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ndb Migration Notes

This is a collection of assumptions, API / implementation differences and comments about the ndb rewrite process.

The primary differences come from:

  • Absence of "legacy" APIs provided by Google App Engine (e.g. google.appengine.api.datastore_types) as well as other environment specific features (e.g. the APPLICATION_ID environment variable)
  • Differences in Datastore APIs between the versions provided by Google App Engine and Google Clould Platform.
  • Presence of new features in Python 3 like keyword only arguments and async support

Bootstrapping

The biggest difference is in establishing a runtime context for your NDB application. The Google App Engine Python 2.7 runtime had a strong assumption that all code executed inside a web framework request-response cycle, in a single thread per request. In order to decouple from that assumption, Cloud NDB implements explicit clients and contexts. This is consistent with other Cloud client libraries.

The Client class has been introduced which by and large works the same as Datastore's Client class and uses google.auth for authentication. You can pass a credentials parameter to Client or use the GOOGLE_APPLICATION_CREDENTIALS environment variable (recommended). See [https://cloud.google.com/docs/authentication/getting-started] for details.

Once a client has been obtained, you still need to establish a runtime context, which you can do using the Client.context method.

from google.cloud import ndb

# Assume GOOGLE_APPLICATION_CREDENTIALS is set in environment
client = ndb.Client()

with client.context() as context:
    do_stuff_with_ndb()

Memcache

Because the Google App Engine Memcache service is not a part of the Google Cloud Platform, it was necessary to refactor the "memcache" functionality of NDB. The concept of a memcache has been generalized to that of a "global cache" and defined by the GlobalCache interface, which is an abstract base class. NDB provides a single concrete implementation of GlobalCache, RedisCache, which uses Redis.

In order to enable the global cache, a GlobalCache instance must be passed into the context. The Bootstrapping example can be amended as follows:

from google.cloud import ndb

# Assume GOOGLE_APPLICATION_CREDENTIALS is set in environment.
client = ndb.Client()

# Assume REDIS_CACHE_URL is set in environment (or not).
# If left unset, this will return `None`, which effectively allows you to turn
# global cache on or off using the environment.
global_cache = ndb.RedisCache.from_environment()

with client.context(global_cache=global_cache) as context:
    do_stuff_with_ndb()

context.Context had a number of methods that were direct pass-throughs to GAE Memcache. These are no longer implemented. The methods of context.Context that are affected are: memcache_add, memcache_cas, memcache_decr, memcache_delete, memcache_get, memcache_gets, memcache_incr, memcache_replace, memcache_set.

Differences (between old and new implementations)

