When using DNS cache, optionally fall back to cached value if the att…

…empt after expiration fails

src/greplin/net/dnsCache.py

@@ -30,9 +30,10 @@ class CachingDNS(object):
   implements(interfaces.IResolverSimple)
 
 
-  def __init__(self, original, timeout = 60):
+  def __init__(self, original, timeout = 60, useFallback = True):
     self._original = original
     self._timeout = timeout
+    self._fallback = {} if useFallback else None
     self._cache = lazymap.DeferredMap(self.__fetchHost)
 
 
@@ -49,12 +50,17 @@ def getHostByName(self, name, *args):
       if isinstance(self._cache[key], Failure):
         del self._cache[key]
       # Check for a cache hit.
-      elif time.time() > self._cache[key][1] + self._timeout:
+      elif time.time() >= self._cache[key][1] + self._timeout:
         # Ensure the item hasn't expired.
+        if self._fallback is not None:
+          self._fallback[key] = self._cache[key][0]
         del self._cache[key]
       else:
         # If the item is in cache and not expired, return it immediately.
         return defer.succeed(self._cache[key][0])
 
     # If it wasn't already in the cache, this always returns a deferred.
-    return self._cache[key].addCallback(lambda x: x[0])
+    result = self._cache[key].addCallback(lambda x: x[0])
+    if self._fallback and key in self._fallback:
+      result.addErrback(lambda _: self._fallback[key])
+    return result

src/greplin/net/dnsCache_test.py

@@ -0,0 +1,70 @@
+# Copyright 2011 The greplin-twisted-utils Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""DNS resolver that uses a short lived local cache to improve performance."""
+
+import mox
+import unittest
+
+from greplin.net import dnsCache
+
+from twisted.internet import defer, error
+from twisted.python import failure
+
+
+
+class CachingDNSTest(unittest.TestCase):
+  """Tests for caching DNS."""
+
+  def testFallback(self):
+    """Test a cache with fallback."""
+    original = mox.MockAnything()
+    original.getHostByName('google.com').AndReturn(
+        defer.succeed('1.2.3.4'))
+    original.getHostByName('google.com').AndReturn(
+        defer.fail(failure.Failure(error.DNSLookupError('Fake DNS failure'))))
+    original.getHostByName('google.com').AndReturn(
+        defer.succeed('9.8.7.6'))
+    mox.Replay(original)
+
+    cache = dnsCache.CachingDNS(original, timeout = 0)
+    result = cache.getHostByName('google.com')
+    self.assertEquals(result.result, '1.2.3.4')
+    result = cache.getHostByName('google.com')
+    self.assertEquals(result.result, '1.2.3.4')
+    result = cache.getHostByName('google.com')
+    self.assertEquals(result.result, '9.8.7.6')
+    mox.Verify(original)
+
+
+  def testNoFallback(self):
+    """Test a cache with fallback."""
+    original = mox.MockAnything()
+    original.getHostByName('google.com').AndReturn(
+        defer.succeed('1.2.3.4'))
+    original.getHostByName('google.com').AndReturn(
+        defer.fail(failure.Failure(error.DNSLookupError('Fake DNS failure'))))
+    original.getHostByName('google.com').AndReturn(
+        defer.succeed('9.8.7.6'))
+    mox.Replay(original)
+
+    cache = dnsCache.CachingDNS(original, timeout = 0, useFallback = False)
+    result = cache.getHostByName('google.com')
+    self.assertEquals(result.result, '1.2.3.4')
+    result = cache.getHostByName('google.com')
+    self.assertTrue(isinstance(result.result, failure.Failure))
+    result.addErrback(lambda _: None) # Consume the error.
+    result = cache.getHostByName('google.com')
+    self.assertEquals(result.result, '9.8.7.6')
+    mox.Verify(original)