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:
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.