Buffered Queues

Another group of events that use the queue tables includes the apply error transactions. When the Apply process fails to apply transactions, they are immediately moved into the error queue and are stored into the associated queue table. This keeps the buffers free for the current events that are being processed. This makes sense in terms of efficiency as the DBA may chose to attend to the error transactions at later time while keeping the Apply process moving forward.

What is SYS.AnyData?

Events of different data types can be staged into a SYS.AnyData queue.  The SYS.AnyData type is defined at the database level. Almost any type of payload can be wrapped in a SYS.AnyData queue. To do this, the convertdata_type static functions of the SYS.AnyData type can be used.  In this case, data type is the type of object to wrap. These functions take the object as input and return a SYS.AnyData object. Since the database table columns routinely have different data types in their definition, the Streams processes deal the columns a uniform data type during the data flow. At the end of Streams flow, they are converted back to the original data type.  The transmission of a single, uniform data type facilitates the smooth, consistent, and efficient exchange of data.

Users and applications may enqueue events into a SYS.AnyData queue or into a typed queue. Remember, a typed queue can stage events of one specific data type only.