Dynamically change Mapnik XML without restarting renderd

Question

I have multiple tables of data in my PostgreSQL database that have wkb_geometry columns used to generate layers. I would like to allow users to select which layers on a map they will see by accessing UI components such as checkboxes. When doing so, based on the selected UI components, it should send a request to my tile server and update the Mapnik XML file accordingly before rendering the tiles. How can I do this without having to restart renderd each time and how can I make sure that concurrent accesses won't overwrite changes to my XML file?

I do not want to cache these tiles as the same URL could have different parameters (layers turned on) so how would I avoid caching these requests?

Answer 1

I ended up editing the source code for mod_tile to force the map to refresh its XML source each time a tile is rendered. I did so by editing the gen_tile.cpp file and rebuilding mod_tile.

I added this code to the function "void *render_thread(void * arg)"

I placed it inside of the following if statement

...
if (check_xyz(item->mx, item->my, req->z, &(maps[i]))) {

    ######## START OF MY CODE #######
    try {
        maps[i].map = Map(256,256,maps[i].map.srs());
        mapnik::load_map(maps[i].map, maps[i].xmlfile);
    }
    catch (std::exception const& ex) {
        syslog(LOG_ERR, "An error occurred");
        maps[iMaxConfigs].ok = 0;
        break;
    } catch (...) {
        syslog(LOG_ERR, "An unknown error occurred ");
        maps[iMaxConfigs].ok = 0;
        break;
    }
    ######## END OF MY CODE ########

    metaTile tiles(req->xmlname, req->options, item->mx, item->my, req->z);
...

I did not solve the issue of caching the requests and have created a script that removes the cache directory if the request coming in is for different layers than the previous request.