Difference between 15 digit and 18 digit ID in Salesforce

• 15 digit case-sensitive version 
• 15 digit ID is referenced in the UI
• 18 digit case-insensitive 
• 18 digit id is referenced through in the API
• Why do we need 18 digit IDs, because many Legacy system [ex: excel sheets] work with Case insensitive IDs
• The id displayed by a report is the 15-digit case-sensitive id. The ids returned by web services api are 18-digit id. For updates web service accept both the 15-digit and 18-digit ids.

BreakDown of 15 Characters - 

As there is a combination of lower and upper case letters the casing of the 15 character Id has significance. E.g. 50130000000014c is a different ID from 50130000000014C.

Within a 15 character Id the breakdown is:

• First 3 characters - The first 3 characters are the key prefix that identify the object type. There are some exceptions to this where multiple objects all share the same key prefix! 
• There are a number of fixed key prefixes that are common across all of Salesforce .  Custom objects in managed packages can have a different keyprefix in each installed org.(Cross check once)
• The 4th and 5th characters - Reserved. Currently used for the instance id / POD Identifier. Indicates which pod/instance the record was created on.
• 6th character - Reserved. Will be 0 until such time that Salesforce has a need for it. See more
• Remaining 9 characters - basically a really big number. Like 62^9 big.

To this you can add an optional 3 character suffix that will make the Id unique case-insensitive.

 This is useful when working with programs that can't maintain the case of the ID (E.g. Excel VLookup).

See More 1 

SOAP API and REST API

* SOAP API and REST API are two commonly used API's to expose your data from force.com platform to the other platforms(JAVA , .NET ,etc) or to allow external application to invoke Apex methods.

What is SOAP ?

The Simple Object Access Protocol (SOAP) is an attempt to define a standard for creating web service APIs. It is a pattern, a web service architecture, which specifies the basic rules to be considered while designing web service platforms. It typically uses HTTP as a layer 7 protocol, although this is not mandatory. The SOAP message itself consists of an envelope, inside of which are the SOAP headers and body, the actual information we want to send. It is based on the standard XML format, designed especially to transport and store structured data. SOAP may also refer to the format of the XML that the envelope uses.

SOAP is a mature standard and is heavily used in many systems, but it does not use many of the functionality build in HTTP. While some consider it slow, it provides a heavy set of functionality which is a necessity in many cases. It might now be the best solution for browser-based clients, due to its custom format.

 What is REST ?

The Representational State Transfer (REST) is another architectural pattern (resource-oriented), an alternative to SOAP. Unlike SOAP, RESTful applications use the HTTP build-in headers (with a variety of media-types) to carry meta information and use the GET, POST, PUT and DELETE verbs to perform CRUD operations. REST is resource-oriented and uses clean URLs (or RESTful URLs).

For example :

http://www.developingthefuture.com/index.php?page=foo

becomes

http://www.developingthefuture.com/foo

This kind of URLs syntax greatly improves both visibility and usability. It doesn’t use any query strings, and it also provides certain SEO benefits (the crawlers just love plain text). The body of the REST message can be XML, JSON or any other format, although JSON is usually the preferred choice. On the other hand, you can’t use JSON with SOAP. Or at least not entirely, because SOAP uses XML by definition for it’s envelope. It should also mentioned that REST is, by design, protocol-agnostic, although it is usually used with HTTP. 

SOAP API

1)Supports data in the form of XML only 2)Requires WSDL for the integration 3)Use SOAP API in any language that supports Web services.

REST API

1)Supports both XML and JSON format 2)Preferred for mobile and web apps since JSON being Lighter the app runs smoother and faster

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(Courtesy Groundwire Consulting)