vsm-dictionary-cacher

vsm-dictionary-cacher

Summary

vsm-dictionary-cacher augments a given VSM-dictionary with a layer of caching functionality.

This speeds up requests for string-matches, refTerms, and dictInfos; and fixedTerms-preloading.

Use

Install like (after also installing a vsm-dictionary-... of choice) :

npm install vsm-dictionary-cacher

Then use like:

const Dictionary       = require('vsm-dictionary-local');  // ...or any other VsmDictionary implementation.
const cacher           = require('vsm-dictionary-cacher');
const CachedDictionary = cacher(Dictionary);  // This makes a cache-enabled subclass.


var dict = new CachedDictionary();  // This makes an instance.


// This will query the Dictionary as normal, bypassing the cache.
dict.getMatchesForString('abc', {filter: {dictID: ['Foo']}}, (err, res) => {
  console.dir(res);

  // This will get the result from the cache, instead of re-running the query.
  dict.getMatchesForString('abc', {filter: {dictID: ['Foo']}}, (err, res) => {
    console.dir(res);
  });

  // These will *not* get their result from the cache.
  dict.getMatchesForString('abc', {filter: {dictID: ['BAR']}}, (err, res) => {});
  dict.getMatchesForString('QQQ', {filter: {dictID: ['Foo']}}, (err, res) => {});


  // And similar behavior for the other three cached functions:
  // - dict.getRefTerms({}, cb)
  // - dict.getDictInfos({}, cb)
  // - dict.loadFixedTerms([], {}, cb)

});

Specify options like:

const CachedDictionary = cacher(Dictionary, { maxItems: 1000 });

Details

This package provides a factory function that accepts any VsmDictionary (sub)class, and returns a (further) subclass of it,
which inserts cache handling code into several functions:

• It speeds up requests for string-matches, to getMatchesForString(), in three ways:

  • It stores results from requests to this function in a cache.
    These results are returned for subsequent requests that use same search-string & options, instead of re-running the query.

    This helps e.g. vsm-autocomplete avoid making duplicate requests to an online dictionary server.
    It creates a more responsive autocomplete when the user types, and then backspaces.

  • It also prevents sending a second (or more) query to the underlying datastore, if this query has the same search-string & options as an ongoing query, whose results haven't arrived yet.
    Instead, it puts such identical queries in a queue, and when the first one's result comes in, it shares its error+result with the queued ones, almost immediately (=on individual, next event-loops).

    This helps e.g. vsm-autocomplete avoid making duplicate requests
    when a user types and backspaces quickly, before any results could come in.

    Note: when a query on the underlying storage fails, then no item will be added to the cache, and no attempt to re-query will be made by the queued ones.
    This also means that on error, the same error would be returned by all queued requests.

  • It can also remember for which strings there were no 'normal entry'-type matches.
    Then for subsequently queried strings, that start with such a 'no matches' string, it can immediately return an empty list for the 'entry-matches' too. (But it still checks for refTerm/number/etc-type matches).

    This helps e.g. vsm-autocomplete avoid making unnecessary requests for search-strings, for which a substring already returned no entry-matches.

• It maintains a cache for getRefTerms().

  • There are usually only a small number of refTerms, and they are just Strings.
    Therefore, at a first call for anything, it queries all of them, and puts them into a cache that is used for all further lookups.

    This makes e.g. vsm-autocomplete not launch two queries per search-string (i.e. one for entries, and one for refTerms).
    And for cacheEmpty-hits, it will even serve all data from either cache or computation.

  • It also catches and prevents any concurrent calls, until this cache data is received.

• It partially maintains a cache for getDictInfos().

  • It caches all dictInfo-objects that are returned from any queries to the underlying datastore, no matter what options were used.

  • Then it may use this cache:

    • It uses it: only for requests that filter for a list of dictIDs (i.e. having a options.filter.id),
      or for requests that ask all dictInfos (i.e. having no options.filter).

