markersopenlayers-2popupreferences
The examples given in the documentation of OpenLayers show very basic ways of utilising the different features. This example shows how to add shadows to the marker. This example shows how to create basic popups.
Looking at the sample code, I have no idea how to combine the two together, nor display different icons for the markers. Can anyone show me a tutorial or example that combines the various aspects of custom markers, shadows and popups using JSON data?
I would personally go with features over markers. Point features (and all features) allow for labeling and can be represented in different shapes and colors like circles, x, square and so forth. Not to mention all the additional intrinsic functionality and flexibility features have over markers such as geographical calculations, transformations, click/highlight/select handlers and on and on.
I hate to be pushy, but it would be beneficial if you used something like GeoServer or MapServer on the back end. You could set a WFS strategy that works on your 5-30 second (or whatever) interval timer and allow it to fetch and render the features automatically. In case you're wondering, it already does this in ajax form. So you don't have to write the code to query the database, parse, convert, etc... on 300 objects on your map. Openlayers will render the latest WFS data on your Geoserver which is pointed to PostGIS. You would save yourself a lot of time and effort reinventing that wheel. Openlayers WFS protocol can be set to query based on your viewport on a temporal filter.
Though features don't perform well when there are a lot of them. 300 might be pushing it. If you see that performance is an issue, you can easily switch to using WMS (only if you have Geoserver or Mapserver). You can also use a cluster strategy to minimize un necessary features on your map.
Few pointers to get you started. You've already done some research, so you know the right places to look for more info:
Don't use a Markers layer. Do use a Vector layer.
Vector layer can be styled. Each of your points can have attributes e.g. "color:blue" or "severity:moderate" and based on those attributes you can Style the points with different icons or shapes
Attach the on-click handler to the LAYER not individual points. The click handler will get a parameter that is the Feature being clicked on. From there you can retrieve attributes attached to the feature.
Inside click-handler show a Bubble and you control what goes in the bubble HTML -> could be point ID (rather pointless) or some other meta-data that you store in the point attribute e.g. "title: point1" "title: point2"
Alternatively you can also do all of the above by adding a KML layer that's properly styled, and the Placemarks have appropriate and tags.
Here are some samples:
Best Answer
the OpenLayers Cookbook (http://www.packtpub.com/openlayers-create-gis-web-applications-cookbook/book) has some samples creating markers by hand.
And you can freely download the source code: http://packtlib.packtpub.com/library/9781849517843
Cheers.