Spec for more reserved queue attributes

Change-Id: I52b1086c00bee9d16469a907a6ee887e14322145

specs/mitaka/more-reserved-queue-attributes.rst

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+==============================
+More reserved queue attributes
+==============================
+
+https://blueprints.launchpad.net/zaqar/+spec/more-reserved-queue-attributes
+
+Currently Zaqar supports setting metadata/attributes when user creating queue.
+However, the metadata/attributes are not used very much by Zaqar itself.
+Now we only support '_flavor' but it would be nice if we can support
+more attributes to make the queue more flexible.
+
+Problem description
+===================
+
+Now Zaqar supports max_messages_post_size, default_message_ttl,
+default_claim_ttl and default_claim_grace. All of them defined in Zaqar's
+configuration file. That means it's global setting, all queues have to share
+the same configurations. It's not really flexible from the end user
+perspective. So it would be nice if we can let end user set them and use
+the default value in configuration file if they are not defined.
+
+Proposed change
+===============
+
+The changes are not complicated, which may change two actions: Post messages
+and create claim for a queue.
+
+1. Post Message:
+   At line stable/liberty/zaqar/transport/wsgi/v2_0/messages.py#L182, we need
+   to get the queue's metadata so as to get the queue's max_messages_post_size
+   and ttl defined by user when creating the queue. If those aren't exist,
+   then just use the global configuration defined in configuration file.
+
+2. Create Claim:
+   Similar thing like above, we need to get the queue's metadata at line
+   stable/liberty/zaqar/transport/wsgi/v2_0/claims.py#L81 and then use that
+   to do the check.
+
+Drawbacks
+---------
+
+It may introduce a tiny performance impact for message posting and claim
+creating. For example, instead of just checking if the queue is existing or
+not, Zaqar need to get the queue's metadata so that to get those attributes.
+But actually, we can use 'get_metadata' replace 'exists' to check if the queue
+exist or not, and they're calling the same collection.find_one() method, so
+technically, there is no performance impact.
+
+Alternatives
+------------
+
+Just keep current way, let all the queues share the same configurations.
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  flwang (flwang@catalyst.net.nz)
+
+Work Items
+----------
+
+1. Support max_messages_size and ttl as queue attributes
+2. Support claim_ttl and claim_grace as queue attributes
+
+
+Dependencies
+============
+
+N/A
+