewall
    software can watch for POSTs whose Content-Type
    is text/xml.
  • Discoverability. We wanted a clean, extensible
    format that's very simple. It should be possible
    for an HTML coder to be able to look at a file
    containing an XML-RPC procedure call, understand
    what it's doing.
  • Easy to implement. We also wanted it to be an
    easy to implement protocol that could quickly be
    adapted to run in other environments or on other
    operating systems.

24
  • Disadvantages of xml-rpc
  • XML-RPC is impoverished in the type of data it
    can transmit and obese in its message size
  • XML-RPC transmits messages lacking statefulness
    and incurs channel bottlenecks.
  • Compared to SOAP, XML-RPC lacks both important
    security mechanisms and a robust object model
  • As a data representation, XML-RPC is slow,
    cumbersome, and incomplete compared to native
    programming language mechanisms like Java's

25
  • A Perl Client example
  • use FrontierClient
  • Make an object to represent the XML-RPC server.
  • server_url 'http//xmlrpc-c.sourceforge.net/api
    /sample.php'
  • server FrontierClient-gtnew(url gt
    server_url)
  • Call the remote server and get our result.
  • result server-gtcall('sample.sumAndDifference'
    , 5, 3)
  • sum result-gt'sum'
  • difference result-gt'difference'
  • print "Sum sum, Difference difference\n"

26
  • Reference
  • Demo
  • http//www.scottandrew.com/xml-rpc/blogger/
  • http/www.xmlrpc.com
  • http//www.cs.nyu.edu/rgrimm/teaching/sp03-web/032
    503.pdf
  • http//hissa.nist.gov/rbac/titlehce/node19.html
  • http//nscp.upenn.edu/aix4.3html/aixprggd/progcomc
    /ch8_rpc.htm
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