[storm] Cache for nonexistent result
Ivan Zakrevskyi
ivan.zakrevskyi at rebelmouse.com
Fri Jan 16 23:33:31 UTC 2015
> For "Read Committed" and "Read Uncommitted" my patch is not safe, because this levels should not to have "Non-repeatable reads". But for existent object storm also can not provide "repeatable reads". So, it's not mater, will "Non-repeatable" be applied reads for existent object or for nonexistent object.
-- sorry. For "Read Committed" and "Read Uncommitted" my patch is not
safe, because this levels should to have "Non-repeatable reads". But
for existent object storm also can not provide "Non-repeatable reads".
So, it's not mater, will "Non-repeatable reads" be prevent for
existent object or for nonexistent object.
2015-01-17 1:12 GMT+02:00 Ivan Zakrevskyi <ivan.zakrevskyi at rebelmouse.com>:
> Thanks for answer.
>
> If you add object to DB by this store, you will can get this object,
> because store.get() looking object first in store._alive and then in
> store.nonexistent_cache.
>
> If you will create object by other store (for example "master" store
> or by concurrent thread), then you can get this object after
> transaction has been stared (but before first reading), if your DB
> transaction-level is read commited. So, it's not exactly serializable
> isolation level or repeteable reads.
>
> But if you have already tried get this object in current transaction,
> and will try to get it again, you will get None even if object has
> been already created by concurrent thread. Of course, you can call
> store.get() with exists=True argument to bypass nonexistent_cache, or
> you can even reset nonexistent_cache (it's a public attribute).
>
> On the other hand, suppose that an object exists, and you have already
> got this object in current transaction. After it, suppose, object was
> changed in DB by concurrent thread. But these changes will not affect
> your object. I think in this case it does not matter what type of
> object, None or Model instance. Since the object has been read, it can
> not be changed even if it has been modified by a parallel process.
>
> My patch does not affect store.find(), and, hence, selection. I'm not
> sure, that phantom reads is possible here, except that
> store.get_multi(). There is rather a "Non-repeatable reads", than
> "Phantom reads". Because it can hide changes of certain row (with
> specified primary key), but not of selection.
>
> So, for "Repeatable Read" and "Serializable" my patch is safe (only
> need add reset of store.nonexistent_cache on commit).
>
> For "Read Committed" and "Read Uncommitted" my patch is not safe,
> because this levels should not to have "Non-repeatable reads". But for
> existent object storm also can not provide "repeatable reads". So,
> it's not mater, will "Non-repeatable" be applied reads for existent
> object or for nonexistent object.
>
> Of course, my patch is temporary solution. There is can be more
> elegant solutions on library level. But it really reduce many DB
> queries for nonexistent primary keys.
>
>
>
> 2015-01-16 23:20 GMT+02:00 Free Ekanayaka <free at 64studio.com>:
>>
>> See:
>>
>> http://en.wikipedia.org/wiki/Isolation_%28database_systems%29
>>
>> for reference.
>>
>> On Fri, Jan 16, 2015 at 10:19 PM, Free Ekanayaka <free at 64studio.com> wrote:
>>>
>>> Hi Ivan,
>>>
>>> it feels what you suggest would work safely on for transactions set the serializable isolation level, not repeteable reads down to read uncommitted (since phantom reads could occur there, and the non-existing cache would hide new results).
>>>
>>> Cheers
>>>
>>> On Fri, Jan 16, 2015 at 5:55 PM, Ivan Zakrevskyi <ivan.zakrevskyi at rebelmouse.com> wrote:
>>>>
>>>> Hi, all. Thanks for answer. I'll try to explain.
>>>>
>>>> Try to get existent object.
>>>>
>>>> In [2]: store.get(StTwitterProfile, (1,3))
>>>> base.py:50 =>
>>>> u'(0.001) SELECT ... FROM twitterprofile WHERE twitterprofile.context_id = %s AND twitterprofile.user_id = %s LIMIT 1; args=(1, 3)'
>>>> Out[2]: <users.orm.TwitterProfile at 0x7f1e93b6d450>
>>>>
>>>> In [3]: store.get(StTwitterProfile, (1,3))
>>>> Out[3]: <users.orm.TwitterProfile at 0x7f1e93b6d450>
>>>>
>>>> In [4]: store.get(StTwitterProfile, (1,3))
>>>> Out[4]: <users.orm.TwitterProfile at 0x7f1e93b6d450>
>>>>
>>>> You can see, that storm made only one query.
