I ran into a great question from a developer on twitter the other day:

It’s a pretty common question among freelancers when they work or do a proposal for a real estate client. Unfortunately most clients don’t know the difference between IDX, MLS and RETS and use them interchangeably in conversation (if they’re even aware of the terms at all). Let me break see if I can break down the definitions a little as I understand them.

Multiple Listing Service (MLS)

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

The practice of cooperation amount real estate brokerages predates the internet and goes all the way back to the 1800's. The MLS was set up as a way for brokerages to compensate each other if they assisted in selling a property. From the US National Association of Realtors:

In the late 1800s, real estate brokers regularly gathered at the offices of their local associations to share information about properties they were trying to sell. They agreed to compensate other brokers who helped sell those properties, and the first MLS was born, based on a fundamental principle that’s unique to organized real estate: Help me sell my inventory and I’ll help you sell yours

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), the two most common being CoreLogic and FlexMLS.

Due to the differences in real estate boards and the fact there’s no single authoritative MLS, there’s 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 to the public.

Before IDX, the MLS was restricted to real estate agents and brokerages (members of the real estate board). 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 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 comma-separated or another formatted text file. 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 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 impossible to perform queries or only get updated listings every night. Furthermore, the data formats and locations of the files would vary from board to board, making it extremely difficult for software to work universally.

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

I hope that’s a good overview of the alphabet soup that is working with real estate data online. If you have any questions about MLS, IDX, or RETS please feel free to contact me ppohler@anecka.com.

Patrick is the owner of the development agency Anecka. He also writes articles and guides for freelancers to help improve their development productivity at PatPohler.com.