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From els...@apache.org
Subject git commit: ACCUMULO-2901 Document cases where replication won't work well
Date Tue, 17 Jun 2014 06:11:57 GMT
Repository: accumulo
Updated Branches:
  refs/heads/master 358c13e66 -> 3ed4b2796


ACCUMULO-2901 Document cases where replication won't work well

the architecture serves itself well for a certain set of problems, but is not a universal
solution in its current implementation. provide a warning for some of those edge cases to
manage user expectations and not mislead them.


Project: http://git-wip-us.apache.org/repos/asf/accumulo/repo
Commit: http://git-wip-us.apache.org/repos/asf/accumulo/commit/3ed4b279
Tree: http://git-wip-us.apache.org/repos/asf/accumulo/tree/3ed4b279
Diff: http://git-wip-us.apache.org/repos/asf/accumulo/diff/3ed4b279

Branch: refs/heads/master
Commit: 3ed4b2796ac95808af7aef62d93e6607cedc4d62
Parents: 358c13e
Author: Josh Elser <elserj@apache.org>
Authored: Mon Jun 16 23:10:24 2014 -0700
Committer: Josh Elser <elserj@apache.org>
Committed: Mon Jun 16 23:10:24 2014 -0700

----------------------------------------------------------------------
 docs/src/main/asciidoc/chapters/replication.txt | 49 ++++++++++++++++++++
 1 file changed, 49 insertions(+)
----------------------------------------------------------------------


http://git-wip-us.apache.org/repos/asf/accumulo/blob/3ed4b279/docs/src/main/asciidoc/chapters/replication.txt
----------------------------------------------------------------------
diff --git a/docs/src/main/asciidoc/chapters/replication.txt b/docs/src/main/asciidoc/chapters/replication.txt
index 03bf67b..8755e24 100644
--- a/docs/src/main/asciidoc/chapters/replication.txt
+++ b/docs/src/main/asciidoc/chapters/replication.txt
@@ -312,3 +312,52 @@ Finally, we can enable replication on this table.
 ----
 root@primary> config -t my_table -s table.replication=true
 ----
+
+=== Extra considerations for use
+
+While this feature is intended for general-purpose use, its implementation does carry some
baggage. Like any software,
+replication is a feature that operates well within some set of use cases but is not meant
to support all use cases.
+For the benefit of the users, we can enumerate these cases.
+
+==== Latency
+
+As previously mentioned, the replication feature uses the Write-Ahead Log files for a number
of reasons, one of which
+is to prevent the need for data to be written to RFiles before it is available to be replicated.
While this can help
+reduce the latency for a batch of Mutations that have been written to Accumulo, the latency
is at least seconds to tens
+of seconds for replication once ingest is active. For a table which replication has just
been enabled on, this is likely
+to take a few minutes before replication will begin.
+
+Once ingest is active and flowing into the system at a regular rate, replication should be
occurring at a similar rate, 
+given sufficient computing resources. Replication attempts to copy data at a rate that is
to be considered low latency
+but is not a replacement for custom indexing code which can ensure near real-time referential
integrity on secondary indexes.
+
+==== Table-Configured Iterators
+
+Accumulo Iterators tend to be a heavy hammer which can be used to solve a variety of problems.
In general, it is highly
+recommended that Iterators which are applied at major compaction time are both idempotent
and associative due to the
+non-determinism in which some set of files for a Tablet might be compacted. In practice,
this translates to common patterns,
+such as aggregation, which are implemented in a manner resilient to duplication (such as
using a Set instead of a List).
+
+Due to the asynchronous nature of replication and the expectation that hardware failures
and network partitions will exist,
+it is generally not recommended to not configure replication on a table which has Iterators
set which are not idempotent.
+While the replication implementation can make some simple assertions to try to avoid re-replication
of data, it is not
+presently guaranteed that all data will only be sent to a peer once. Data will be replicated
at least once. Typically,
+this is not a problem as the VersioningIterator will automaticaly deduplicate this over-replication
because they will
+have the same timestamp; however, certain Combiners may result in inaccurate aggregations.
+
+As a concrete example, consider a table which has the SummingCombiner configured to sum all
values for
+multiple versions of the same Key. For some key, consider a set of numeric values that are
written to a table on the
+primary: [1, 2, 3]. On the primary, all of these are successfully written and thus the current
value for the given key
+would be 6, (1 + 2 + 3). Consider, however, that each of these updates to the peer were done
independently (because
+other data was also included in the write-ahead log that needed to be replicated). The update
with a value of 1 was
+successfully replicated, and then we attempted to replicate the update with a value of 2
but the remote server never
+responded. The primary does not know whether the update with a value of 2 was actually applied
or not, so the
+only recourse is to re-send the update. After we receive confirmation that the update with
a value of 2 was replicated,
+we will then replicate the update with 3. If the peer did never apply the first update of
'2', the summation is accurate.
+If the update was applied but the acknowledgement was lost for some reason (system failure,
network partition), the
+update will be resent to the peer. Because addition is non-idempotent, we have created an
inconsistency between the
+primary and peer. As such, the SummingCombiner wouldn't be recommended on a table being replicated.
+
+While there are changes that could be made to the replication implementation which could
attempt to mitigate this risk,
+presently, it is not recommended to configure Iterators or Combiners which are not idempotent
to support cases where
+inaccuracy of aggregations is not acceptable.


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