Скачать презентацию Ace 104 Lecture 5 The Flavor of SOAP
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Ace 104 Lecture 5 The Flavor of SOAP: A study of the key concepts without all of the detail
Some facts about SOAP • SOAP no longer stands for Simple Object Access Protocol – This was dropped in 2003 with publication of v 1. 2 – Considered to be a misleading name • Originally was XML-RPC – Created in 1998 as a very lightweight protocol for rpc – Was basis for development of SOAP several years later – SOAP currently maintained by W 3 C XML working group • SOAP and XML-RPC are based entirely on XML
SOAP as a messaging protocol • SOAP is fundamentally a stateless, one-way message exchange paradigm • However, applications can create more complex interaction patterns (e. g. , request/response, request/multiple responses, etc. ) by combining such oneway exchanges with features provided by an underlying protocol and/or application-specific information. • SOAP is silent on the semantics of any application-specific data it conveys, as it is on issues such as the routing of SOAP messages, reliable data transfer, firewall traversal, etc. • However, SOAP provides the framework by which application-specific information may be conveyed in an extensible manner. Also, SOAP provides a full description of the required actions taken by a SOAP node on receiving a SOAP message.
SOAP • SOAP is made up of three major parts – A generic XML messaging framework – An data encoding standard – An RPC (remote procedure call) framework • It is possible to use just the messaging framework or messaging framework/encoding standard without using the RPC mechanism (though latter is where much of power/usefulness lies).
Web Services • Note that classic Web Services are made up of three parts – SOAP – WSDL (Web Services Descriptor Language) – UDDI (Universal Description, Discovery, and Integration) • All three are based on XML • SOAP simply defines the structure of the XML document used to transfer the message • WSDL and UDDI are covered next
SOAP: Messaging framework • Just defines a generic message XML schema • Virtually any type of message you can think of can be packaged as a SOAP message. • However, doing so without RPC mechanisms takes only very small advantage of the features defined in the SOAP standard
General (Basic) Structure SOAP Message SOAP Envelope • Envelope SOAP Header – Defines the content of the message • Header (optional) – Contains destination information, versioning, extensions – Good place for security • Body – Contains payload SOAP Body Payload Document(s) SOAP Fault
General (Basic) Structure SOAP Message
SOAP encoding • In order to build SOAP messages from our language of choice, we need to know how to serialize data -- ie the rules for representing an integer, string, or floating point number. The serialization of data inside a SOAP message is referred to as encoding • The encoding. Style attribute defined by the SOAP specification is used to identify the encoding rules used in a particular message. • SOAP does this in a language agnostic way, much like CORBA (but not in binary form) • If the encoding. Style attribute does not appear in the message, the receiver cannot make any assumptions about how data will be represented within the message
SOAP Encoding • You may be wondering about the relationship between encoding and XML Schemas – Encoding can make use of XML Schemas. – The SOAP Specification defines a single set of (recommended) encoding rules call SOAP encoding is based on XML Schemas and as such it closely models many of the standard types and constructs. The value is http: //schemas. xmlsoap. org/soap/encoding/, which points to the XML Schema that defines the encoding rules. – SOAP encoding rules use XML Schemas heavily, relying on the XML Schema datatypes namespace and the type attribute. – The key difference is that encoding does not mandate XML Schemas. – Encoding rules are simply identified by a URI. The rules implied by that URI could be backed up by nothing more than a verbal agreement, or possibly some written documentation. – This allows developers who do not necessarily need the capabilities of XML Schemas to forego their use and start sending messages with encoding rules based on an accepted URI.
SOAP Encoding • For example, SOAP stipulates that an array of three integers be represented as:
SOAP RPC • The third part of SOAP is an RPC mechanism that turns messages into method calls • We have a generic message structure + data. It requires just a little more work to turn the message into a function call. • Must be a standard way to represent parameters and return values, exceptions, etc. • Note that the encoding and rpc mechanisms are only important if SOAP is being automatically generated/read from the application level (see next slide) in a general way
SOAP RPC cartoon VB application Java application Invoice VB-Structure Invoice Java-Structure SOAP client SOAP Server SOAP Message The client application thinks its making a procedure call to a remote module
Sample RPC rules • This is intended just to give you a flavor. Best to allow applications to do this automatically: Consider the following three method signatures // Reverse the string, s, and return the new string.
string Reverse. String ( [in] string s ); // Reverse the string, s, and return the new string.
void Reverse. String ( [in] string s, [out] string s. Rev ); // Reverse the string, s, passed in by reference.
