Google App Engine

NDB Context class

An application can use a Context to control its caching policy. Context also offers convenient asynchronous APIs for Memcache and URL Fetch integrated into NDB's asynchronous facilities; in particular, the Memcache API supports auto-batching.

Cache Management Instance Methods

clear_cache()
Clears the in-context cache.
flush()
Flush all auto-batcher queues (sending their work to the servers). The flush happens asynchronously.

Returns a Future. To wait for the flushing to finish, call this object's wait() method.

get_cache_policy()
Returns the cache policy function. This policy function takes a Key instance argument and returns a bool indicating whether the entity with this key should be cached in the in-context cache. May be None (meaning use default_cache_policy).
get_datastore_policy()
Returns the current context Datastore policy function. This policy function takes a Key instance argument and returns a bool indicating if it should use the Datastore. May be None (meaning use default_datastore_policy).
get_memcache_policy()
Returns the current memcache policy function. This policy function takes a Key instance argument and returns a bool indicating if it should be cached. May be None (meaning use default_memcache_policy).
get_memcache_timeout_policy()
Returns the current memcache timeout policy function. This policy function takes a Key instance argument and returns the desired memcache timeout in seconds (or returns zero to use the default timeout). May be None, which uses default_memcache_timeout_policy.
set_cache_policy(func)
Sets the cache policy function.

Arguments

func
This function takes a Key instance argument and returns a bool indicating whether the entity with this key should be cached in the in-context cache. May be None.
set_datastore_policy(func)
Sets the Datastore policy function.

Arguments

func
This function takes a Key instance argument and returns a bool indicating whether the entity with this key should be stored in the persistent Datastore. May be None.
set_memcache_policy(func)
Sets the memcache policy function.

Arguments

func
This function takes a Key instance argument and returns a bool indicating whether the entity with this key should be cached in the in-context cache. May be None.
set_memcache_timeout_policy(func)
Sets the memcache timeout policy function.

Arguments

func
This function takes a Key instance argument and returns the desired memcache timeout in seconds (or returns zero to use the default timeout). May be None.

Memcache and Urlfetch Instance Methods

memcache_add(key, value, time, namespace)
Async auto-batching memcache add().
memcache_cas(key, value, time, namespace)
Async auto-batching memcache Client cas() (compare-and-swap).
memcache_decr(key, delta, initial_value, namespace)
Async auto-batching memcache decr().
memcache_delete(key, seconds, namespace)
Async auto-batching memcache delete().
memcache_get(key, namespace, use_cache)
Async auto-batching memcache get().

Returns a Future whose return value is the value retrieved from memcache or None.

memcache_gets(key, namespace, use_cache)
Async auto-batching memcache gets().
memcache_incr(key, delta, initial_value, namespace)
Async auto-batching memcache incr().
memcache_replace(key, value, time, namespace)
Async auto-batching memcache replace().
memcache_set(key, value, time, namespace, use_cache)
Async auto-batching memcache set().
urlfetch(url, payload, method, headers, allow_truncated, follow_redirects, validate_certificate, deadline, callback)
Async auto-batching urlfetch fetch().

Transaction Management Methods

call_on_commit(callback)
Queues a callback to call upon successful commit of a transaction.

If not in a transaction, the callback is called immediately.

In a transaction, multiple callbacks may be registered and will be called once the transaction commits in the order in which they were registered. If the transaction fails, the callbacks will not be called.

If a callback raises an exception, it bubbles up normally. This means: If the callback is called immediately, any exception it raises will bubble up immediately. If the call is postponed until commit, remaining callbacks will be skipped and the exception will bubble up through the transaction() call. (However, the transaction is already committed at that point.)

Arguments

callback
Callback function. This callback function takes no parameters and returns nothing.
in_transaction()
Returns True if this Context represents a transaction, False otherwise.

Static Methods

default_cache_policy(key)
Checks for a _use_cache class variable on the entity's model class; if there is one, return it. Otherwise, return None.
default_datastore_policy(key)
Checks for a _use_datastore class variable on the entity's model class; if there is one, return it. Otherwise, return None.
default_memcache_policy(key)
Checks for a _use_memcache class variable on the entity's model class; if there is one, return it. Otherwise, return None.
default_memcache_timeout_policy(key)
Checks for a _memcache_timeout class variable on the entity's model class; if there is one, return it. Otherwise, return None.

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