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Triple backticks were inadvertantly used in place of double backticks. Change-Id: I0703335ddaa6c1c238053e2283b0a76127c84b43
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78 lines
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..
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Licensed under the Apache License, Version 2.0 (the "License"); you may
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not use this file except in compliance with the License. You may obtain
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a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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License for the specific language governing permissions and limitations
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under the License.
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==========
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Glossary
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==========
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Messaging Service Concepts
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==========================
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The Messaging Service is a multi-tenant, message queue implementation that
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utilizes a RESTful HTTP interface to provide an asynchronous communications
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protocol, which is one of the main requirements in today’s scalable applications.
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.. glossary::
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Queue
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Queue is a logical entity that groups messages. Ideally a queue is created
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per work type. For example, if you want to compress files, you would create
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a queue dedicated for this job. Any application that reads from this queue
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would only compress files.
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Message
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Message is sent through a queue and exists until it is deleted by a recipient
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or automatically by the system based on a TTL (time-to-live) value.
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Claim
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Claim is a mechanism to mark messages so that other workers will not process the same message.
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Worker
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Worker is an application that reads one or multiple messages from the queue.
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Producer
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Producer is an application that creates messages in one or multiple queues.
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Publish - Subscribe
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Publish - Subscribe is a pattern where all worker applications have access
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to all messages in the queue. Workers can not delete or update messages.
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Producer - Consumer
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Producer - Consumer is a pattern where each worker application that reads
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the queue has to claim the message in order to prevent duplicate processing.
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Later, when the work is done, the worker is responsible from deleting the
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message. If message is not deleted in a predefined time (claim TTL), it can
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be claimed by other workers.
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Message TTL
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Message TTL is time-to-live value and defines how long a message will be accessible.
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Claim TTL
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Claim TTL is time-to-live value and defines how long a message will be in
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claimed state. A message can be claimed by one worker at a time.
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Queues Database
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Queues database stores the information about the queues and the messages
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within these queues. Storage layer has to guarantee durability and availability of the data.
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Sharding
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If sharding enabled, queuing service uses multiple queues databases in order
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to scale horizontally. A shard (queues database) can be added anytime without
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stopping the service. Each shard has a weight that is assigned during the
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creation time but can be changed later. Sharding is done by queue which
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indicates that all messages for a particular queue can be found in the same shard (queues database).
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Catalog Database
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If sharding is enabled, catalog database has to be created. Catalog database
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maintains ``queues`` to ``queues database`` mapping. Storage layer has
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to guarantee durability and availability of data.
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