void Reverse. String ( [in, out] string s ); See next slide for SOAP rpc encoding
Sample rpc rules The first version reverses the string and returns the result as the return value of the method
SOAP transport • Recall that SOAP is just a generic message envelope • Augmented by encodingstyle and simple rpc rules, it becomes a powerful middleware layer for remote procedure calls, if one chooses to use it that way • Now we must consider how to transport SOAP messages -- this is the final ingredient in making it something useful
SOAP protocol bindings • Question: how are SOAP messages transmitted? • Answer: using existing protocols (http, SMTP, etc. ) • This has some obvious advantages vs. defining its own protocol – Piggybacks on security model, general robustness • This has some disadvantages also – What are these? • SOAP defines bindings to different protocols that specify how SOAP is used with that protocol to send messages. – http is most popular
Inside http • http is a simple, flexible protocol • Some examples GET http: //people. cs. uchicago. edu/~asiegel/lottery/lotto. html POST /path/script. cgi HTTP/1. 0 From: frog@jmarshall. com User-Agent: HTTPTool/1. 0 Content-Type: application/x-www-form-urlencoded Content-Length: 32 home=Cosby&favorite+flavor=flies POST /path/script. cgi HTTP/1. 0 From: frog@jmarshall. com User-Agent: HTTPTool/1. 0 Content-Type: text/xml Content-Length: 32
Testing http • Good idea to play around with http by connecting to server and issuing http commands • There a two typical ways to do this: – Using telnet, which allows arbitrary commands to be passed to a server • telnet people. cs. uchicago. edu 80 • Note that expect can be useful in automating this – Using a socket library in a programming language (see sock. py on website) • Question: how does the server obtain the uploaded data in each case?
Role of SOAP • Note that the http + XML is the important thing here • SOAP only helps standardize the meaning of the messages that are sent – In terms of datatypes for rpc – In terms of headers, faults, etc. • Note that it is still possible to bypass SOAP and define your own xmlbased protocol, retaining many of the advantages of SOAP.
Sorting out the API’s • In Java the following directly related API’s are available: – SAAJ • SOAP with Attachments API for Java • Provides a relatively low-level interface that allows one to programmatically construct/decompose SOAP messages and send to web server • Intended more tool writers. Good for learning. – JAX-RPC • Java API form XML-based RPC • Java’s rmi framework over SOAP – Compare RMI, CORBA, etc. • Makes developer unaware of SOAP internals – Apache XML-RPC for Java • A framework for remote procedure calls using XML-RPC • Recall that XML-RPC is an alternative protocol to SOAP
Looking deeper into SOAP
Envelope • MUST be the root element of the SOAP message • MUST be associated with SOAP envelope namespace – http: //schemas. xmlsoap. org/soap/envelope – http: //www. w 3. org/2001/06/soap-envelope in SOAP 1. 2 (Oct 15, 2002) • SOAP serialization namespace – Encoding Style attributes can contain a URI describing how the data should be serialized. – Two usual styles (more on this later) • "SOAP Section 5" encoding: http: //www. w 3. org/2001/06/soap-encoding • Literal encoding: (no namespace used – or set to empty string) • SOAP message MUST NOT contain – DTD – Processing Instructions.
Envelope versioning • Version determined by the namespace associated with the Envelope element – SOAP 1. 1 Envelope version: http: //schemas. xmlsoap. org/soap/envelope – If any other namespace used, assume it's a version problem – Versioning problems must generate a SOAP Fault • Example SOAP fault: HTTP/1. 0 500 Internal Server Error Content-Type: text/html; charset="utf-8" Content-length: 311
Envelope Versioning Fault in SOAP 1. 2 • SOAP 1. 2 (Oct 15, 2002) has defined an Upgrade element in the header for the Note that 1. 2 Envelope Version Fault versioning fault: Response is versioned 1. 1 (or whatever incoming request is)
Header • • • Optional If present, must immediately follow the SOAP Envelope XML element followed by any header entries Uses same namespace as Envelope Often contains meta-information regarding the method call. Examples: – – Security • No security mechanisms yet, but soon Transaction IDs
Header • • • actor attribute defines the URI for which the header elements are intended (i. e. who should process a Header element) must. Understand attribute how to process (default is “ 0” if not present) encoding. Style attribute used to describe how data (such as binary integers) are marshaled into characters in the XML document
actor Attribute • The SOAP message often gets passed through several intermediaries before being processed – For example, a SOAP proxy service might get the message before the target SOAP service • Header may contain information for both – intermediary service – target service • actor attribute specifies which service should process a specific Header element • actor attribute is replaced by role attribute in SOAP 1. 2
Intermediary Services • SOAP requires that an intermediary strip off Header elements specified for that intermediary before passing the message to the next service in the chain. • If information in a Header element targeted for an intermediary is also needed by another service in the chain – The intermediary service may insert additional Header elements with an actor attribute that specifies the downstream service • In fact, any service may insert any Header elements that it deems necessary • If a Header element has no actor attribute – It is assumed to be destined for the final recipient – This is equivalent to adding an actor attribute with the URL of the final recipient
