124 lines
4.1 KiB
Python
124 lines
4.1 KiB
Python
# Copyright 2012 Pinterest.com
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#
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# Licensed under the Apache License, Version 2.0 (the "License");
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# you may not use this file except in compliance with the License.
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# You may obtain a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS,
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# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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# See the License for the specific language governing permissions and
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# limitations under the License.
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"""
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A client for falling back to older memcached servers when performing reads.
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It is sometimes necessary to deploy memcached on new servers, or with a
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different configuration. In theses cases, it is undesirable to start up an
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empty memcached server and point traffic to it, since the cache will be cold,
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and the backing store will have a large increase in traffic.
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This class attempts to solve that problem by providing an interface identical
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to the Client interface, but which can fall back to older memcached servers
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when reads to the primary server fail. The approach for upgrading memcached
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servers or configuration then becomes:
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1. Deploy a new host (or fleet) with memcached, possibly with a new
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configuration.
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2. From your application servers, use FallbackClient to write and read from
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the new cluster, and to read from the old cluster when there is a miss in
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the new cluster.
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3. Wait until the new cache is warm enough to support the load.
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4. Switch from FallbackClient to a regular Client library for doing all
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reads and writes to the new cluster.
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5. Take down the old cluster.
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Best Practices:
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---------------
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- Make sure that the old client has "ignore_exc" set to True, so that it
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treats failures like cache misses. That will allow you to take down the
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old cluster before you switch away from FallbackClient.
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"""
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class FallbackClient(object):
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def __init__(self, caches):
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assert len(caches) > 0
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self.caches = caches
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def close(self):
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"Close each of the memcached clients"
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for cache in self.caches:
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cache.close()
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def set(self, key, value, expire=0, noreply=True):
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self.caches[0].set(key, value, expire, noreply)
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def add(self, key, value, expire=0, noreply=True):
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self.caches[0].add(key, value, expire, noreply)
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def replace(self, key, value, expire=0, noreply=True):
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self.caches[0].replace(key, value, expire, noreply)
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def append(self, key, value, expire=0, noreply=True):
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self.caches[0].append(key, value, expire, noreply)
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def prepend(self, key, value, expire=0, noreply=True):
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self.caches[0].prepend(key, value, expire, noreply)
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def cas(self, key, value, cas, expire=0, noreply=True):
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self.caches[0].cas(key, value, cas, expire, noreply)
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def get(self, key):
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for cache in self.caches:
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result = cache.get(key)
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if result is not None:
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return result
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return None
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def get_many(self, keys):
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for cache in self.caches:
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result = cache.get_many(keys)
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if result:
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return result
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return []
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def gets(self, key):
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for cache in self.caches:
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result = cache.gets(key)
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if result is not None:
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return result
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return None
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def gets_many(self, keys):
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for cache in self.caches:
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result = cache.gets_many(keys)
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if result:
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return result
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return []
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def delete(self, key, noreply=True):
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self.caches[0].delete(key, noreply)
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def incr(self, key, value, noreply=True):
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self.caches[0].incr(key, value, noreply)
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def decr(self, key, value, noreply=True):
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self.caches[0].decr(key, value, noreply)
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def touch(self, key, expire=0, noreply=True):
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self.caches[0].touch(key, expire, noreply)
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def stats(self):
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# TODO: ??
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pass
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def flush_all(self, delay=0, noreply=True):
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self.caches[0].flush_all(delay, noreply)
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def quit(self):
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# TODO: ??
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pass
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