>>>>
>>>> Ok, now try get nonexistent twitter profile for given context:
>>>>
>>>> In [5]: store.get(StTwitterProfile, (10,3))
>>>> base.py:50 =>
>>>> u'(0.001) SELECT ... FROM twitterprofile WHERE twitterprofile.context_id = %s AND twitterprofile.user_id = %s LIMIT 1; args=(1, 10)'
>>>>
>>>> In [6]: store.get(StTwitterProfile, (10,3))
>>>> base.py:50 =>
>>>> u'(0.001) SELECT ... FROM twitterprofile WHERE twitterprofile.context_id = %s AND twitterprofile.user_id = %s LIMIT 1; args=(1, 10)'
>>>>
>>>> In [7]: store.get(StTwitterProfile, (10,3))
>>>> base.py:50 =>
>>>> u'(0.001) SELECT ... FROM twitterprofile WHERE twitterprofile.context_id = %s AND twitterprofile.user_id = %s LIMIT 1; args=(1, 10)'
>>>>
>>>> Storm sends a query to the database each time.
>>>>
>>>> For example, we have a some util:
>>>>
>>>> def myutil(user_id, *args, **kwargs):
>>>> context_id = get_context_from_mongodb_redis_memcache_environment_etc(user_id, *args, **kwargs)
>>>> twitter_profile = store.get(TwitterProfile, (context_id, user_id))
>>>> return twitter_profile.some_attr
>>>>
>>>> In this case, Storm will send a query to the database each time.
>>>>
>>>> The similar situation for non-existent relation.
>>>>
>>>> In [20]: u = store.get(StUser, 10)
>>>> base.py:50 =>
>>>> u'(0.001) SELECT ... FROM user WHERE user.id = %s LIMIT 1; args=(10,)'
>>>>
>>>>
>>>> In [22]: u.profile
>>>> base.py:50 =>
>>>> u'(0.001) SELECT ... FROM userprofile WHERE userprofile.user_id = %s LIMIT 1; args=(10,)'
>>>>
>>>> In [23]: u.profile
>>>> base.py:50 =>
>>>> u'(0.001) SELECT ... FROM userprofile WHERE userprofile.user_id = %s LIMIT 1; args=(10,)'
>>>>
>>>> In [24]: u.profile
>>>> base.py:50 =>
>>>> u'(0.001) SELECT ... FROM userprofile WHERE userprofile.user_id = %s LIMIT 1; args=(10,)'
>>>>
>>>> I've created a temporary patch, to reduce number of DB queries (see bellow). But I am sure that a solution can be more elegant (on library level).
>>>>
>>>>
>>>> class NonexistentCache(list):
>>>>
>>>> _size = 1000
>>>>
>>>> def add(self, val):
>>>> if val in self:
>>>> self.remove(val)
>>>> self.insert(0, val)
>>>> if len(self) > self._size:
>>>> self.pop()
>>>>
>>>>
>>>> class Store(StoreOrig):
>>>>
>>>> def __init__(self, database, cache=None):
>>>> StoreOrig.__init__(self, database, cache)
>>>> self.nonexistent_cache = NonexistentCache()
>>>>
>>>> def get(self, cls, key, exists=False):
>>>> """Get object of type cls with the given primary key from the database.
>>>>
>>>> This method is patched to cache nonexistent values to reduce number of DB-queries.
>>>> If the object is alive the database won't be touched.
>>>>
>>>> @param cls: Class of the object to be retrieved.
>>>> @param key: Primary key of object. May be a tuple for composed keys.
>>>>
>>>> @return: The object found with the given primary key, or None
>>>> if no object is found.
>>>> """
>>>>
>>>> if self._implicit_flush_block_count == 0:
>>>> self.flush()
>>>>
>>>> if type(key) != tuple:
>>>> key = (key,)
>>>>
>>>> cls_info = get_cls_info(cls)
>>>>
>>>> assert len(key) == len(cls_info.primary_key)
>>>>
>>>> primary_vars = []
>>>> for column, variable in zip(cls_info.primary_key, key):
>>>> if not isinstance(variable, Variable):
>>>> variable = column.variable_factory(value=variable)
>>>> primary_vars.append(variable)
>>>>
>>>> primary_values = tuple(var.get(to_db=True) for var in primary_vars)
>>>>
>>>> # Patched
>>>> alive_key = (cls_info.cls, primary_values)
>>>> obj_info = self._alive.get(alive_key)
>>>> if obj_info is not None and not obj_info.get("invalidated"):
>>>> return self._get_object(obj_info)
>>>>
>>>> if obj_info is None and not exists and alive_key in self.nonexistent_cache:
>>>> return None
>>>> # End of patch
>>>>
>>>> where = compare_columns(cls_info.primary_key, primary_vars)
>>>>
>>>> select = Select(cls_info.columns, where,
>>>> default_tables=cls_info.table, limit=1)
>>>>
>>>> result = self._connection.execute(select)
>>>> values = result.get_one()
>>>> if values is None:
>>>> # Patched
>>>> self.nonexistent_cache.add(alive_key)
>>>> # End of patch
>>>> return None
>>>> return self._load_object(cls_info, result, values)
>>>>
>>>> def get_multi(self, cls, keys, exists=False):
>>>> """Get objects of type cls with the given primary key from the database.
