Multiple Listing Service (MLS)

Multiple Listing Service is a set of services that allow brokerages operating in a local real estate board to fairly share information about properties and help order and disseminate information about local properties to clients.

In the United States, there’s no single or nationwide MLS, each MLS is controlled by a local association of brokerages & realtors. Database and software for accessing the MLS data are usually provided by private software companies (MLS vendors).

Due to the differences in real estate boards and the fact that there’s no single authoritative MLS, there is no common data format between different boards. There are certain data standards that are emerging, which brings us to IDX & RETS.

Internet Data Exchange (IDX)

An Internet Data Exchange is simply a site that allows public consumers to search for MLS properties in an area. Also going by Broker Reciprocity, IDX encompasses the policies, rules, and software governing how listings are shared online. Before IDX, the MLS was restricted to real estate agents and brokerages. As the popularity of the Internet grew, there was a huge demand for the regular consumer to do their own MLS searches without agent assistance.

The actual implementation of IDX varies from board to board. Most IDX options are:

• iFrame / widget: The search tools and property listings run externally on the IDX software vendor’s site and are shared on brokerage and agent sites.
• CSV / Flat File data feed: The MLS listings are accessible on an FTP server as a formatted text file. The software must be written to connect and downlod the listings nightly.
• RETS: A special framework for allowing an efficient download of MLS data (see below).

Real Estate Transaction Standard (RETS)

RETS was introduced as the the Real Estate Standards Organization to help standardize the way software interacts with MLS data. Prior to RETS, listing data was stored in flat files on FTP servers and it was difficult to perform queries or get updated listings nightly. Furthermore, the data formats and locations of the files would vary from board to board, making it extremely difficult for a single software to work universally.

RETS is XML based and there’s a wide variety of tools and libraries for working with RETS data.