must. Understand Attribute • Also put on a Header element • If its value is "1" – recipient is required to understand make proper use of the information supplied by that element – intended for situations where recipient can't do its job unless it knows what to do with the specific information supplied by this particular element • Examples of use – Client is upgraded to a new version which includes extra information – username – security
must. Understand Attribute • If the recipient does not understand this element – Must respond with a SOAP Fault HTTP/1. 0 500 Internal Server Error Content-Type: text/xml; charst="utf-8" Content-length: 287
Some examples Taken from W 3 C primer
Sample SOAP message for travel reservation
Sample SOAP reply
Reply continuing conversational exchange
RPC Notice that previous did not include rpc capability To invoke a SOAP RPC, the following information is needed: 1. The address of the target SOAP node. 2. The procedure or method name. 3. The identities and values of any arguments to be passed to the procedure or 4. method together with any output parameters and return value. 5. 4. A clear separation of the arguments used to identify the Web resource which 6. is the actual target for the RPC, as contrasted with those that convey data or 7. control information used for processing the call by the target resource. 5. The message exchange pattern which will be employed to convey the RPC, 6. together with an identification of the so-called "Web Method” (on which more later 7. to be used. 6. Optionally, data which may be carried as a part of SOAP header blocks.
SOAP RPC request with a mandatory header and two input (or "in") parameters
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RPC response with two output (or "out") parameters ? xml version='1. 0' ? >
RPC response with a "return" value and two "out" parameters
Extra Slides In progress, read to get a sense of underlying details
Marshalling / Serialization VB application Data here is binary Must Marshall or Serialize Java application Invoice VB-Structure Data here is ASCII SOAP client Data here Invoice Java-Structure is binary SOAP Server Must Un. Marshall or De. Serialize SOAP Message • • To be interoperable, we use XML is ASCII, not binary End points use binary Must Marshall (Serialize) and Un. Marshall (De. Serialize) on the ends
Body • Message to exchange. – • Immediate child element of SOAP Envelope XML element – • • • follows Header, if present Uses same namespace as Envelope and Header Contains serialized method arguments. Remote method name – – • • Most often for RPC calls and error reporting. Used to name the method call’s XML element Must immediately follow the SOAP body opening XML tag. SOAP Fault goes in the Body (of a response) too The only Body elements actually defined in the SOAP specification are the SOAP Fault elements – Other elements are user defined
Example • A simple SOAP XML document requesting the price of soap (leaving off the required namespaces declarations)
Client/Server… • In order for SOAP to work – – Client must have code running that is responsible for building the SOAP request. Server must also be responsible for • Understanding the SOAP request • Invoking the specified method • Building the response message • Returning it to the client. • These details are up to you. • There already exist SOAP implementations for languages such as C++, Perl, VB, and Java.
Binding • SOAP is transport independent – SOAP usually transported over HTTP – SOAP can be transported over any protocol • e. g. SMTP (e-mail) • GSI (Globus Secure Transport) • HTTPS • pure sockets • HTTP is the default binding
SOAPAction HTTP header • When using SOAP over HTTP, must include SOAPAction header • SOAPAction HTTP request header field indicates that it is a SOAP HTTP request (contains a SOAP message) • The value • An HTTP client must use SOAPAction header field when issuing a SOAP HTTP Request. • An HTTP server must not process an HTTP request as a SOAP HTTP request if it does not contain a SOAPAction header field. • It may be used by firewalls to filter request messages • It may be used by servers to facilitate dispatching of SOAP messages to internal message handlers • It should not be used as an insecure form of access authorization. – – Indicates the intent of the request in a manner readily accessible to the HTTP server. Is a URI Is up to the application – not specified by SOAP specs Doesn't have to be resolvable
SOAPAction HTTP header • Example POST /xt/services/Color. Request HTTP/1. 0 Content Length: 442 Host: localhost Content-type: text/xml; charset=utf-8 SOAPAction: "/get. Color"
SOAP Messages with Attachments • SOAP messages often have attachments, such as pictures • The attachments don't have to be XML encoded, but may be binary • The SOAP message becomes the root of a Multipart/Related MIME structure • The SOAP message refers to the attachment using a URI with the cid: protocol – cid = "content ID"
SOAP Messages with Attachments MIME-version: 1. 0 Content-Type: Multipart/Related; . . . --MIME_boundary Content-Type: text/xml; . . .
