Introduce SessionIdGenerationStrategy

Closes gh-11

spring-session-docs/modules/ROOT/nav.adoc

@@ -17,6 +17,7 @@
 ** XML Configuration
 * xref:configurations.adoc[Configurations]
 ** xref:configuration/redis.adoc[Redis]
+** xref:configuration/common.adoc[Common Configurations]
 * xref:http-session.adoc[HttpSession Integration]
 * xref:web-socket.adoc[WebSocket Integration]
 * xref:web-session.adoc[WebSession Integration]

spring-session-docs/modules/ROOT/pages/configuration/common.adoc

@@ -0,0 +1,57 @@
+[[common-configurations]]
+= Common Configurations
+
+This section contains common configurations that applies to all or most Spring Session modules.
+It contains configuration examples for the following use cases:
+
+- I need to <<changing-how-session-ids-are-generated,change the way that Session IDs are generated>>
+
+[[changing-how-session-ids-are-generated]]
+== Changing How Session IDs Are Generated
+
+By default, Spring Session uses `UuidSessionIdGenerationStrategy` which, in turn, uses a `java.util.UUID` to generate a session id.
+There might be scenarios where it may be better to include other characters to increase entropy, or you may want to use a different algorithm to generate the session id.
+To change this, you can provide a custom `SessionIdGenerationStrategy` bean:
+
+.Changing How Session IDs Are Generated
+[tabs]
+======
+Java::
++
+[source,java,role="primary"]
+----
+@Bean
+public SessionIdGenerationStrategy sessionIdGenerationStrategy() {
+    return new MySessionIdGenerationStrategy();
+}
+
+class MySessionIdGenerationStrategy implements SessionIdGenerationStrategy {
+
+    @Override
+    public String generate() {
+        // ...
+    }
+
+}
+----
+======
+
+After exposing your `SessionIdGenerationStrategy` bean, Spring Session will use it to generate session ids.
+
+If you are manually configuring your `SessionRepository` bean (instead of using `@EnableRedisHttpSession`, for example), you can set the `SessionIdGenerationStrategy` directly on the `SessionRepository` implementation:
+
+.Setting `SessionIdGenerationStrategy` directly into `SessionRepository` implementation
+[tabs]
+======
+Java::
++
+[source,java,role="primary"]
+----
+@Bean
+public RedisSessionRepository redisSessionRepository(RedisOperations redisOperations) {
+    RedisSessionRepository repository = new RedisSessionRepository(redisOperations)
+    repository.setSessionIdGenerationStrategy(new MySessionIdGenerationStrategy());
+    return repository;
+}
+----
+======

spring-session-docs/modules/ROOT/pages/whats-new.adoc

@@ -1 +1,3 @@
 = What's New
+
+- xref:configuration/common.adoc#changing-how-session-ids-are-generated[docs] - https://github.com/spring-projects/spring-session/issues/11[gh-11] - Introduce `SessionIdGenerationStrategy` to allow custom session id generation