  • The "standard" exceptions from App Engine are no longer available. Instead, we'll create "shims" for them in google.cloud.ndb.exceptions to match the class names and emulate behavior.
  • There is no replacement for google.appengine.api.namespace_manager which is used to determine the default namespace when not passed in to Key()
  • The Key() constructor (and helpers) make a distinction between unicode and str types (in Python 2). These are now unicode->str and str->bytes. However, google.cloud.datastore.Key() (the actual type we use under the covers), only allows the str type in Python 3, so much of the "type-check and branch" from the original implementation is gone. This may cause some slight differences.
  • Key.from_old_key() and Key.to_old_key() always raise NotImplementedError. Without the actual types from the legacy runtime, these methods are impossible to implement. Also, since this code won't run on legacy Google App Engine, these methods aren't needed.
  • Key.app() may not preserve the prefix from the constructor (this is noted in the docstring)
  • Key.__eq__ previously claimed to be "performance-conscious" and directly used self.__app == other.__app and similar comparisons. We don't store the same data on our Key (we just make a wrapper around google.cloud.datastore.Key), so these are replaced by functions calls self.app() == self.app() which incur some overhead.
  • The verification of kind / string ID fails when they exceed 1500 bytes. The original implementation didn't allow in excess of 500 bytes, but it seems the limit has been raised by the backend. (FWIW, Danny's opinion is that the backend should enforce these limits, not the library.)
  • Property.__creation_counter_global has been removed as it seems to have been included for a feature that was never implemented. See Issue #175 for original rationale for including it and Issue #6317 for discussion of its removal.
  • ndb uses "private" instance attributes in many places, e.g. Key.__app. The current implementation (for now) just uses "protected" attribute names, e.g. Key._key (the implementation has changed in the rewrite). We may want to keep the old "private" names around for compatibility. However, in some cases, the underlying representation of the class has changed (such as Key) due to newly available helper libraries or due to missing behavior from the legacy runtime.
  • query.PostFilterNode.__eq__ compares self.predicate to other.predicate rather than using self.__dict__ == other.__dict__
  • __slots__ have been added to most non-exception types for a number of reasons. The first is the naive "performance" win and the second is that this will make it transparent whenever ndb users refer to non-existent "private" or "protected" instance attributes
  • I dropped Property._positional since keyword-only arguments are native Python 3 syntax and dropped Property._attributes in favor of an approach using inspect.signature()
  • A bug in Property._find_methods was fixed where reverse=True was applied before caching and then not respected when pulling from the cache
  • The Property._find_methods_cache has been changed. Previously it would be set on each Property subclass and populated dynamically on first use. Now Property._FIND_METHODS_CACHE is set to {} when the Property class is created and there is another level of keys (based on fully-qualified class name) in the cache.
  • BlobProperty._datastore_type has not been implemented; the base class implementation is sufficient. The original implementation wrapped a byte string in a google.appengine.api.datastore_types.ByteString instance, but that type was mostly an alias for str in Python 2
  • BlobProperty._validate used to special case for "too long when indexed" if isinstance(self, TextProperty). We have removed this check since the implementation does the same check in TextProperty._validate.
  • The BlobProperty constructor only sets _compressed if explicitly passed. The original set _compressed always (and used False as default). In the exact same fashion the JsonProperty constructor only sets _json_type if explicitly passed. Similarly, the DateTimeProperty constructor only sets _auto_now and _auto_now_add if explicitly passed.
  • TextProperty(indexed=True) and StringProperty(indexed=False) are no longer supported (see docstrings for more info)
  • model.GeoPt is an alias for google.cloud.datastore.helpers.GeoPoint rather than an alias for google.appengine.api.datastore_types.GeoPt. These classes have slightly different characteristics.
  • The Property() constructor (and subclasses) originally accepted both unicode and str (the Python 2 versions) for name (and kind) but we only accept str.
  • The Parameter() constructor (and subclasses) originally accepted int, unicode and str (the Python 2 versions) for key but we only accept int and str.
  • When a Key is used to create a query "node", e.g. via MyModel.my_value == some_key, the underlying behavior has changed. Previously a FilterNode would be created with the actual value set to some_key.to_old_key(). Now, we set it to some_key._key.
  • The google.appengine.api.users.User class is missing, so there is a replacement in google.cloud.ndb.model.User that is also available as google.cloud.ndb.User. This does not support federated identity and has new support for adding such a user to a google.cloud.datastore.Entity and for reading one from a new-style Entity
  • The UserProperty class no longer supports auto_current_user(_add)
  • Model.__repr__ will use _key to describe the entity's key when there is also a user-defined property named key. For an example, see the class docstring for Model.
  • Future.set_exception no longer takes tb argument. Python 3 does a good job of remembering the original traceback for an exception and there is no longer any value added by manually keeping track of the traceback ourselves. This method shouldn't generally be called by user code, anyway.
  • Future.state is omitted as it is redundant. Call Future.done() or Future.running() to get the state of a future.
  • StringProperty properties were previously stored as blobs (entity_pb2.Value.blob_value) in Datastore. They are now properly stored as strings (entity_pb2.Value.string_value). At read time, a StringProperty will accept either a string or blob value, so compatibility is maintained with legacy databases.
  • The QueryOptions class from google.cloud.ndb.query, has been reimplemented, since google.appengine.datastore.datastore_rpc.Configuration is no longer available. It still uses the same signature, but does not support original Configuration methods.
  • Because google.appengine.datastore.datastore_query.Order is no longer available, the ndb.query.PropertyOrder class has been created to replace it.
  • Transaction propagation is no longer supported. This was a feature of the older Datastore RPC library which is no longer used. Starting a new transaction when a transaction is already in progress in the current context will result in an error, as will passing a value for the propagation option when starting a transaction.
  • The xg option for transactions is ignored. Previously, setting this to True, allowed writes up 5 entity groups in a transaction, as opposed to only being able to write to a single entity group. In Datastore, currently, writing up to 25 entity groups in a transaction is supported by default and there is no option to change this.
  • Datastore API does not support Entity Group metadata queries anymore, so google.cloud.ndb.metadata.EntityGroup and google.cloud.ndb.metadata.get_entity_group_version both throw a google.cloud.ndb.exceptions.NoLongerImplementedError exception when used.
  • The batch_size and prefetch_size arguments to Query.fetch and Query.fetch_async are no longer supported. These were passed through directly to Datastore, which no longer supports these options.
  • The index_list method of QueryIterator is not implemented. Datastore no longer returns this data with query results, so it is not available from the API in this way.
  • The produce_cursors query option is deprecated. Datastore always returns cursors, where it can, and NDB always makes them available when possible. This option can be passed in but it will be ignored.
  • The max argument to Model.allocate_ids and Model.allocate_ids_async is no longer supported. The Google Datastore API does not support setting a maximum ID, a feature that GAE Datastore presumably had.
  • model.get_indexes() and model.get_indexes_async() are no longer implemented, as the support in Datastore for these functions has disappeared from GAE to GCP.
  • The max_memcache_items option is no longer supported.
  • The force_writes option is no longer supported.
  • The blobstore module is no longer supported.
  • The pass_batch_into_callback argument to Query.map and Query.map_async is no longer supported.
  • The merge_future argument to Query.map and Query.map_async is no longer supported.
  • Key.urlsafe() output is subtly different: the original NDB included a GAE Datastore-specific "location prefix", but that string is neither necessary nor available on Cloud Datastore. For applications that require urlsafe() strings to be exactly consistent between versions, use Key.to_legacy_urlsafe(location_prefix) and pass in your location prefix as an argument. Location prefixes are most commonly "s~" (or "e~" in Europe) but the easiest way to find your prefix is to base64 decode any urlsafe key produced by the original NDB and manually inspect it. The location prefix will be consistent for an App Engine project and its corresponding Datastore instance over its entire lifetime.
  • Key.urlsafe outputs a "bytes" object on Python 3. This is consistent behavior and actually just a change in nomenclature; in Python 2, the "str" type referred to a bytestring, and in Python 3 the corresponding type is called "bytes". Users may notice a difficulty in incorporating urlsafe() strings in JSON objects in Python 3; that is due to a change in the json.JSONEncoder default behavior between Python 2 and Python 3 (in Python 2, json.JSONEncoder accepted bytestrings and attempted to convert them to unicode automatically, which can result in corrupted data and as such is no longer done) and does not reflect a change in NDB behavior.

