Add Query Cache solution

solutions/system_design/query_cache/query_cache_snippets.py

@@ -0,0 +1,89 @@
+# -*- coding: utf-8 -*-
+
+class QueryApi(object):
+
+    def __init__(self, memory_cache, reverse_index_cluster):
+        self.memory_cache = memory_cache
+        self.reverse_index_cluster = reverse_index_cluster
+
+    def parse_query(self, query):
+        """Remove markup, break text into terms, deal with typos,
+        normalize capitalization, convert to use boolean operations.
+        """
+        ...
+
+    def process_query(self, query):
+        query = self.parse_query(query)
+        results = self.memory_cache.get(query)
+        if results is None:
+            results = self.reverse_index_cluster.process_search(query)
+            self.memory_cache.set(query, results)
+        return results
+
+
+class Node(object):
+
+    def __init__(self, query, results):
+        self.query = query
+        self.results = results
+
+
+class LinkedList(object):
+
+    def __init__(self):
+        self.head = None
+        self.tail = None
+
+    def move_to_front(self, node):
+        ...
+
+    def append_to_front(self, node):
+        ...
+
+    def remove_from_tail(self):
+        ...
+
+
+class Cache(object):
+
+    def __init__(self, MAX_SIZE):
+        self.MAX_SIZE = MAX_SIZE
+        self.size = 0
+        self.lookup = {}
+        self.linked_list = LinkedList()
+
+    def get(self, query)
+        """Get the stored query result from the cache.
+
+        Accessing a node updates its position to the front of the LRU list.
+        """
+        node = self.lookup[query]
+        if node is None:
+            return None
+        self.linked_list.move_to_front(node)
+        return node.results
+
+    def set(self, results, query):
+        """Set the result for the given query key in the cache.
+
+        When updating an entry, updates its position to the front of the LRU list.
+        If the entry is new and the cache is at capacity, removes the oldest entry
+        before the new entry is added.
+        """
+        node = self.map[query]
+        if node is not None:
+            # Key exists in cache, update the value
+            node.results = results
+            self.linked_list.move_to_front(node)
+        else:
+            # Key does not exist in cache
+            if self.size == self.MAX_SIZE:
+                # Remove the oldest entry from the linked list and lookup
+                self.lookup.pop(self.linked_list.tail.query, None)
+                self.linked_list.remove_from_tail()
+            else:
+                self.size += 1
+            # Add the new key and value
+            new_node = Node(query, results)
+            self.linked_list.append_to_front(new_node)
+            self.lookup[query] = new_node