What is SOAP?
"Simple Object Access Protocol" - SOAP is a method and format for sending XML from one server to another via HTTP. SOAP allows you to have remote database servers which supply information to power your website. The best part about SOAP is that it is universal and secure. This means that big companies like SoftLayer can open their databases for you to gather the information that you need to keep your customers up to date. This kind of complete transparency is what makes SOAP (and any other supported API) an invaluable asset to SoftLayer customers. In order to fully understand SOAP, you will need to know about markup languages, namespaces and a little about the protocols used to implement it.
The term "markup language" is derived from when teachers/editors would make corrections to a written article, using shorthand for what needed to be corrected. A markup language is just a document with a few special symbols used to separate some text as "markup;" this text can then be used by other programs to perform tasks. Markup languages are declarative programming languages, so they contain data and talk about data, but they don't actually have instructions about what to do with the data. HTML, XML and LaTeX are all variations of markup languages.
Markup languages can be used for all kinds of different things, but the most common markup language used in web design is Hyper Text or HTML. HTML uses predetermined markup "tags" to describe data for browsers to display; it is the basic building block of all websites. SOAP uses XML to transfer data from server to server.
What is XML?
"Extensible markup language" - XML is a markup language dedicated to presenting data to different applications. Unlike HTML, XML has no dedicated markup tags; instead, you can create any type of tag you want. In XML, the tags describe and define the data that they contain. Applications can easily pull just the data that they need, no matter what is in the document. In this way an XML can be expanded without causing application errors, making it "extensible." Due to the free form of XML tags, sometimes you want multiple tags to have the same name; this is possible with XML namespaces.
How do XML Namespaces work?
Namespaces are used to group XML elements so that parsing programs won't confuse different elements that have the same name. Namespaces have to be defined by a "universal resource identifier" or URI; essentially, they have to be universally unique. This is why the WC3 chose to use website URLs as the standard naming convention. This can lead to some confusion when looking at XML namespaces, though. Information is not transferred to or from the websites; they don't even have to resolve, parsing programs read the URLs as basic text strings. SOAP uses a few pre-determined namespaces to define key information, but the most important thing about SOAP is its ability to work as an RPC-type protocol.
"Remote procedure calls" are connections created between two or more servers for the sole purpose of enacting programs or procedures on a remote server. RPCs start at the client, sending a request to the remote server with a procedure name and perameters for the procedure. The request has to be in a very strict format, that way the host server doesn't have to know anything about the language that the client is using. SOAP makes RPCs over HTTP because it is the most accessible protocol; very few firewalls block HTTP, and SOAP makes use of headers similar to HTTP headers to provide encoding and security information.
So, What is SOAP?
In short, SOAP is a very strictly formatted XML document which uses XML namespaces to define key elements of data, sent via HTTP, in order to enact procedures on a remote server, and sometimes receive data in response to those procedures.