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From "Wilfred Spiegelenburg (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-15621) S3Guard: Implement time-based (TTL) expiry for Authoritative Directory Listing
Date Wed, 03 Oct 2018 06:13:00 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-15621?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16636502#comment-16636502
] 

Wilfred Spiegelenburg commented on HADOOP-15621:
------------------------------------------------

[~gabor.bota] and [~fabbri]: this checkin has broken the build.

Based on the commit message two new files have been missed on checkin:
hadoop-tools/hadoop-aws/src/main/java/org/apache/hadoop/fs/s3a/s3guard/ExpirableMetadata.java
hadoop-tools/hadoop-aws/src/test/java/org/apache/hadoop/fs/s3a/ITestS3GuardTtl.java

Can you please check, I cannot reopen this jira as i do not seem to have the permission in
the hadoop-common project

> S3Guard: Implement time-based (TTL) expiry for Authoritative Directory Listing
> ------------------------------------------------------------------------------
>
>                 Key: HADOOP-15621
>                 URL: https://issues.apache.org/jira/browse/HADOOP-15621
>             Project: Hadoop Common
>          Issue Type: Sub-task
>          Components: fs/s3
>    Affects Versions: 3.0.0-beta1
>            Reporter: Aaron Fabbri
>            Assignee: Gabor Bota
>            Priority: Major
>             Fix For: 3.3.0
>
>         Attachments: HADOOP-15621.001.patch, HADOOP-15621.002.patch
>
>
> Similar to HADOOP-13649, I think we should add a TTL (time to live) feature to the Dynamo
metadata store (MS) for S3Guard.
> This is a similar concept to an "online algorithm" version of the CLI prune() function,
which is the "offline algorithm".
> Why: 
>  1. Self healing (soft state): since we do not implement transactions around modification
of the two systems (s3 and metadata store), certain failures can lead to inconsistency between
S3 and the metadata store (MS) state. Having a time to live (TTL) on each entry in S3Guard
means that any inconsistencies will be time bound. Thus "wait and restart your job" becomes
a valid, if ugly, way to get around any issues with FS client failure leaving things in a
bad state.
>  2. We could make manual invocation of `hadoop s3guard prune ...` unnecessary, depending
on the implementation.
>  3. Makes it possible to fix the problem that dynamo MS prune() doesn't prune directories
due to the lack of true modification time.
> How:
>  I think we need a new column in the dynamo table "entry last written time". This is
updated each time the entry is written to dynamo.
>  After that we can either
>  1. Have the client simply ignore / elide any entries that are older than the configured
TTL.
>  2. Have the client delete entries older than the TTL.
> The issue with #2 is it will increase latency if done inline in the context of an FS
operation. We could mitigate this some by using an async helper thread, or probabilistically
doing it "some times" to amortize the expense of deleting stale entries (allowing some batching
as well).
> Caveats:
>  - Clock synchronization as usual is a concern. Many clusters already keep clocks close
enough via NTP. We should at least document the requirement along with the configuration knob
that enables the feature.



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