zaqar/doc/source/glossary.rst
pengfei wang 1060dd8745 Rename shard to pool
Most of the work done in https://review.openstack.org/#/c/96463/
and here are for those left.

Change-Id: I9249747557b0784982f2fe75c1f13fd866a16616
Closes-Bug: #1333152
2014-06-24 14:19:37 +08:00

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..
Licensed under the Apache License, Version 2.0 (the "License"); you may
not use this file except in compliance with the License. You may obtain
a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
License for the specific language governing permissions and limitations
under the License.
==========
Glossary
==========
Messaging Service Concepts
==========================
The Messaging Service is a multi-tenant, message queue implementation that
utilizes a RESTful HTTP interface to provide an asynchronous communications
protocol, which is one of the main requirements in todays scalable applications.
.. glossary::
Queue
Queue is a logical entity that groups messages. Ideally a queue is created
per work type. For example, if you want to compress files, you would create
a queue dedicated for this job. Any application that reads from this queue
would only compress files.
Message
Message is sent through a queue and exists until it is deleted by a recipient
or automatically by the system based on a TTL (time-to-live) value.
Claim
Claim is a mechanism to mark messages so that other workers will not process the same message.
Worker
Worker is an application that reads one or multiple messages from the queue.
Producer
Producer is an application that creates messages in one or multiple queues.
Publish - Subscribe
Publish - Subscribe is a pattern where all worker applications have access
to all messages in the queue. Workers can not delete or update messages.
Producer - Consumer
Producer - Consumer is a pattern where each worker application that reads
the queue has to claim the message in order to prevent duplicate processing.
Later, when the work is done, the worker is responsible from deleting the
message. If message is not deleted in a predefined time (claim TTL), it can
be claimed by other workers.
Message TTL
Message TTL is time-to-live value and defines how long a message will be accessible.
Claim TTL
Claim TTL is time-to-live value and defines how long a message will be in
claimed state. A message can be claimed by one worker at a time.
Queues Database
Queues database stores the information about the queues and the messages
within these queues. Storage layer has to guarantee durability and availability of the data.
Pooling
If pooling enabled, queuing service uses multiple queues databases in order
to scale horizontally. A pool (queues database) can be added anytime without
stopping the service. Each pool has a weight that is assigned during the
creation time but can be changed later. Pooling is done by queue which
indicates that all messages for a particular queue can be found in the same pool (queues database).
Catalog Database
If pooling is enabled, catalog database has to be created. Catalog database
maintains ``queues`` to ``queues database`` mapping. Storage layer has
to guarantee durability and availability of data.