Parallelize hillshade generation using PostGIS. Much worse than generating hillshades from GeoTIFF inputs.

generate_hillshades.js

#!/usr/bin/env node
"use strict";

var os = require("os");

var async = require("async"),
    env = require("require-env"),
    pg = require("pg"),
    ProgressBar = require("progress");

var DATABASE_URL = env.require("DATABASE_URL"),
    // TODO make limit configurable; this assumes that the client has the same
    // number of cores as the server, which won't always be the case
    CONCURRENCY = os.cpus().length;

pg.connect(DATABASE_URL, function(err, client, done) {
  if (err) {
    throw err;
  }

  return client.query("SELECT rid FROM dem", function(err, result) {
    done();

    if (err) {
      throw err;
    }

    var bar = new ProgressBar("hillshading [:bar] :percent :etas", {
      complete: "=",
      incomplete: " ",
      total: result.rows.length
    });

    return async.eachLimit(result.rows, CONCURRENCY, function(row, next) {
      // use a connection for each task to use multiple processes on pg's end
      return pg.connect(DATABASE_URL, function(err, client, done) {
        if (err) {
          return next(err);
        }

        // TODO populate overview tables
        return client.query([
          "INSERT INTO hillshade (rid, rast) (",
          "  SELECT",
          "    focus.rid,",
          "    ST_Clip(ST_HillShade(ST_Union(dem.rast), 1, '8BUI'), ST_Envelope(focus.rast)) rast",
          "  FROM dem focus",
          "  JOIN dem ON dem.rast && ST_Buffer(ST_Envelope(focus.rast), 1)",
          "  WHERE focus.rid = $1",
          "  GROUP BY focus.rid, focus.rast",
          ")"
        ].join("\n"), [row.rid], function(err) {
          done();

          bar.tick();

          return next.apply(null, arguments);
        });
      });
    }, function(err) {
        if (err) {
          console.error(err);
          throw err;
        }

        clearInterval(status);
        done();

        pg.end();
    });
  });
});