What happens when you try to give map users a rich display with thousands of points in a web browser? Generally, one of two things occurs. One, the map looks like a carpet of marker symbols. Two, depending on device and bandwidth, a browser-side clustering algorithm can delay the display for an agonizingly long time. The problem is acute in dynamic map systems where automated processes or users generate content because the system architects cannot know in advance how many map entities will be created. Think of mapping tweets, potholes in a major city, or locations of coffee places in Portland Ð map displays have to show dense data effectively. Our presentation shows how a Leaflet plug-in is used on a node.js server to solve this problem. We will demonstrate how the Leaflet.markercluster plug-in is used to generate clusters. Once generated, the clusters are stored in MongoDB and retrieved using a MongoDB geospatial index.