Oracle Advanced Queuing (AQ)¶
Oracle Advanced Queuing is a highly configurable and scalable messaging feature of Oracle Database. It has interfaces in various languages, letting you integrate multiple tools in your architecture.
cx_Oracle 7.2 introduced an updated interface for Oracle Advanced Queuing.
There are Advanced Queuing examples in the GitHub examples directory.
Creating a Queue¶
Before being used, queues need to be created in the database, for example in SQL*Plus:
begin dbms_aqadm.create_queue_table('MY_QUEUE_TABLE', 'RAW'); dbms_aqadm.create_queue('DEMO_RAW_QUEUE', 'MY_QUEUE_TABLE'); dbms_aqadm.start_queue('DEMO_RAW_QUEUE'); end; /
This examples creates a RAW queue suitable for sending string or raw bytes messages.
To send messages in Python you connect and get a queue. The queue can be used for enqueuing, dequeuing, or both as needed.
queue = connection.queue("DEMO_RAW_QUEUE")
Now messages can be queued using
Queue.enqOne(). To send three
PAYLOAD_DATA = [ "The first message", "The second message", "The third message" ] for data in PAYLOAD_DATA: queue.enqOne(connection.msgproperties(payload=data)) connection.commit()
Since the queue sending the messages is a RAW queue, the strings in this
example will be internally encoded to bytes using
before being enqueued.
Dequeuing is performed similarly. To dequeue a message call the method
Queue.deqOne() as shown. Note that if the message is expected to be a
string, the bytes must be decoded using
queue = connection.queue("DEMO_RAW_QUEUE") msg = queue.deqOne() connection.commit() print(msg.payload.decode(connection.encoding))
Using Object Queues¶
Named Oracle objects can be enqueued and dequeued as well. Given an object
CREATE OR REPLACE TYPE udt_book AS OBJECT ( Title VARCHAR2(100), Authors VARCHAR2(100), Price NUMBER(5,2) ); /
And a queue that accepts this type:
begin dbms_aqadm.create_queue_table('BOOK_QUEUE_TAB', 'UDT_BOOK'); dbms_aqadm.create_queue('DEMO_BOOK_QUEUE', 'BOOK_QUEUE_TAB'); dbms_aqadm.start_queue('DEMO_BOOK_QUEUE'); end; /
You can queue messages:
booksType = connection.gettype("UDT_BOOK") queue = connection.queue("DEMO_BOOK_QUEUE", booksType) book = booksType.newobject() book.TITLE = "Quick Brown Fox" book.AUTHORS = "The Dog" book.PRICE = 123 queue.enqOne(connection.msgproperties(payload=book)) connection.commit()
Dequeuing is done like this:
booksType = connection.gettype("UDT_BOOK") queue = connection.queue("DEMO_BOOK_QUEUE", booksType) msg = queue.deqOne() connection.commit() print(msg.payload.TITLE) # will print Quick Brown Fox
Changing Queue and Message Options¶
Enqueue options can be set. For example, to make it so that an explicit
commit() on the connection is not needed to commit
queue = connection.queue("DEMO_RAW_QUEUE") queue.enqOptions.visibility = cx_Oracle.ENQ_IMMEDIATE
Dequeue options can also be set. For example, to specify not to block on dequeuing if no messages are available:
queue = connection.queue("DEMO_RAW_QUEUE") queue.deqOptions.wait = cx_Oracle.DEQ_NO_WAIT
Message properties can be set when enqueuing. For example, to set an expiration of 60 seconds on a message:
This means that if no dequeue operation occurs within 60 seconds that the message will be dropped from the queue.
Bulk Enqueue and Dequeue¶
messages = [ "The first message", "The second message", "The third message", ] queue = connection.queue("DEMO_RAW_QUEUE") queue.enqMany(connection.msgproperties(payload=m) for m in messages) connection.commit()
Queue.enqMany() in parallel on different connections
acquired from the same pool may fail due to Oracle bug 29928074. Ensure that
this function is not run in parallel, use standalone connections or connections
from different pools, or make multiple calls to
Queue.deqMany() call is not affected.
To dequeue multiple messages at one time, use
takes an argument specifying the maximum number of messages to dequeue at one
for m in queue.deqMany(maxMessages=10): print(m.payload.decode(connection.encoding))
Depending on the queue properties and the number of messages available to dequeue, this code will print out from zero to ten messages.