1 # Build Cache wrapper for JSON files
7 The `json_cache` package exports a single constructor
9 which must be called with the `new` operator. The resulting cache object stores
10 the JavaScript instantiation of JSON files loaded and parsed from disk.
12 See the `build_cache` package for more information. The `JSONCache` object is
13 essentially a wrapper object which routes the request between the `build_cache`
14 package and the asynchronous `fs.readFile()` and `JSON.parse` APIs of Node.js,
15 to ensure that the file is retrieved from either RAM or disk as required.
19 Suppose one has a `JSONCache` instance named `jc`. It behaves somewhat like an
20 ES6 `Map` object, except that it only has the `jc.get()` function, because new
21 objects are added to the cache by attempting to `get` them.
23 The interface for the `JSONCache`-provided instance function `jc.get()` is:
25 `await jc.get(key)` — retrieves the object stored under `key`, where
26 `key` is the on-disk pathname to a JSON file. A JavaScript object is returned,
28 `JSON.parse(await util.promisify(fs.readFile)(key, {encoding: 'utf-8'}))`,
29 except that the pathname and the parsed result are cached for future reuse.
31 Before returning the cached copy, the existence and modification time of the
32 JSON file on disk is checked to make sure that the cache is up-to-date.
33 Otherwise, if the file doesn't exist an `ENOENT` exception is thrown, or if the
34 file exists it is loaded and parsed and the cache updated for next time.
38 The `diag` argument to the constructor is a `bool`, which if `true` causes
39 messages to be printed via `console.log()` for all activities except for the
40 common case of retrieval when the object is already up-to-date. A `diag` value
41 of `undefined` is treated as `false`, thus it can be omitted in the usual case.
45 It is intended that we will shortly add a timer function (or possibly just a
46 function that the user should call periodically) to flush built templates from
47 the cache after a stale time. There is otherwise no way to delete an object
48 from the cache, except by first deleting it on disk, then trying to `get` it.
52 The development version can be cloned, downloaded, or browsed with `gitweb` at:
53 https://git.ndcode.org/public/json_cache.git
57 All of our NPM packages are MIT licensed, please see LICENSE in the repository.
61 The caching system is under active development (and is part of a larger project
62 that is also under development) and thus the API is tentative. Please go ahead
63 and incorporate the system into your project, or try out our example webserver
64 built on the system, subject to the caution that the API could change. Please
65 send us your experience and feedback, and let us know of improvements you make.
67 Contact: Nick Downing <nick@ndcode.org>