Privatization

App Engine NDB exposed some internal utilities as part of the public API. A few bits of the nominally public API have been found to be de facto private. These are pieces that are omitted from public facing documentation and which have no apparent use outside of NDB internals. These pieces have been formally renamed as part of the private API:

  • eventloop has been renamed to _eventloop.
  • tasklets.get_return_value has been renamed to tasklets._get_return_value and is no longer among top level exports.
  • tasklets.MultiFuture has been renamed to tasklets._MultiFuture, removed from top level exports, and has a much simpler interface.

These options classes appear not to have been used directly by users and are not implemented—public facing API used keyword arguments instead, which are still supported:

  • ContextOptions
  • TransactionOptions

The following pieces appear to have been only used internally and are no longer implemented due to the features they were used for having been refactored:

  • Query.run_to_queue
  • tasklets.add_flow_exception
  • tasklets.make_context
  • tasklets.make_default_context
  • tasklets.QueueFuture
  • tasklets.ReducingFuture
  • tasklets.SerialQueueFuture
  • tasklets.set_context

A number of functions in the utils package appear to have only been used internally and have been made obsolete either by API changes, internal refactoring, or new features of Python 3, and are no longer implemented:

  • utils.code_info()
  • utils.decorator()
  • utils.frame_info()
  • utils.func_info()
  • utils.gen_info()
  • utils.get_stack()
  • utils.logging_debug()
  • utils.positional()
  • utils.tweak_logging()
  • utils.wrapping() (use functools.wraps instead)
  • utils.threading_local()

Bare Metal

One of the largest classes of differences comes from the use of the current Datastore API, rather than the legacy App Engine Datastore. In general, for users coding to the public interface, this won't be an issue, but users relying on pieces of the ostensibly private API that are exposed to the bare metal of the original datastore implementation will have to rewrite those pieces. Specifically, any function or method that dealt directly with protocol buffers will no longer work. The Datastore .protobuf definitions have changed significantly from the barely public API used by App Engine to the current published API. Additionally, this version of NDB mostly delegates to google.cloud.datastore for parsing data returned by RPCs, which is a significant internal refactoring.

