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https://issues.apache.org/jira/browse/SOLR-1301?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12806547#action_12806547
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Ted Dunning commented on SOLR-1301:
-----------------------------------
It is critical to put indexes in the task local area on both local and hdfs
storage areas not just because of task cleanup, but also because a task may be
run more than once. Hadoop handles all the race conditions that would
otherwise happen as a result.
> Solr + Hadoop
> -------------
>
> Key: SOLR-1301
> URL: https://issues.apache.org/jira/browse/SOLR-1301
> Project: Solr
> Issue Type: Improvement
> Affects Versions: 1.4
> Reporter: Andrzej Bialecki
> Fix For: 1.5
>
> Attachments: commons-logging-1.0.4.jar,
> commons-logging-api-1.0.4.jar, hadoop-0.19.1-core.jar, hadoop.patch,
> log4j-1.2.15.jar, README.txt, SOLR-1301.patch, SOLR-1301.patch,
> SOLR-1301.patch, SOLR-1301.patch, SOLR-1301.patch, SolrRecordWriter.java
>
>
> This patch contains a contrib module that provides distributed indexing
> (using Hadoop) to Solr EmbeddedSolrServer. The idea behind this module is
> twofold:
> * provide an API that is familiar to Hadoop developers, i.e. that of
> OutputFormat
> * avoid unnecessary export and (de)serialization of data maintained on HDFS.
> SolrOutputFormat consumes data produced by reduce tasks directly, without
> storing it in intermediate files. Furthermore, by using an
> EmbeddedSolrServer, the indexing task is split into as many parts as there
> are reducers, and the data to be indexed is not sent over the network.
> Design
> ----------
> Key/value pairs produced by reduce tasks are passed to SolrOutputFormat,
> which in turn uses SolrRecordWriter to write this data. SolrRecordWriter
> instantiates an EmbeddedSolrServer, and it also instantiates an
> implementation of SolrDocumentConverter, which is responsible for turning
> Hadoop (key, value) into a SolrInputDocument. This data is then added to a
> batch, which is periodically submitted to EmbeddedSolrServer. When reduce
> task completes, and the OutputFormat is closed, SolrRecordWriter calls
> commit() and optimize() on the EmbeddedSolrServer.
> The API provides facilities to specify an arbitrary existing solr.home
> directory, from which the conf/ and lib/ files will be taken.
> This process results in the creation of as many partial Solr home directories
> as there were reduce tasks. The output shards are placed in the output
> directory on the default filesystem (e.g. HDFS). Such part-NNNNN directories
> can be used to run N shard servers. Additionally, users can specify the
> number of reduce tasks, in particular 1 reduce task, in which case the output
> will consist of a single shard.
> An example application is provided that processes large CSV files and uses
> this API. It uses a custom CSV processing to avoid (de)serialization overhead.
> This patch relies on hadoop-core-0.19.1.jar - I attached the jar to this
> issue, you should put it in contrib/hadoop/lib.
> Note: the development of this patch was sponsored by an anonymous contributor
> and approved for release under Apache License.
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