This tutorial focuses on the boto interface to the Simple Queue Service from Amazon Web Services. This tutorial assumes that you have boto already downloaded and installed.
The first step in accessing SQS is to create a connection to the service. The recommended method of doing this is as follows:
>>> import boto.sqs >>> conn = boto.sqs.connect_to_region( ... "us-west-2", ... aws_access_key_id='<aws access key'>, ... aws_secret_access_key='<aws secret key>')
At this point the variable conn will point to an SQSConnection object in the US-WEST-2 region. Bear in mind that just as any other AWS service, SQS is region-specific. In this example, the AWS access key and AWS secret key are passed in to the method explicitely. Alternatively, you can set the environment variables:
and then simply call:
>>> import boto.sqs >>> conn = boto.sqs.connect_to_region("us-west-2")
In either case, conn will point to an SQSConnection object which we will use throughout the remainder of this tutorial.
Once you have a connection established with SQS, you will probably want to create a queue. In its simplest form, that can be accomplished as follows:
>>> q = conn.create_queue('myqueue')
The create_queue method will create (and return) the requested queue if it does not exist or will return the existing queue if it does. There is an optional parameter to create_queue called visibility_timeout. This basically controls how long a message will remain invisible to other queue readers once it has been read (see SQS documentation for more detailed explanation). If this is not explicitly specified the queue will be created with whatever default value SQS provides (currently 30 seconds). If you would like to specify another value, you could do so like this:
>>> q = conn.create_queue('myqueue', 120)
This would establish a default visibility timeout for this queue of 120 seconds. As you will see later on, this default value for the queue can also be overridden each time a message is read from the queue. If you want to check what the default visibility timeout is for a queue:
>>> q.get_timeout() 30
To retrieve a list of the queues for your account in the current region:
>>> conn.get_all_queues() [ Queue(https://queue.amazonaws.com/411358162645/myqueue), Queue(https://queue.amazonaws.com/411358162645/another_queue), Queue(https://queue.amazonaws.com/411358162645/another_queue2) ]
This will leave you with a list of all of your boto.sqs.queue.Queue instances. Alternatively, if you wanted to only list the queues that started with 'another':
>>> conn.get_all_queues(prefix='another') [ Queue(https://queue.amazonaws.com/411358162645/another_queue), Queue(https://queue.amazonaws.com/411358162645/another_queue2) ]
If you wish to explicitly retrieve an existing queue and the name of the queue is known, you can retrieve the queue as follows:
>>> my_queue = conn.get_queue('myqueue') Queue(https://queue.amazonaws.com/411358162645/myqueue)
This leaves you with a single boto.sqs.queue.Queue, which abstracts the SQS Queue named ‘myqueue’.
Once you have a queue setup, presumably you will want to write some messages to it. SQS doesn’t care what kind of information you store in your messages or what format you use to store it. As long as the amount of data per message is less than or equal to 256Kb, SQS won’t complain.
So, first we need to create a Message object:
>>> from boto.sqs.message import Message >>> m = Message() >>> m.set_body('This is my first message.') >>> status = q.write(m)
The write method returns a True if everything went well. If the write didn’t succeed it will either return a False (meaning SQS simply chose not to write the message for some reason) or an exception if there was some sort of problem with the request.
The technique above will work only if you use boto’s default Message payload format; however, you may have a lot of specific requirements around the format of the message data. For example, you may want to store one big string or you might want to store something that looks more like RFC822 messages or you might want to store a binary payload such as pickled Python objects.
The way boto deals with this issue is to define a simple Message object that treats the message data as one big string which you can set and get. If that Message object meets your needs, you’re good to go. However, if you need to incorporate different behavior in your message or handle different types of data you can create your own Message class. You just need to register that class with the boto queue object so that it knows that, when you read a message from the queue, it should create one of your message objects rather than the default boto Message object. To register your message class, you would:
>>> import MyMessage >>> q.set_message_class(MyMessage) >>> m = MyMessage() >>> m.set_body('This is my first message.') >>> status = q.write(m)
where MyMessage is the class definition for your message class. Your message class should subclass the boto Message because there is a small bit of Python magic happening in the __setattr__ method of the boto Message class.
So, now we have a message in our queue. How would we go about reading it? Here’s one way:
>>> rs = q.get_messages() >>> len(rs) 1 >>> m = rs >>> m.get_body() u'This is my first message'
The get_messages method also returns a ResultSet object as described above. In addition to the special attributes that we already talked about the ResultSet object also contains any results returned by the request. To get at the results you can treat the ResultSet as a sequence object (e.g. a list). We can check the length (how many results) and access particular items within the list using the slice notation familiar to Python programmers.
At this point, we have read the message from the queue and SQS will make sure that this message remains invisible to other readers of the queue until the visibility timeout period for the queue expires. If you delete the message before the timeout period expires then no one else will ever see the message again. However, if you don’t delete it (maybe because your reader crashed or failed in some way, for example) it will magically reappear in my queue for someone else to read. If you aren’t happy with the default visibility timeout defined for the queue, you can override it when you read a message:
This means that regardless of what the default visibility timeout is for the queue, this message will remain invisible to other readers for 60 seconds.
The get_messages method can also return more than a single message. By passing a num_messages parameter (defaults to 1) you can control the maximum number of messages that will be returned by the method. To show this feature off, first let’s load up a few more messages.
>>> for i in range(1, 11): ... m = Message() ... m.set_body('This is message %d' % i) ... q.write(m) ... >>> rs = q.get_messages(10) >>> len(rs) 10
Don’t be alarmed if the length of the result set returned by the get_messages call is less than 10. Sometimes it takes some time for new messages to become visible in the queue. Give it a minute or two and they will all show up.
If you want a slightly simpler way to read messages from a queue, you can use the read method. It will either return the message read or it will return None if no messages were available. You can also pass a visibility_timeout parameter to read, if you desire:
>>> m = q.read(60) >>> m.get_body() u'This is my first message'
As stated above, messages are never deleted by the queue unless explicitly told to do so. To remove a message from a queue:
>>> q.delete_message(m) 
If I want to delete the entire queue, I would use:
This will delete the queue, even if there are still messages within the queue.
The above tutorial covers the basic operations of creating queues, writing messages, reading messages, deleting messages, and deleting queues. There are a few utility methods in boto that might be useful as well. For example, to count the number of messages in a queue:
>>> q.count() 10
This can be handy but is command as well as the other two utility methods I’ll describe in a minute are inefficient and should be used with caution on queues with lots of messages (e.g. many hundreds or more). Similarly, you can clear (delete) all messages in a queue with:
Be REAL careful with that one! Finally, if you want to dump all of the messages in a queue to a local file:
>>> q.dump('messages.txt', sep='\n------------------\n')
This will read all of the messages in the queue and write the bodies of each of the messages to the file messages.txt. The option sep argument is a separator that will be printed between each message body in the file.