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HADOOP-15400. Improve S3Guard documentation on Authoritative Mode implementation. (Contributed by Gabor Bota)

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Sean Mackrory 2018-08-07 13:55:59 -06:00
parent 38784f95fe
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hadoop-tools/hadoop-aws/src/site/markdown/tools/hadoop-aws/s3guard.md
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@ -100,7 +100,51 @@ More settings will may be added in the future.
Currently the only Metadata Store-independent setting, besides the
implementation class above, is the *allow authoritative* flag.
It is recommended that you leave the default setting here:
The _authoritative_ expression in S3Guard is present in two different layers, for
two different reasons:
* Authoritative S3Guard
* S3Guard can be set as authoritative, which means that an S3A client will
avoid round-trips to S3 when **getting directory listings** if there is a fully
cached version of the directory stored in metadata store.
* This mode can be set as a configuration property
`fs.s3a.metadatastore.authoritative`
* All interactions with the S3 bucket(s) must be through S3A clients sharing
the same metadata store.
* This is independent from which metadata store implementation is used.
* Authoritative directory listings (isAuthoritative bit)
* Tells if the stored directory listing metadata is complete.
* This is set by the FileSystem client (e.g. s3a) via the `DirListingMetadata`
class (`org.apache.hadoop.fs.s3a.s3guard.DirListingMetadata`).
(The MetadataStore only knows what the FS client tells it.)
* If set to `TRUE`, we know that the directory listing
(`DirListingMetadata`) is full, and complete.
* If set to `FALSE` the listing may not be complete.
* Metadata store may persist the isAuthoritative bit on the metadata store.
* Currently only `org.apache.hadoop.fs.s3a.s3guard.LocalMetadataStore`
implementation supports authoritative bit.
More on Authoritative S3Guard:
* It is not treating the MetadataStore (e.g. dynamodb) as the source of truth
in general.
* It is the ability to short-circuit S3 list objects and serve listings from
the MetadataStore in some circumstances.
* For S3A to skip S3's list objects on some path, and serve it directly from
the MetadataStore, the following things must all be true:
1. The MetadataStore implementation persists the bit
`DirListingMetadata.isAuthorititative` set when calling
`MetadataStore#put` (`DirListingMetadata`)
1. The S3A client is configured to allow metadatastore to be authoritative
source of a directory listing (`fs.s3a.metadatastore.authoritative=true`).
1. The MetadataStore has a **full listing for path** stored in it. This only
happens if the FS client (s3a) explicitly has stored a full directory
listing with `DirListingMetadata.isAuthorititative=true` before the said
listing request happens.
This configuration only enables authoritative mode in the client layer. It is
recommended that you leave the default setting here:
```xml
<property>
@ -109,9 +153,8 @@ It is recommended that you leave the default setting here:
</property>
```
Setting this to `true` is currently an experimental feature. When true, the
S3A client will avoid round-trips to S3 when getting directory listings, if
there is a fully-cached version of the directory stored in the Metadata Store.
Note that a MetadataStore MAY persist this bit. (Not MUST).
Setting this to `true` is currently an experimental feature.
Note that if this is set to true, it may exacerbate or persist existing race
conditions around multiple concurrent modifications and listings of a given