>>>>
>>>> If the object is alive the database won't be touched.
>>>>
>>>> @param cls: Class of the object to be retrieved.
>>>> @param key: Collection of primary key of objects (that may be a tuple for composed keys).
>>>>
>>>> @return: The object found with the given primary key, or None
>>>> if no object is found.
>>>> """
>>>> result = {}
>>>> missing = {}
>>>> if self._implicit_flush_block_count == 0:
>>>> self.flush()
>>>>
>>>> for key in keys:
>>>> key_orig = key
>>>> if type(key) != tuple:
>>>> key = (key,)
>>>>
>>>> cls_info = get_cls_info(cls)
>>>>
>>>> assert len(key) == len(cls_info.primary_key)
>>>>
>>>> primary_vars = []
>>>> for column, variable in zip(cls_info.primary_key, key):
>>>> if not isinstance(variable, Variable):
>>>> variable = column.variable_factory(value=variable)
>>>> primary_vars.append(variable)
>>>>
>>>> primary_values = tuple(var.get(to_db=True) for var in primary_vars)
>>>>
>>>> alive_key = (cls_info.cls, primary_values)
>>>> obj_info = self._alive.get(alive_key)
>>>> if obj_info is not None and not obj_info.get("invalidated"):
>>>> result[key_orig] = self._get_object(obj_info)
>>>> continue
>>>>
>>>> if obj_info is None and not exists and alive_key in self.nonexistent_cache:
>>>> result[key_orig] = None
>>>> continue
>>>>
>>>> missing[primary_values] = key_orig
>>>>
>>>> if not missing:
>>>> return result
>>>>
>>>> wheres = []
>>>> for i, column in enumerate(cls_info.primary_key):
>>>> wheres.append(In(cls_info.primary_key[i], tuple(v[i] for v in missing)))
>>>> where = And(*wheres) if len(wheres) > 1 else wheres[0]
>>>>
>>>> for obj in self.find(cls, where):
>>>> key_orig = missing.pop(tuple(var.get(to_db=True) for var in get_obj_info(obj).get("primary_vars")))
>>>> result[key_orig] = obj
>>>>
>>>> for primary_values, key_orig in missing.items():
>>>> self.nonexistent_cache.add((cls, primary_values))
>>>> result[key_orig] = None
>>>>
>>>> return result
>>>>
>>>> def reset(self):
>>>> StoreOrig.reset(self)
>>>> del self.nonexistent_cache[:]
>>>>
>>>>
>>>>
>>>> 2015-01-16 9:03 GMT+02:00 Free Ekanayaka <free at 64studio.com>:
>>>>>
>>>>> Hi Ivan
>>>>>
>>>>> On Thu, Jan 15, 2015 at 10:23 PM, Ivan Zakrevskyi <ivan.zakrevskyi at rebelmouse.com> wrote:
>>>>>>
>>>>>> Hi all.
>>>>>>
>>>>>> Storm has excellent caching behavior, but stores in Store._alive only existent objects. If object does not exists for some key, storm makes DB-query again and again.
>>>>>>
>>>>>> Are you planning add caching for keys of nonexistent objects to prevent DB-query?
>>>>>
>>>>>
>>>>> If an object doesn't exist in the cache it meas that either it was not yet loaded at all, or it was loaded but it's now mark as "invalidated" (for example the transaction in which it was loaded fresh has terminated).
>>>>>
>>>>> So I'm note sure what you mean in you question, but I don't think anything more that could be cached (in terms of key->object values).
>>>>>
>>>>> Cheers
>>>>>
>>>>
>>>>
>>>> --
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>>>> storm at lists.canonical.com
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>>>>
>>>
>>
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