    • • Then it collects all dictInfos with a cache-hit in the dictInfos-cache,
      • and sends a query only for the dictInfos that had a cache-miss, and that are not marked as being-queried by another concurrent getDictInfos() call,
      • after marking these as being-queried now too.
      + Note: queries to the underlying datastore are made, explicitly unpaginated (i.e. with options = { perPage: Number.MAX_VALUE, ... }).

    • When all dictInfos that it depends on have come in, (possibly as partial results from several other concurrent calls), then it finally returns its own, complete, assembled result.

    • The implementation of this is not trivial. But it ensures that for a large amount of concurrent requests (which may be launched when an app starts up), no dictID is queried twice.
      And it works even if clearCache() (see below) is called during this process.

      A vsm-autocomplete needs a corresponding dictInfo for each of its string-matches.
      So if its string-matches already came from cache-hits, then the above makes all its other data also come only from cache.

      If a vsm-box with a template, or several vsm-boxes loaded on a same page, would launch multiple concurrent requests for dictInfos, then this caching may result in a lot less queries to the underlying datastore.

• It enhances the cache management of loadFixedTerms().

  • • Note: this function (of the VsmDictionary parent class) queries a VsmDictionary subclass's getEntries(), and puts the processed results in its own simple cache (in the VsmDictionary parent class).
    • But if a web page would contain multiple vsm-boxes based on a template, then each of them may call the (shared) VsmDictionary's loadFixedTerms() and launch a query. This would query and add results to the VsmDictionary's fixedTermsCache, no matter whether these results were in there already.

  • • It maintains a list of fixedTerms (idts) that have ever been queried.
    • It removes, from a request's idts-argument, any that were queried before,
    • and then launches the query only for the remaining idts,
    • after marking them as 'pending/having-been-queried-now',
    • so that concurrent calls can be prevented from requesting anything twice.

    Note: the options.z, for z-object-pruning, is not taken into account here, because VsmDictionary's fixedTerms-cache does so neither.

    This prevents that loadFixedTerms() is called multiple times for the same data, when loading multiple vsm-boxes with the same template.

Options

An options object can be given as second argument to the factory function (see example above), with these optional properties:

• maxItems: {Number}:
  This limits the amount of items kept in the string-match cache (only). One item equals the result of one getMatchesForString() query (which is often a list of match-objects).
  When adding a new item to a full cache, the least recently added or accessed item gets removed first.
  Default is 0, which means unlimited storage.
  • Note: this pertains to string-match-objects only;
    so not to refTerms (they are small and few), not to dictInfos (same reason, and the result of one query is spread out over multiple cache-items, which is hard to manage), and not to fixedTerms (same).
• predictEmpties: {Boolean}:
  If true, then it keeps a list of strings (per options-object) for which getEntryMatchesForString() returned no results (i.e.: { items: [] }).
  Then for any subsequent query (with same options) for a string that starts with any such empty-returning-string, we can assume that no results will be returned either.
  E.g. if a search for 'ab' returned no matching entries, then neither will 'abc'. So it can avoid running that query and immediately return the empty { items: [] } for 'abc'.
  Default is true.
  • Note: maxItems does not apply to this collection of strings either.
    But the collection gets cleared, like everything else, by a call to clearCache() (see below).
  • Note: this is handled in getEntryMatchesForString(), not in getMatchesForString(), because the latter may still add 'extra' matches (refTerm/number/fixedTerm), other than the 'entry'-type matches.
    • Example 1: after a call for "i" would give no entry-matches (and "i" ends up in the 'cacheEmpties'), a subsequent call for "it" should still return "it" as a refTerm-match.
      (Note that a refTerm only matches for a full, not partial, string match).
    • Example 2: after a call for "1e" gave no results, a subsequent call for the valid number-string "1e5" should still return it as a result.

Functions

An extra function is added to the VsmDictionary subclass:

• clearCache():
  This removes all data from the cache layer, including e.g. the list used by predictEmpties.