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https://issues.apache.org/jira/browse/HDFS-13186?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16785546#comment-16785546
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Ewan Higgs commented on HDFS-13186:
-----------------------------------
[~fabbri]
{quote}What is the motivation for this? Even if not part of FileSystem it is
more surface area we need to deal with.{quote}
The motivation for this is to be able to write files to FileSystems in parallel
and surviving upload failures without having to restart the entire upload. One
immediate use case is that Tiered Storage can write files from datanodes to a
synchronization endpoint without having to reassemble the files locally. The NN
can initialize the write and tell the DNs to upload files and when they are
done, the NN will commit the work. Further down the line, it's possible that a
tool like DistCp could be written in terms of this uploader to allow
users/admins to copy data from one HDFS system to another without having to
stream blocks locally to a single worker on a single DN.
> [PROVIDED Phase 2] Multipart Uploader API
> -----------------------------------------
>
> Key: HDFS-13186
> URL: https://issues.apache.org/jira/browse/HDFS-13186
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Reporter: Ewan Higgs
> Assignee: Ewan Higgs
> Priority: Major
> Fix For: 3.2.0
>
> Attachments: HDFS-13186.001.patch, HDFS-13186.002.patch,
> HDFS-13186.003.patch, HDFS-13186.004.patch, HDFS-13186.005.patch,
> HDFS-13186.006.patch, HDFS-13186.007.patch, HDFS-13186.008.patch,
> HDFS-13186.009.patch, HDFS-13186.010.patch
>
>
> To write files in parallel to an external storage system as in HDFS-12090,
> there are two approaches:
> # Naive approach: use a single datanode per file that copies blocks locally
> as it streams data to the external service. This requires a copy for each
> block inside the HDFS system and then a copy for the block to be sent to the
> external system.
> # Better approach: Single point (e.g. Namenode or SPS style external client)
> and Datanodes coordinate in a multipart - multinode upload.
> This system needs to work with multiple back ends and needs to coordinate
> across the network. So we propose an API that resembles the following:
> {code:java}
> public UploadHandle multipartInit(Path filePath) throws IOException;
> public PartHandle multipartPutPart(InputStream inputStream,
> int partNumber, UploadHandle uploadId) throws IOException;
> public void multipartComplete(Path filePath,
> List<Pair<Integer, PartHandle>> handles,
> UploadHandle multipartUploadId) throws IOException;{code}
> Here, UploadHandle and PartHandle are opaque handlers in the vein of
> PathHandle so they can be serialized and deserialized in hadoop-hdfs project
> without knowledge of how to deserialize e.g. S3A's version of a UpoadHandle
> and PartHandle.
> In an object store such as S3A, the implementation is straight forward. In
> the case of writing multipart/multinode to HDFS, we can write each block as a
> file part. The complete call will perform a concat on the blocks.
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