应用程序引擎数据存储当然有downtime。但是,我希望有一个“故障安全”put,它在面对数据存储错误时更加稳健(请参阅下面的动机)。当数据存储不可用时,似乎任务队列是延迟写入的明显位置。我不知道任何其他解决方案(除了通过urlfetch将数据发送给第三方)。应用程序引擎上的故障安全数据存储更新
动机:我真的需要要被放置在数据存储实体 - 只是显示错误信息给用户不会做。例如,也许有一些副作用发生,不容易撤销(可能与第三方网站有一些互动)。
我想出了一个简单的包装(我认为)提供了一个合理的“故障安全”放(见下文)。你有没有看到这方面的问题,或有一个更强大的实施的想法? (注:由于张贴在由尼克·约翰逊和萨克森德鲁斯答案的建议,这个帖子被编辑了一些改进的代码)的任务
import logging
from google.appengine.api.labs.taskqueue import taskqueue
from google.appengine.datastore import entity_pb
from google.appengine.ext import db
from google.appengine.runtime.apiproxy_errors import CapabilityDisabledError
def put_failsafe(e, db_put_deadline=20, retry_countdown=60, queue_name='default'):
"""Tries to e.put(). On success, 1 is returned. If this raises a db.Error
or CapabilityDisabledError, then a task will be enqueued to try to put the
entity (the task will execute after retry_countdown seconds) and 2 will be
returned. If the task cannot be enqueued, then 0 will be returned. Thus a
falsey value is only returned on complete failure.
Note that since the taskqueue payloads are limited to 10kB, if the protobuf
representing e is larger than 10kB then the put will be unable to be
deferred to the taskqueue.
If a put is deferred to the taskqueue, then it won't necessarily be
completed as soon as the datastore is back up. Thus it is possible that
e.put() will occur *after* other, later puts when 1 is returned.
Ensure e's model is imported in the code which defines the task which tries
to re-put e (so that e can be deserialized).
"""
try:
e.put(rpc=db.create_rpc(deadline=db_put_deadline))
return 1
except (db.Error, CapabilityDisabledError), ex1:
try:
taskqueue.add(queue_name=queue_name,
countdown=retry_countdown,
url='/task/retry_put',
payload=db.model_to_protobuf(e).Encode())
logging.info('failed to put to db now, but deferred put to the taskqueue e=%s ex=%s' % (e, ex1))
return 2
except (taskqueue.Error, CapabilityDisabledError), ex2:
return 0
请求处理程序:
from google.appengine.ext import db, webapp
# IMPORTANT: This task deserializes entity protobufs. To ensure that this is
# successful, you must import any db.Model that may need to be
# deserialized here (otherwise this task may raise a KindError).
class RetryPut(webapp.RequestHandler):
def post(self):
e = db.model_from_protobuf(entity_pb.EntityProto(self.request.body))
e.put() # failure will raise an exception => the task to be retried
我不要期待这个用于每把 - 大部分时间,显示错误信息就好了。对于每一个放置都使用它是很有诱惑力的,但是我认为有时如果我告诉他们他们的改变将在以后出现(并且继续向他们显示旧数据直到数据存储备份和延期卖出执行)。
一个相关的问题:是否有数据存储和任务队列停机之间有任何相关性?(http://stackoverflow.com/questions/3800252/datastore-and-task-queue-downtime-correlation) – 2010-09-26 23:25:45