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cqlengine supports batch queries using the BatchQuery class. Batch queries can be started and stopped manually, or within a context manager. To add queries to the batch object, you just need to precede the create/save/delete call with a call to batch, and pass in the batch object.
You can only create, update, and delete rows with a batch query, attempting to read rows out of the database with a batch query will fail.
from cassandra.cqlengine.query import BatchQuery #using a context manager with BatchQuery() as b: now = datetime.now() em1 = ExampleModel.batch(b).create(example_type=0, description="1", created_at=now) em2 = ExampleModel.batch(b).create(example_type=0, description="2", created_at=now) em3 = ExampleModel.batch(b).create(example_type=0, description="3", created_at=now) # -- or -- #manually b = BatchQuery() now = datetime.now() em1 = ExampleModel.batch(b).create(example_type=0, description="1", created_at=now) em2 = ExampleModel.batch(b).create(example_type=0, description="2", created_at=now) em3 = ExampleModel.batch(b).create(example_type=0, description="3", created_at=now) b.execute() # updating in a batch b = BatchQuery() em1.description = "new description" em1.batch(b).save() em2.description = "another new description" em2.batch(b).save() b.execute() # deleting in a batch b = BatchQuery() ExampleModel.objects(id=some_id).batch(b).delete() ExampleModel.objects(id=some_id2).batch(b).delete() b.execute()
Typically you will not want the block to execute if an exception occurs inside the with block. However, in the case that this is desirable, it’s achievable by using the following syntax:
with BatchQuery(execute_on_exception=True) as b: LogEntry.batch(b).create(k=1, v=1) mystery_function() # exception thrown in here LogEntry.batch(b).create(k=1, v=2) # this code is never reached due to the exception, but anything leading up to here will execute in the batch.
If an exception is thrown somewhere in the block, any statements that have been added to the batch will still be executed. This is useful for some logging situations.
In order to allow secondary tasks to be chained to the end of batch, BatchQuery instances allow callbacks to be registered with the batch, to be executed immediately after the batch executes.
Multiple callbacks can be attached to same BatchQuery instance, they are executed in the same order that they are added to the batch.
The callbacks attached to a given batch instance are executed only if the batch executes. If the batch is used as a context manager and an exception is raised, the queued up callbacks will not be run.
def my_callback(*args, **kwargs): pass batch = BatchQuery() batch.add_callback(my_callback) batch.add_callback(my_callback, 'positional arg', named_arg='named arg value') # if you need reference to the batch within the callback, # just trap it in the arguments to be passed to the callback: batch.add_callback(my_callback, cqlengine_batch=batch) # once the batch executes... batch.execute() # the effect of the above scheduled callbacks will be similar to my_callback() my_callback('positional arg', named_arg='named arg value') my_callback(cqlengine_batch=batch)
Failure in any of the callbacks does not affect the batch’s execution, as the callbacks are started after the execution of the batch is complete.
By default, queries in cqlengine are LOGGED, which carries additional overhead from UNLOGGED. To explicitly state which batch type to use, simply:
from cassandra.cqlengine.query import BatchType with BatchQuery(batch_type=BatchType.Unlogged) as b: LogEntry.batch(b).create(k=1, v=1) LogEntry.batch(b).create(k=1, v=2)
cassandra- Exceptions and Enums
cassandra.cluster- Clusters and Sessions
cassandra.policies- Load balancing and Failure Handling Policies
cassandra.graph- Graph Statements, Options, and Row Factories
cassandra.metadata- Schema and Ring Topology
cassandra.metrics- Performance Metrics
cassandra.query- Prepared Statements, Batch Statements, Tracing, and Row Factories
cassandra.pool- Hosts and Connection Pools
cassandra.protocol- Protocol Features
cassandra.encoder- Encoders for non-prepared Statements
cassandra.decoder- Data Return Formats
cassandra.concurrent- Utilities for Concurrent Statement Execution
cassandra.connection- Low Level Connection Info
cassandra.timestamps- Timestamp Generation
gevent-compatible Event Loop
cassandra.io.twistedreactor- Twisted Event Loop
cassandra.cqlengine.models- Table models for object mapping
cassandra.cqlengine.columns- Column types for object mapping models
cassandra.cqlengine.query- Query and filter model objects
cassandra.cqlengine.connection- Connection management for cqlengine
cassandra.cqlengine.management- Schema management for cqlengine
cassandra.cqlengine.usertype- Model classes for User Defined Types
cassandra.datastax.graph- Graph Statements, Options, and Row Factories