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http-disk-cache

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An HTTP client that maintains a persistent disk-based cache. This module was written for internal use with Project Atlas but may be of use to others.

Installation

npm install http-disk-cache

Example

var cache = new HTTPCache('/cache/root/directory');

cache.getContents('http://example.com/url', function(err, buf, path) {
  if (err) {
    console.error("Error:", err);
  } else {
    console.log("Saved", buf.length, "bytes to path", path);
  }
};

cache.openReadStream('http://example.com/url', function(err, readStream) {
  if (err) { /* handle error */ }
  else { readStream.pipe(destination); }
});

API

Class: HTTPCache

new HTTPCache(cacheRoot)

Creates an HTTP cache directory which will work out of cacheRoot. The directory is created if it doesn't exist.

cache.getContents(url, callback)

Main URL request method for HTTPCache.

url may be a string URL or an object with a url key and any of the following options:

  • headers: An object of headers to pass along with the request
  • encoding: If specified, callback will get a String instead of a Buffer

Callback arguments:

  • err: Optional error message
  • contents: A String if the encoding option was specified, otherwise a Buffer
  • filename: The path to the contents on disk (contents are always saved to disk regardless of cache expiry)

cache.openReadStream(url[, onProgress], callback)

Creates a readable stream for the contents of url.

url may be a string URL or an object with a url key and any of the following options:

  • headers: An object of headers to pass along with the request

onProgress arguments:

  • numBytes: Number of bytes read in the last chunk.

Callback arguments:

  • err: Optional error message
  • stream: A readable stream containing the content of the URL
  • filename: The path to the contents on disk (contents are always saved to disk regardless of cache expiry)

cache.assertCached(url[, onProgress], callback)

Checks whether url is already in the cache and, if not, fetches and caches it. url may be a string or an object with the same options as openReadStream(). This is used to warm up the cache and is less resource-intensive than the other methods.

onProgress arguments:

  • numBytes: Number of bytes read in the last chunk.

Callback arguments:

  • err: Optional error message

cache.abortAllInFlightRequests()

Calls abort() on any in-progress HTTP requests that may have been initiated by openReadStream() or assertCached(). The callbacks for those methods will get called with an error.

cache.getContentPathname(url[, options])

Return a path to a requested URL assuming that it's already been cached.

Options:

  • absolute: Return an absolute path instead of a path relative to the cache root (default: false)

cache.setContentsSync(url, contents)

Synchronously set the contents for url. url must begin with cache:. Throws on error.

cache.getContentsSync(url, contents)

Synchronously get the contents for url. Throws on error.

url may be a string URL or an object with a url key and any of the following options:

  • headers: An object of headers to pass along with the request
  • encoding: If specified, will return a String instead of a Buffer

Returns a buffer, unless encoding is provided.

cache.reset()

reset must be called if you wish to reload any expired assets. (Otherwise, assets always persist for the lifetime of the cache, even if they would not otherwise be cached at all.)

cache.clean(shouldCleanCb, finalCb)

Iterates over all .meta files in the cache. For each one, the meta data is loaded and parsed, and the corresponding content file is stated.

shouldCleanCb is then called as shouldCleanCb(curFileNum, totalFiles, metadata, stat, resultCb) where:

  • curFileNum: A running count of how many files we have processed.
  • totalFiles: The total number of files that are to be processed. (Will not change in between calls to shouldCleanCb).
  • metadata: The parsed metadata contained in the meta file for the current cache entry.
  • stat: An fs.Stat object for the content file corresponding to the current meta file.
  • resultCb: a callback which takes a single argument.

shouldCleanCb should call resultCb with 'REMOVE' to signal that the entry should be removed from the cache, or 'KEEP' to indicate that the file should be kept.

Notes

The on-disk cache is structured as follows:

  • URLs are canonicalized and hashed via the MD5 algorithm.
  • The hex digest of the hash is split into 3 pieces: characters 0-1, characters 2-3, and characters 4-31, and used to construct a pathname relative to the cache root, e.g. 79/da/646f91932de1ed0267ed1abdb741
  • The contents of the cached object are stored at that pathname. Additionally, metadata about the cached object is stored at that pathname with the suffix .meta.

A cached object is valid iff:

  • The .meta file contains a valid JSON object.
  • The expiration time stored in the .meta file under the expiry property has not elapsed.
  • The contents stored in the cache object file have an MD5 sum equal to that stored in the .meta file as the contentMD5 property.

A valid cached object may always be served in response to an openReadStream request. An invalid cached object may be deleted at any time, and in practice, will be deleted when it is next accessed while attempting to meet a request.

Fetched resources are always saved to disk regardless of expiry. This is ease integration with things such as SDL functions which load images or dynamic libraries from disk.

Enable debug output by setting the DEBUG environment variable to include http-disk-cache

License

MIT