Cache exact count operation in CompositeDatasource

Author: rubensworks

lib/datasources/CompositeDatasource.js

@@ -2,7 +2,8 @@
 
 /** A CompositeDatasource delegates queries to an consecutive list of datasources. */
 
-var Datasource = require('./Datasource');
+var Datasource = require('./Datasource'),
+    LRU        = require('lru-cache');
 
 // Creates a new CompositeDatasource
 function CompositeDatasource(options) {
@@ -28,6 +29,7 @@ function CompositeDatasource(options) {
       this._datasourceNames.push(datasourceName);
     }
   }
+  this._countCache = LRU({ max: 1000, maxAge: 1000 * 60 * 60 * 3 });
 }
 Datasource.extend(CompositeDatasource);
 
@@ -53,12 +55,21 @@ CompositeDatasource.prototype._getDatasourceById = function(datasourceIndex) {
 
 // Count the amount of triple in the query result to get an exact count.
 CompositeDatasource.prototype._getExactCount = function(datasource, query, cb) {
+  // Try to find a cache match
+  var cacheKey = query.subject + "|" + query.predicate + "|" + query.object;
+  var cache = this._countCache, count = cache.get(cacheKey);
+  if (count) return setImmediate(cb, count);
+
+  // Otherwise, count all the triples manually
   var emptyQuery = { offset: 0, subject: query.subject, predicate: query.predicate, object: query.object };
   var exactCount = 0;
   var countingTripleStream = { push: function(triple) {
     if (triple) {
       exactCount++;
     } else {
+      // Cache large values; small ones are calculated fast anyway
+      if (exactCount > 1000)
+        cache.set(cacheKey, exactCount);
       cb(exactCount);
     }
   }};