SOAP (see below for name and origins) is a protocol for exchanging XML-based messages over computer networks, normally using HTTP/HTTPS. SOAP forms the foundation layer of the web services protocol stack providing a basic messaging framework upon which abstract layers can be built.

History

SOAP once stood for 'Simple Object Access Protocol' but this acronym was dropped with Version 1.2 of the standard, as it was considered to be misleading. Version 1.2 became a W3C Recommendation on June 24, 2003. The acronym is sometimes confused with SOA, or Service-oriented architecture; however SOAP is quite different from SOA.

SOAP was originally designed by Dave Winer, Don Box, Bob Atkinson, and Mohsen Al-Ghosein in 1998, with backing from Microsoft (where Atkinson and Al-Ghosein worked at the time), as an object-access protocol. The SOAP specification is currently maintained by the XML Protocol Working Group of the World Wide Web Consortium.

Transport methods

SOAP makes use of an Internet application layer protocol as a transport protocol. Critics have argued that this is an abuse of such protocols, as it is not their intended purpose and therefore not a role they fulfill well. Backers of SOAP have drawn analogies to successful uses of protocols at various levels for tunneling other protocols.^{[citation needed]}

Both SMTP and HTTP are valid application layer protocols used as Transport for SOAP, but HTTP has gained wider acceptance as it works well with today's Internet infrastructure; specifically, HTTP works well with network firewalls. SOAP may also be used over HTTPS(which is the same protocol as HTTP at the application level, but usesan encrypted transport protocol underneath) in either simple or mutual authentication; this is the advocated WS-I method to provide web service security as stated in the WS-I Basic Profile 1.1. This is a major advantage over other distributed protocols like GIOP/IIOP or DCOM which are normally filtered by firewalls. XML was chosen as the standard message format because of its widespread use by major corporations and open source development efforts. Additionally, a wide variety of freely available tools significantly eases the transition to a SOAP-based implementation..

The somewhat lengthy syntax of XMLcan be both a benefit and a drawback. While it promotes readability forhumans, it can retard processing speed and be cumbersome. For example, CORBA, GIOP, ICE, and DCOM use much shorter, binary message formats. On the other hand, hardware appliances are available to accelerate processing of XML messages. [1][2]. Binary XML is also being explored as a means for streamlining the throughput requirements of XML.

Technical critique

Numerous commentators and specialists have discussed the technical advantages and disadvantages of SOAP relative to alternative technologies, and relative to the context of its intended use.

Advantages

• Using SOAP over HTTP allows for easier communication through proxies and firewalls than previous remote execution technology.
• SOAP is versatile enough to allow for the use of different transport protocols. The standard stacks use HTTP as a transport protocol, but other protocols are also usable (e.g. SMTP, RSS).
• SOAP is platform independent.
• SOAP is language independent.
• SOAP is simple and extensible.

Disadvantages

• Because of the verbose XML format, SOAP can be considerably slower than competing middleware technologies such as CORBA. This may not be an issue when only small messages are sent[3]. To improve performance for the special case of XML with embedded binary objects, Message Transmission Optimization Mechanismwas introduced. Further, to improve the performance of XML in general,there are emerging non-extractive XML processing models, e.g., VTD-XML.
• When relying on HTTP as a transport protocol and not using WS-Addressing or an ESB,the roles of the interacting parties are fixed. Only one party (the client) can use the services of the other. Developers must use polling instead of notification in these common cases.
• Most uses of HTTP as a transport protocol are done in ignorance of how the operation would be modelled in HTTP. This is by design (with analogy to how different protocols sit on top of each other in the IPstack) but the analogy is imperfect (because the application protocols used as transport protocols are not really transport protocols).Because of this, there is no way to know if the method used is appropriate to the operation. This makes good analysis of the operation at the application-protocol level problematic at best with results that are sub-optimal (if the POST-based binding is used for an application which in HTTP would be more naturally modelled as a GET operation).

When to switch?

There are three scenarios where a context switch needs to occur.

Multitasking

Most commonly, within some scheduling scheme, one process needs to be switched out of the CPU so another process can run. Within a preemptive multitasking operating system, the scheduler allows every task to run for some certain amount of time, called its time slice.

If a process does not voluntarily yield the CPU (for example, by performing an I/O operation), a timer interrupt fires, and the operating system schedules another process for execution instead. This ensures that the CPU cannot be monopolized by any one processor-intensive application.

Interrupt handling

Some architectures (like the Intel x86 architecture) are interrupt driven. This means that if the CPU requests data from a disk, for example, it does not need to busy-wait until the read is over, it can issue the request and continue with some other execution; when the read is over, the CPU can be interrupted and presented with the read. For interrupts, a program called an interrupt handler is installed, and it is the interrupt handler that handles the interrupt from the disk.

Clearly, there is a necessity to switch contexts when issuing interrupts - the state of the currently running process must be suspended so that the interrupt handler can be executed.

User and kernel mode switching

When a transition between user mode and kernel mode is required in an operating system, a context switch is not necessary; a mode transition is not by itself a context switch. However, depending on the operating system, a context switch may also take place at this time.