dougal-software/lib/www/server/api/middleware/event/cache.js

const { listen } = require('../../../ws/db');

// Event responses take a long time as we are querying a view
// which is the union of other views and non-optimised tables,
// so to speed things up a bit for the user we cache the
// results here.
// We do this by indexing each result by its ETag value and
// storing the ID of the project it belongs to as well as the
// timestamp of the request. If the events for a project are
// modified in any way (addition/deletion/change) we immediately
// invalidate all cached responses for that project, else we
// delete them when they're older than maxAge (plus a delay).
// When the user sends a request with an ETag, we search for
// the ETag in our cache and return that, if present, instead
// of hitting the database.

const cache = {};
const maxAge = 90*60*1000; // 1.5 hours
setInterval(() => {
	const now = Date.now();
	for (const key in cache) {
		const value = cache[key];
		if ((now - value.tstamp) > maxAge) {
// 			console.log("CLEARING", key);
			delete cache[key];
		}
	}
}, 5*60*1000); // Run every five minutes

listen(["event"], (data) => {
	for (const key in cache) {
		const value = cache[key];
		if (value.pid == data.payload.pid) {
			delete cache[key];
		}
	}
});

function get (req, res, next) {
	try {
// 		console.log(cache);
		const etag = req.get('if-none-match');
// 		console.log("ETag", etag);
		if (etag && cache[etag]) {
// 			console.log("In cache");
			if (cache[etag].headers) {
				for (const header in cache[etag].headers) {
					const value = cache[etag].headers[header];
					if (header && value) {
						res.set(header, value);
					}
				}
			}
			// 304s have no body
			// https://tools.ietf.org/html/rfc7232#section-4.1
			res.status(304).send();
			next('route');
		} else {
// 			console.log("Not in cache");
			next();
		}
	} catch (err) {
		next(err);
	}
}

function save (req, res, next) {
	const etag = res.getHeader("etag");
	if (etag) {
		cache[etag] = {
			headers: {
				"Content-Type": res.getHeader("content-type") || "application/json"
			},
			pid: req.params.project,
			tstamp: Date.now()
		}
// 		console.log("CACHE", cache);
	}
	
	next();
}

module.exports = { get, save };