From 54f3a5c837551a2f3b619e14c6a737c25f698187 Mon Sep 17 00:00:00 2001
From: Mahmoud Ben Hassine <mbenhassine@pivotal.io>
Date: Thu, 25 Jan 2018 11:02:02 +0100
Subject: [PATCH] BATCH-2465: add note about the importance to have a unique
 key constraint on the sortKey

---
 spring-batch-docs/asciidoc/readersAndWriters.adoc          | 3 +++
 .../batch/item/database/JdbcPagingItemReader.java          | 6 ++++--
 .../database/support/AbstractSqlPagingQueryProvider.java   | 7 +++++--
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/spring-batch-docs/asciidoc/readersAndWriters.adoc b/spring-batch-docs/asciidoc/readersAndWriters.adoc
index 60eff6ff5..bcd663167 100644
--- a/spring-batch-docs/asciidoc/readersAndWriters.adoc
+++ b/spring-batch-docs/asciidoc/readersAndWriters.adoc
@@ -2247,6 +2247,9 @@ The `SqlPagingQueryProviderFactoryBean` requires that you specify a `select` cla
 `from` clause. You can also provide an optional `where` clause. These clauses and the
 required `sortKey` are used to build an SQL statement.
 
+NOTE: It is important to have a unique key constraint on the `sortKey` to guarantee that
+ no data is lost between executions.
+
 After the reader has been opened, it passes back one item per call to `read` in the same
 basic fashion as any other `ItemReader`. The paging happens behind the scenes when
 additional rows are needed.
diff --git a/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/JdbcPagingItemReader.java b/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/JdbcPagingItemReader.java
index 6a5d8195a..06fffaba0 100644
--- a/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/JdbcPagingItemReader.java
+++ b/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/JdbcPagingItemReader.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2006-2013 the original author or authors.
+ * Copyright 2006-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -51,7 +51,8 @@ import org.springframework.util.ClassUtils;
  * needed as {@link #read()} method is called, returning an object corresponding
  * to current position. On restart it uses the last sort key value to locate the
  * first page to read (so it doesn't matter if the successfully processed items
- * have been removed or modified).
+ * have been removed or modified). It is important to have a unique key constraint
+ * on the sort key to guarantee that no data is lost between executions.
  * </p>
  * 
  * <p>
@@ -71,6 +72,7 @@ import org.springframework.util.ClassUtils;
  * @author Thomas Risberg
  * @author Dave Syer
  * @author Michael Minella
+ * @author Mahmoud Ben Hassine
  * @since 2.0
  */
 public class JdbcPagingItemReader<T> extends AbstractPagingItemReader<T> implements InitializingBean {
diff --git a/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/support/AbstractSqlPagingQueryProvider.java b/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/support/AbstractSqlPagingQueryProvider.java
index 01398f004..d33987570 100644
--- a/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/support/AbstractSqlPagingQueryProvider.java
+++ b/spring-batch-infrastructure/src/main/java/org/springframework/batch/item/database/support/AbstractSqlPagingQueryProvider.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2006-2012 the original author or authors.
+ * Copyright 2006-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -42,11 +42,14 @@ import org.springframework.util.StringUtils;
  * Provides properties and preparation for the mandatory "selectClause" and
  * "fromClause" as well as for the optional "whereClause". Also provides
  * property for the mandatory "sortKeys".  <b>Note:</b> The columns that make up 
- * the sort key must be a true key and not just a column to order by.
+ * the sort key must be a true key and not just a column to order by. It is important
+ * to have a unique key constraint on the sort key to guarantee that no data is lost
+ * between executions.
  * 
  * @author Thomas Risberg
  * @author Dave Syer
  * @author Michael Minella
+ * @author Mahmoud Ben Hassine
  * @since 2.0
  */
 public abstract class AbstractSqlPagingQueryProvider implements PagingQueryProvider {