Encoding • One type of encoding specified in "section 5" of the SOAP spec • No default encoding (not even "section 5" encoding) • Encoding rules exist to define mapping between abstract data types and XML syntax (binary to character mapping) • Encoding style is specified with the encoding. Style attribute
Encoding • The encoding. Style attribute can be placed on any element – allowing mixed encoding styles • Two values most often used (anything possible): – "SOAP Section 5" encoding: http: //www. w 3. org/2001/06/soap-encoding – Literal encoding: (no namespace used – or set to empty string) • Also can do base 64 encoding • Can turn it off currently scoped style using an empty string as URL ("") – parent scoped style becomes default again
Response • No Special HTTP Response headers (doesn't use SOAPAction: header) • Only special SOAP element is the Fault element and its children • Otherwise, looks like a normal SOAP message
Fault • The
Fault Code (faultcode) • One of Two required elements in Fault element – Other required element is faultstring • Must be associated with SOAP envelope namespace • Server error code could be something like a back-end database couldn't be reached – Might try resending without modification • Fault codes are extensible using "dot" notation – Server. Bridge. Keeper. Absent. Thrown. In. Gorge
HTTP Headers with Faults • Response code can only be 2 xx or 500 • If message is received and understood, the response should use 2 xx
HTTP Headers with Faults • If message cannot be processed for any reason – – – server does not understand the message is improperly formatted message is missing information message cannot be processed for any other reason Response should be "500 Internal Server Error" • 500 response should be followed by a SOAP envelope which includes its own fault code • Reasoning: the error is internal to the server as far as HTTP is concerned
Example Fault with HTTP and SOAP HTTP/1. 0 500 Internal Server Error Content-Type: text/xml; charst="utf-8" Content-length: 287
Data Encoding • When sending data over a network – Data must comply with the underlying transmission protocol – Data must be formatted in such a way that both the sending and receiving entities understand its meaning • Even if endpoints are different platforms or languages • Model for SOAP encoding is based on XML data encoding • Encoding style given in Section 5 of the SOAP specification used to be most common encoding style used – Commonly called "SOAP-Section-5 encoding" – namespace: http: //schemas. xmlsoap. org/encoding/ – Commonly aliased as SOAP-ENC:
Data Encoding and Schemas • In SOAP, Schemas are used as references to definitions of data elements – Aren't used to validate SOAP message data in standard SOAP processing • However, there's nothing stopping you from doing that – References to Schemas are often used as namespaces in order to disambiguate a serialized data element
Data Encoding and Schemas • SOAP Section 5 uses all of the build-in data types defined in the "XML Schema Part 2 Datatypes" specification (at w 3 c. org) – These data types need to be disambiguated • namespace: http: //www. w 3. org/2001/XMLSchema • Commonly aliased as xsd: • used with the data type names • e. g. xsd: string – A datum is given a data type using the type attribute • This attribute must also be disambiguated • namespace: http: //www. w 3. org/2001/XMLSchema-instance • Commonly aliased as xsi: • e. g.
Other Common Namespaces • Envelope – namespace: http: //schemas. xmlsoap. org/soap/envelope/ – Common aliases: env or SOAP-ENV or SOAP or soap • Example (we've seen this before)
Data Types • The data type for a given value is never undefined in SOAP • SOAP distinguished between simple types and complex types – A simple type does not contain any named parts, it just contains a single piece of data • Example: string • Example: int – A complex type contains multiple pieces of data that have some relation to each other • Similar to structs or classes or arrays • Individual pieces of data may be accessed by using – an ordinal position in a sequence of values (like arrays) – values that are keys to an associative array (like hash tables) – the names of the constituent parts (like C structs) • There is always a way to distinguish a specific data value within a complex value – referred to as the "accessor" • A names subcomponent of a complex type may be a complex type itself
Built-in Data Types
References • We briefly saw how to declare these in DTDs • Let's see how to use these
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References • May not save space if the data items are small • Should save space if the data items are larger • Are necessary to link objects. – Graphs – Organizational structure (like round table in previous slide) – Students' class enrollment
Indicating Type • Every element that contains a value must also indicate the type of the data • In SOAP, these types may be specified – By using the xsi: type attribute • with a valid type identifier, such as xsd: string or xsd: int – By being an element of an array that already constrains the type of its contents – By being related to some type that is defined in an XML Schema • So far, we've only seen examples of the first method • SOAP (by itself – without WSDL), usually uses the first method (and the second) • There are two different string type declarations: – xsd: string from XML Schema – SOAP-ENC: string from SOAP Section 5 • This one allows for id and href attributes
Indicating Type • Example of dynamically declaring the type inside the XML document itself