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The RESO Web API Specification is available and the process for RESO Web API Certification is underway. All National Association of REALTORS® affiliated MLSs are required to adopt the RESO Web API no later than June 30, 2016 and compliance with this policy may be demonstrated through RESO Web API Certification.

Along with the release of our Web API Certification comes a new streamlined process. The good news is that the path for an MLS technology partner or real estate software firm to become Web API Certified once RESO Data Dictionary Certified is significantly truncated. They’ve already done the heavy lifting, so we’ve created an express lane of sorts to make Web API Certification faster, easier and cheaper!

The Certification will be powered in part through the already available OData Service Validator and the Open ID Connect Certification tool sets. RESO will utilize these open source validation and certification tools within the RESO Web API Certification Platform.

Understanding what an API is and does is important to appreciate the impact the new RESO Web API will have on the industry.

The acronym “API” is bantered about more and more these days in real estate technology circles and that’s why it’s important to make sure that everyone – not just the geeks in the real estate industry – understand what APIs are and the value they can deliver all of us.

What is an API?

The acronym stands for “application-programming interface.” From a technical standpoint, it is a range of programming standards and instructions released publically that allow software developers to access a web-based software application or tool. From a practical standpoint at RESO, an API makes everyone’s life simpler as it reduces the amount of time it takes for software developers to connect to, retrieve and organize listing data within a website, web application or tool.

Because RESO supports the widely adopted, RESTful (Representational State Transfer) this helps expand development for web, mobile, social and other HTTP-based applications.


There are three big reasons that we believe software firms that work with listing data and MLS technology partners will be quick to implement the RESO Web API:

  • Speedier development, more robust features: Web API offers a quicker path for software development. RETS, while fully functional, can be heavy to develop against and is proprietary, where the Web API leverages Global Technology Standards.
  • Faster path to listing data deployment: For a software developer, having unified data standards for every product – whether it is a website, web app or tool – reduces their development time, speeding up their time to market. The new Web API accomplishes this.
  • Simplification: A Web API means eliminating the need to have multiple software solutions for multiple markets. For firms working with MLSs in multiple marketing, the new RESO Web API simplifies the development process.

Web API Certification will also be covered as part of the RESO Certification Workshop + Tech Bar – sort of a RESO “Tech Bar” – at our upcoming RESO Spring Conference, April 19-21. It’s where you can get your questions answered directly from leading MLS vendors about Web API and the RESO Data Dictionary.

Let’s take a deeper look at the advantages of implementing the new RESO Web API.

Speedier development, more robust features

The new Web API is based upon global technology platforms created by Microsoft, SAP, Oracle and other leading software firms. Because it follows global technology standards, it is powered by a huge set of worldwide technology tools surrounding the OData Standard. This gives developers greater flexibility and allows for more robust features. For example, OData is built into Excel as a data source. By simply entering the server address and your credentials, one can pull data from an MLS into Excel. RETS is not supported in that manner natively at all. To accomplish this in RETS, one has to use a third-party tool (created by EZRETS, for example), which attempts to provide an Excel plug-in for RETS. However, this is not as easy to use or as quick as having built-in Excel support features through the RESO Web API. With Odata support built in directly, developers have access to a huge library of tools one can use for quick development with more robust features.

Faster path to deployment

RESO Web API Specification utilizes standard global technology platforms, and this allows software developers to use readily available off-the-shelf platforms and plugins to gain easier access to MLS data. The RESO Web API leverages OData and OpenID Connect standards, which are designed to efficiently transport data. Overall security and authentication is provided through the simplistic OpenID Connect standard developed by industry leading security organizations including Google, PayPal, Microsoft, Verizon, and Symantec.  Having tools available off the shelf reduces software development time and allows a more global technology “plug and play” solution. In the past, developers relying on the RETS 1x transportation standard found implementation far more burdensome. The RESO Web API removes this barrier.


Through the creation of the RESO Web API on top of OData and OpenID Connect standards, plugging into MLS data sets through the RESO Web API is simplified. Brokerages and technology partners can download from extensive libraries of toolsets designed specifically for Odata and OpenID Connect. The ease of implementing authentication was demonstrated at the RESO Fall Conference in 2015, seamlessly utilizing Google Login to provide Single Sign On (SSO) into MLSListings and FlexMLS created by FBS. Utilizing off the shelf tools helps simplify the development process so the focus can be on product innovation. The RESO Web API Specification was created on the premise that OData and OpenID Connect could be extended in a way that fits the needs of the real estate industry. The specification is just that: a manner in which to take globally utilized and supported technologies and extend them for full functionality needs of the real estate industry.

For more information about the RESO Web API, you can find additional information and download the entire standards specification for free on our website here. Information from NAR about the required Web API implementation can be found here. Finally, for specific questions about RESO Web API Certification, just send an email to info[at]

Note: RESO is currently developing a White Paper about data replication and data aggregation through the use of the RESO Web API, which is scheduled for release this fall.

Jeremy Crawford, RESO Executive Director