Github user mikewalch commented on a diff in the pull request:
https://github.com/apache/accumulo/pull/224#discussion_r103748199
--- Diff: docs/src/main/asciidoc/chapters/summaries.txt ---
@@ -0,0 +1,209 @@
+// Licensed to the Apache Software Foundation (ASF) under one or more
+// contributor license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright ownership.
+// The ASF licenses this file to You under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance with
+// the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+== Summary Statistics
+
+=== Overview
+
+Accumulo has the ability to generate summary statistics about data in a
table
+using user define functions. Currently these statistics are only
generated for
+data written to files. Data recently written to Accumulo that is still in
+memory will not contribute to summary statistics.
+
+This feature can be used to inform a user about what data is in their
table.
+Summary statistics can also be used by compaction strategies to make
decisions
+about which files to compact.
+
+Summary data is stored in each file Accumulo produces. Accumulo can gather
+summary information from across a cluster merging it along the way. In
order
+for this to be fast the, summary information should fit in cache. There
is a
+dedicated cache for summary data on each tserver with a configurable size.
In
+order for summary data to fit in cache, it should probably be small.
+
+For information on writing a custom summarizer see the javadoc for
++org.apache.accumulo.core.client.summary.Summarizer+.
+
+=== Configuring
+
--- End diff --
I think this section should better describe how to configure table
summaries and not just list the properties. The prefix property needs more
documentation on how to set. Also it looks like most are optional but prefix
property is required. This would be nice to mention.
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