rcn-frontserver/lib/caching.js

const fs = require("fs"),
    path = require("path"),
    config_path = "./config/caching_dump.json",
    cache_dump = JSON.parse(fs.readFileSync(config_path)),
    cache_dir = "./.cache";
let cache = {};
let logger = require("winston");
if (cache_dump != null && cache_dump["last"] != null) cache = cache_dump["last"]; // read the data from the file dump

/**
 * Sets the logger for logging
 * @param  {Object} newLogger
 */
exports.setLogger = function (newLogger) {
    logger = newLogger;
};

/**
 * Returns the data from files that were cached
 * @param  {String} filename The name of the file that has been cached
 * @return {String}          The data stored in the file
 */
exports.getCached = function (filename) {
    let cf = cache[filename];
    let call_passed = (Date.now() - cf.last_call) / 1000; // calculate the time since the last call of the file
    if (cf.call_count > 10 && call_passed < 60) {
        cf.data = fs.readFileSync(cf.path); // store the file's data into the json
    } else if (call_passed > 3600) {
        cf.call_count = 0; // reset the counter when an hour has passed since the last call
        cf.data = null; // reset the data to free memory
    }
    cf.last_call = Date.now(); // set the last call to now
    cf.call_count += 1; // increase the call count
    if (cf.data != null) return cf.data;
    logger.debug("Returning cached data for %s : %s", filename, cf.path);
    return fs.readFileSync(cf.path); // return either the data or read the file
};

/**
 * Safes the file with it's corresponding data in the cache directory
 * @param  {String} filename The name of the file
 * @param  {String} data     The data form the file
 */
exports.cache = function (filename, data) {
    logger.verbose("Creating cache entry for %s", filename);
    if (!fs.existsSync("./.cache")) fs.mkdirSync("./.cache"); // if the cache folder doesn't exist, create it
    let cache_fn = filename.replace(/[^\w.]/g, "__"); // remove invalid path characters
    let count = 0;
    while (fs.existsSync(filename + count + ".cache")) count++; // check if a file with the same name already exists and increase count
    let cache_path = path.join(cache_dir, cache_fn + count + ".cache"); // create the final file path. Cachedir + cached filename (without invalid) + count + .cache
    logger.debug("Creating file %s", cache_path);
    fs.writeFile(cache_path, data, (err) => {
        if (err !== null) logger.error(err);
        else {
            logger.debug("Created file cache entry for %s", filename);
            cache[filename] = {   // create a cache-entry with the file's path when the file is written (so it won't be accessed before)
                "path": cache_path, // the last call to the file, the count of calls and an
                "last_call": null,  // empty data field to store the file's data if the file
                "call_count": 0,    // was called often
                "data": null,
                "creation_time": Date.now(),
                "changed": false
            }
        }
        fs.watch(filename, (eventType) => { // watch the file for changes
            logger.debug("Change detected on %s", filename);
            if (eventType === 'change') cache[filename].changed = true; // if the event change is detected set the change attribute to  true
        });
    }); // write the data asynchronously to the file
};

/**
 * Returns if the file is already cached. It returns false if changes were detected in the file
 * @param  {String} filename The filename to check
 * @return {Boolean}          Is it cached or not
 */
exports.isCached = function (filename) {
    let cached_entry = cache[filename];
    if (cached_entry) { // check if the cache entry exists
        logger.debug("Found cache entry for %s", filename);
        if (cached_entry.changed) return false; // if a change was detected recache the file
        if (cached_entry.path) { // check if the path variable is set
            logger.debug("Found path entry for %s", filename)
            return fs.existsSync(cached_entry.path); // return if the file exists
        }
    }
    logger.debug("Found no cache entry for %s", filename);
    return false; // return false if the cache entry doesn't exist
}

/**
 * A function that dumps the config into the config file after appending the cache to it.
 */
exports.cleanup = function () {
    logger.verbose("Dumping cache into cache_dump file");
    cache_dump["last"] = cache; // append the cache to the dump object
    fs.writeFileSync(config_path, JSON.stringify(cache_dump)); // write the dump data to the file
};