Oracle Advanced Queuing (AQ)¶
Note
cx_Oracle has a major new release under a new name and homepage python-oracledb.
New projects should install python-oracledb instead of cx_Oracle.
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.
Enqueuing 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 enqone()
. To send three
messages:
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 Connection.encoding
before being enqueued.
Dequeuing Messages¶
Dequeuing is performed similarly. To dequeue a message call the method
deqone()
as shown. Note that if the message is expected to be a
string, the bytes must be decoded using Connection.encoding
.
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
type called UDT_BOOK
:
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:
book_type = connection.gettype("UDT_BOOK")
queue = connection.queue("DEMO_BOOK_QUEUE", book_type)
book = book_type.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:
book_type = connection.gettype("UDT_BOOK")
queue = connection.queue("DEMO_BOOK_QUEUE", book_type)
msg = queue.deqone()
connection.commit()
print(msg.payload.TITLE) # will print Quick Brown Fox
Changing Queue and Message Options¶
Refer to the cx_Oracle AQ API and Oracle Advanced Queuing documentation for details on all of the enqueue and dequeue options available.
Enqueue options can be set. For example, to make it so that an explicit
call to commit()
on the connection is not needed to commit
messages:
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:
queue.enqone(connection.msgproperties(payload="Message", expiration=60))
This means that if no dequeue operation occurs within 60 seconds that the message will be dropped from the queue.
Bulk Enqueue and Dequeue¶
The enqmany()
and deqmany()
methods can be used
for efficient bulk message handling.
enqmany()
is similar to enqone()
but accepts an
array of messages:
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()
Warning
Calling 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
enqone()
instead. The function deqmany()
call
is not affected.
To dequeue multiple messages at one time, use deqmany()
. This
takes an argument specifying the maximum number of messages to dequeue at one
time:
for m in queue.deqmany(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.