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https://issues.apache.org/jira/browse/HADOOP-16279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16841112#comment-16841112
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Gabor Bota commented on HADOOP-16279:
-------------------------------------
{quote}You might be able to resolve HADOOP-14468 then.{quote}
I just commented on the issue. I will resolve it soon if no one has any further
comments on it.
{quote}
AF> why we need more prune() functions added to the MS interface
GB> That prune is for removing expired entries from the ddbms. It uses
last_updated for expiry rather than mod_time.
AF> It seems like an internal implementation detail that doesn't need to be
exposed.
{quote}
True. It is an internal impl question. I think we could even go with merging
the two list internally: so we would prune with mod_time and last_updated.
{quote}
AF> Can we claim that last_updated (metastore write time) >= mod_time?
{quote}
Sure we can. Whenever we access a file's metadata (eg. do a HEAD or a GET) and
the file already existed on S3, the {{last_updated}} field will be updated to
the current time, but the mod_time will be what's in the file descriptor in S3.
This is a very important detail, this is the reason why we use a different
field in the first place for TTL in auth dirs. It tells how fresh the metadata
is.
{quote}
AF> smarter logic that allows you set a policy for handling S3 versus MS
conflicts
GB> So basically what you mean is to add a conflict resolution algorithm when
an entry is expired?
AF> Not so much when entry is expired, but when data from S3 conflicts with
data from MS. For example, MS has tombstone but S3 says file exists.
{quote}
I would say this is out of scope for this issue. We would like to solve only
the metadata expiry with this, and not add policies for conflict resolution.
> S3Guard: Implement time-based (TTL) expiry for entries (and tombstones)
> -----------------------------------------------------------------------
>
> Key: HADOOP-16279
> URL: https://issues.apache.org/jira/browse/HADOOP-16279
> Project: Hadoop Common
> Issue Type: Sub-task
> Components: fs/s3
> Reporter: Gabor Bota
> Assignee: Gabor Bota
> Priority: Major
>
> In HADOOP-15621 we implemented TTL for Authoritative Directory Listings and
> added {{ExpirableMetadata}}. {{DDBPathMetadata}} extends {{PathMetadata}}
> extends {{ExpirableMetadata}}, so all metadata entries in ddb can expire, but
> the implementation is not done yet.
> To complete this feature the following should be done:
> * Add new tests for metadata entry and tombstone expiry to {{ITestS3GuardTtl}}
> * Implement metadata entry and tombstone expiry
> I would like to start a debate on whether we need to use separate expiry
> times for entries and tombstones. My +1 on not using separate settings - so
> only one config name and value.
> ----
> Notes:
> * In HADOOP-13649 the metadata TTL is implemented in LocalMetadataStore,
> using an existing feature in guava's cache implementation. Expiry is set with
> {{fs.s3a.s3guard.local.ttl}}.
> * LocalMetadataStore's TTL and this TTL is different. That TTL is using the
> guava cache's internal solution for the TTL of these entries. This is an
> S3AFileSystem level solution in S3Guard, a layer above all metadata store.
> * This is not the same, and not using the [DDB's TTL
> feature|https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/TTL.html].
> We need a different behavior than what ddb promises: [cleaning once a day
> with a background
> job|https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/howitworks-ttl.html]
> is not usable for this feature - although it can be used as a general
> cleanup solution separately and independently from S3Guard.
> * Use the same ttl for entries and authoritative directory listing
> * All entries can be expired. Then the returned metadata from the MS will be
> null.
> * Add two new methods pruneExpiredTtl() and pruneExpiredTtl(String keyPrefix)
> to MetadataStore interface. These methods will delete all expired metadata
> from the ms.
> * Use last_updated field in ms for both file metadata and authoritative
> directory expiry.
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