  • ModelAdapter is no longer used. In legacy NDB, this was passed to the Datastore RPC client so that calls to Datastore RPCs could yield NDB entities directly from Datastore RPC calls. AFAIK, Datastore no longer accepts an adapter for adapting entities. At any rate, we no longer do it that way.
  • Property._db_get_value, Property._db_set_value, are no longer used. They worked directly with Datastore protocol buffers, work which is now delegated to google.cloud.datastore.
  • Property._db_set_compressed_meaning and Property._db_set_uncompressed_meaning were used by Property._db_set_value and are no longer used.
  • Model._deserialize and Model._serialize are no longer used. They worked directly with protocol buffers, so weren't really salvageable. Unfortunately, there were comments indicating they were overridden by subclasses. Hopefully this isn't broadly the case.
  • model.make_connection is no longer implemented.

Comments

  • There is rampant use (and abuse) of __new__ rather than __init__ as a constructor as the original implementation. By using __new__, sometimes a different type is used from the constructor. It seems that feature, along with the fact that pickle only calls __new__ (and never __init__) is why __init__ is almost never used.
  • The Key.__getnewargs__() method isn't needed. For pickle protocols 0 and 1, __new__ is not invoked on a class during unpickling; the state "unpacking" is handled solely via __setstate__. However, for pickle protocols 2, 3 and 4, during unpickling an instance will first be created via Key.__new__() and then __setstate__ would be called on that instance. The addition of the __getnewargs__ allows the (positional) arguments to be stored in the pickled bytes. All of the work of the constructor happens in __new__, so the call to __setstate__ is redundant. In our implementation __setstate__ is sufficient, hence __getnewargs__ isn't needed.
  • Key parts (i.e. kind, string ID and / or integer ID) are verified when a Reference is created. However, this won't occur when the corresponding protobuf for the underlying google.cloud.datastore.Key is created. This is because the Reference is a legacy protobuf message type from App Engine, while the latest (google/datastore/v1) RPC definition uses a Key.
  • There is a Property._CREATION_COUNTER that gets incremented every time a new Property() instance is created. This increment is not threadsafe. However, ndb was designed for Property() instances to be created at import time, so this may not be an issue.
  • ndb.model._BaseValue for "wrapping" non-user values should probably be dropped or redesigned if possible.
  • Since we want "compatibility", suggestions in TODO comments have not been implemented. However, that policy can be changed if desired.
  • It seems that query.ConjunctionNode.__new__ had an unreachable line that returned a FalseNode. This return has been changed to a RuntimeError just it case it is actually reached.
  • For AND and OR to compare equal, the nodes must come in the same order. So AND(a > 7, b > 6) is not equal to AND(b > 6, a > 7).
  • It seems that query.ConjunctionNode.__new__ had an unreachable line that returned a FalseNode. This return has been changed to a RuntimeError just it case it is actually reached.
  • For AND and OR to compare equal, the nodes must come in the same order. So AND(a > 7, b > 6) is not equal to AND(b > 6, a > 7).
  • The whole bytes vs. str issue needs to be considered package-wide. For example, the Property() constructor always encoded Python 2 unicode to a Python 2 str (i.e. bytes) with the utf-8 encoding. This fits in some sense: the property name in the protobuf definition is a string (i.e. UTF-8 encoded text). However, there is a bit of a disconnect with other types that use property names, e.g. FilterNode.
  • There is a giant web of module interdependency, so runtime imports (to avoid import cycles) are very common. For example model.Property depends on query but query depends on model.
  • Will need to sort out dependencies on old RPC implementations and port to modern gRPC. (Issue #6363)