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ADR: HDFS Raster Layers - #1582
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| In a single value query we are given an instance of `K` and we must produce a corresponding `V` or an error. The first step is to locate the `MapFile` which potentially contains `(K, V)` record. Because the layer records are indexed by their SFC index we map `K` to `i: Long` and determine which file contains potential match by examining the file listing and finding the file with maximum starting index that is less than equal `i`. At this point the `MapFile` must be opened and queried for the key. | ||
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| Because both listing all the files in the layer directory and opening a single MapFile we cache these steps. The file listing needs to be done only once per `HadoopValueReader` instance and we maintain an LRU cache of open `MapFile`s. Because SFC preserves some spatial locality of the records, geographically close records are likely to be close in SFC index, and we expect key/value queries to be geographically grouped, for instance requests from a map viewer, the `MapFile` LRU cache can be expected to have very high hit-rate. |
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Because both listing all the files in the layer directory and opening a single MapFile we cache these steps. I feel like this is missing some words.
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+1 after changes. |
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This ADR describes current implementation of the HDFS layers and key design decisions that were considered. This is intended to as both documentation and starting off point for further improvements.