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| <div class="col-sm-9 col-sm-offset-3 col-md-10 col-md-offset-2 main"> |
| <h1>Hadoop Distributed Data Store</h1> |
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| <h2 id="scm-overview">SCM Overview</h2> |
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| <p>Storage Container Manager or SCM is a very important component of ozone. SCM |
| offers block and container-based services to Ozone Manager. A container is a |
| collection of unrelated blocks under ozone. SCM and data nodes work together |
| to maintain the replication levels needed by the cluster.</p> |
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| <p>It is easier to look at a putKey operation to understand the role that SCM plays.</p> |
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| <p>To put a key, a client makes a call to KSM with the following arguments.</p> |
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| <p>– putKey(keyName, data, pipeline type, replication count)</p> |
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| <ol> |
| <li>keyName - refers to the file name.</li> |
| <li>data - The data that the client wants to write.</li> |
| <li>pipeline type - Allows the client to select the pipeline type. A pipeline |
| refers to the replication strategy used for replicating a block. Ozone |
| currently supports Stand Alone and Ratis as two different pipeline types.</li> |
| <li>replication count - This specifies how many copies of the block replica should be maintained.</li> |
| </ol> |
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| <p>In most cases, the client does not specify the pipeline type and replication |
| count. The default pipeline type and replication count are used.</p> |
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| <p>Ozone Manager when it receives the putKey call, makes a call to SCM asking |
| for a pipeline instance with the specified property. So if the client asked |
| for RATIS replication strategy and a replication count of three, then OM |
| requests SCM to return a set of data nodes that meet this capability.</p> |
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| <p>If SCM can find this a pipeline ( that is a set of data nodes) that can meet |
| the requirement from the client, then those nodes are returned to OM. OM will |
| persist this info and return a tuple consisting of {BlockID, ContainerName, and Pipeline}.</p> |
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| <p>If SCM is not able to find a pipeline, then SCM creates a logical pipeline and then returns it.</p> |
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| <p>SCM manages blocks, containers, and pipelines. To return healthy pipelines, |
| SCM also needs to understand the node health. So SCM listens to heartbeats |
| from data nodes and acts as the node manager too.</p> |
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