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From "Sankar Hariappan (JIRA)" <>
Subject [jira] [Resolved] (HIVE-16676) Enable concurrent RENAME during bootstrap REPL DUMP
Date Sat, 02 Mar 2019 18:16:00 GMT


Sankar Hariappan resolved HIVE-16676.
    Resolution: Won't Do

REPL DUMP is really fast and rename is not a frequent operation. So, closing this.

> Enable concurrent RENAME during bootstrap REPL DUMP
> ---------------------------------------------------
>                 Key: HIVE-16676
>                 URL:
>             Project: Hive
>          Issue Type: Sub-task
>          Components: repl
>    Affects Versions: 2.1.0
>            Reporter: Sankar Hariappan
>            Assignee: Sankar Hariappan
>            Priority: Major
> Currently, concurrent renames are disabled while bootstrap in progress. If enabled, then
lead to below highlighted problems. 
> For bootstrap dump, if the table is renamed after fetching the table names, then new
table will be missing in the dump and so the target database doesn't have both old and new
table. During incremental replication, later RENAME events will be noop as the old table doesn't
exist in target. This leads to data loss as the table never gets replicated in the target.
> To generalise the solution for this issue, the following logic is proposed.
> 1. In Bootstrap dump logic, after successfully dumping all tables to dumpDir, just traverse
the events and if encounter any rename event, then shall do the following.
> - If target table/partition exists in dumpDir, then do nothing. The event load in target
should be idempotent and should apply events only if the event is newer than object.
> - If only source table/partition exists in dumpDir, then check if it is newer than event.
If yes, then dump the target table/partition if it exists in metastore. This is to handle
the scenario of RENAME(A->B)->Create(A)->Dump(A). In this case both A and B should
be there in the dump. In case of RENAME(A->B)->RENAME(B->A)->Dump(A), the target
table B will be missing in metastore and hence nothing to be dumped.
> - If both source and target tables/partitions are missing in dumpDir, then dump the target
table/partition alone if it exists in the metastore. If target table/partition missing in
metastore, it means some future event would’ve dropped or renamed the target table/partition
and hence nothing to be dumped.
> - If both source and target tables/partitions are there in the dumpDir, it means, both
tables/partitions are already disconnected and not a renamed version of another and hence
nothing to be dumped.
> 2. We still use bootstrapBeginReplID as the lastReplID of the bootstrap dump. This ensures
REPL STATUS return it (same as current code) and the next incremental dump won’t miss any
> 3. We won’t combine bootstrap + incremental in a single dump. This is complex as we
won’t be able to predict the previous state of any object while checking the applicability
of any event. Just need to ensure that in case of rename either source or target table exist
in the dump to avoid loss of data.
> 4. It is expected that user will trigger incremental dump/load immediately after bootstrap
to ensure consistent state of the database.
> 5.Incremental REPL LOAD: Modify the event load behaviour in target as follows,
> - If the object exists, apply the event only if the event is newer than the object for
any type of event. In current code, we do blind replace for some event types.
> - If the object is missing, then just ignore the event. No need to create the object
which will be ultimately dropped or renamed by future applied event. In current code, we create
the object with metadata available for CREATE and ALTER events whereas TRUNCATE and RENAME
> Object will be missed in 2 cases. When it is renamed or dropped.
> - If replace/create is allowed, then for events like alter, rename shall lead to inconsistent
> - In case of CREATE_TABLE, we’ll refer to database lastReplId to decide if table needs
to be created or not.
> - In case of ADD_PARTITION, we’ll refer to table object’s lastReplId to decide if
partition to be added or not.
> - ALTER, DROP, RENAME, TRUNCATE and INSERT events should be noop if the corresponding
object is missing.
> 6. Above logic works for RENAME as well. 
> - If rename event found the source table exists in metastore, then check if the event
is newer than the object, and if yes, then apply rename. In this case, it is always guaranteed
that the target table doesn't exist.
> - If source table is missing, then just skip the event as rename would’ve already applied.
It is not necessary to check the target table as it may or may not exist. It will not exist
if it is dropped or renamed. However, it is safe to assume that current rename was already
applied and hence source table is missing.

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