[GitHub] gerlowskija commented on a change in pull request #594: SOLR-13259: Add new section on Reindexing in Solr

[GitBox <gi...@apache.org>]

gerlowskija commented on a change in pull request #594: SOLR-13259: Add new section on Reindexing in Solr
URL: https://github.com/apache/lucene-solr/pull/594#discussion_r261634857
 
 

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 File path: solr/solr-ref-guide/src/reindexing.adoc
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+= Reindexing
+// 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.
+
+There are several types of changes to Solr configuration that require you to reindex your data.
+
+These changes include editing properties of fields or field types; adding fields, field types, or copy field rules;
+upgrading Solr; and some system configuration properties.
+
+It's important to be aware that many changes require reindexing, because there are times when not reindexing
+can have negative consequences for Solr as a system, or for the ability of your users to find what they are looking for.
+
+There is no process in Solr for programmatically reindexing data. When we say "reindex", we mean, literally,
+"index it again". However you got the data into the index the first time, you will run that process again.
+
+Reindexing is recommended during upgrades, so in addition to covering what types of configuration changes should trigger a reindex, this section will also cover strategies for reindexing.
+
+== Changes that Require Reindex
+
+=== Schema Changes
+
+All changes to a collection's schema require reindexing. This is because many of the available options are only
+applied during the indexing process. Solr simply has no way to implement the desired change without reindexing
+the data.
+
+To understand the general reason why reindexing is ever required, it's helpful to understand the relationship between
+Solr's schema and the underlying Lucene index. Lucene does not use a schema, it is a Solr-only concept. When you delete
+a field from Solr's schema, it does not modify Lucene's index in any way. When you add a field to Solr's schema, the
+field does not exist in Lucene's index until a document that contains the field is indexed.
+
+This means that there are many types of schema changes that cannot be reflected in the index simply by modifying
+Solr's schema. This is different from most database models where schemas are used. With regard to indexing, Solr's
+schema acts like a rulebook for indexing documents by telling Lucene how to interpret the data being sent. Once the
+documents are in Lucene, Solr's schema has no control over the underlying data structure.
+
+==== Adding or Deleting Fields
+
+If you add or delete a field from Solr's schema, it's strongly recommended to reindex.
+
+When you add a field, you generally do so with the intent to use the field in some way.
+Since documents were indexed before the field was added, the index will not hold any references to the field for earlier documents.
+If you want to use the new field for faceting, for example, the new field facet will not include any documents that were not indexed with the new field.
+
+There is a slightly different situation when deleting a field.
+In this case, since simply removing the field from the schema doesn't change anything about the index, the field will still be in the index until the documents are reindexed.
+In fact, Lucene may keep a reference to a deleted field _forever_ (see also https://issues.apache.org/jira/browse/LUCENE-1761[LUCENE-1761]).
+This may only be an issue for your environment if you try to add a field that has the same name a deleted field,
 
 Review comment:
   "same name as a"

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