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