[improve][doc] clarify detailed behaviors for failover subscription t…

…ype (#633)

docs/concepts-messaging.md

@@ -521,6 +521,12 @@ A master consumer is picked for a non-partitioned topic or each partition of a p
 
 When the master consumer disconnects, all (non-acknowledged and subsequent) messages are delivered to the next consumer in line.
 
+::: note
+
+In some cases, a partition may have an older active consumer processing messages while a newly switched over active consumer starts receiving new messages. This may lead to message duplication or out-of-order.
+
+:::
+
 ##### Failover | Partitioned topics
 
 For partitioned topics, the broker sorts consumers by priority and lexicographical order of consumer name. 
@@ -533,25 +539,32 @@ A consumer is selected by running a module operation `mod (partition index, cons
  
  For example, in the diagram below, this partitioned topic has 2 partitions and there are 4 consumers. 
  
- Each partition has 1 active consumer and 1 stand-by consumer. 
+ Each partition has 1 active consumer and 3 stand-by consumers. 
  
- - For p0, consumer A is the master consumer, while consumer B would be the next consumer in line to receive messages if consumer A is disconnected.
+ - For P0, Consumer A is the master consumer, while Consumer B, Consumer C, and Consumer D would be the next consumer in line to receive messages if consumer A is disconnected.
 
- - For p1, consumer C is the master consumer, while consumer D would be the next consumer in line to receive messages if consumer C is disconnected.
+ - For P1, Consumer B is the master consumer, while Consumer A, Consumer C, and Consumer D would be the next consumer in line to receive messages if consumer B is disconnected.
 
- ![Failover subscriptions](/assets/pulsar-failover-subscriptions-4.svg)
+ - Moreover, if Consumer A and consumer B are disconnected, then 
+ 
+ - for P0: Consumer C is the active consumer and Consumer D is the stand-by consumer.
+ 
+ - for P1: Consumer D is the active consumer and Consumer C is the stand-by consumer.
+
+ ![Failover subscriptions](/assets/pulsar-failover-subscriptions-5.png)
 
 - If the number of partitions in a partitioned topic is **greater** than the number of consumers:
  
  For example, in the diagram below, this partitioned topic has 9 partitions and 3 consumers. 
  
- - p0, p3, and p6 are assigned to consumer A.
+ - P0, P3, and P6 are assigned to Consumer A. Consumer A is their active consumer. Consumer B and Consumer C are their stand-by consumers.
  
- - p1, p4, and p7 are assigned to consumer B.
+ - P1, P4, and P7 are assigned to Consumer B. Consumer B is their active consumer. Consumer A and Consumer C are their stand-by consumers.
  
- - p2, p5, and p8 are assigned to consumer C.
+ - P2, P5, and P8 are assigned to Consumer C. Consumer C is their active consumer. Consumer A and Consumer B are their stand-by consumers.
  
  ![Failover subscriptions](/assets/pulsar-failover-subscriptions-1.svg)
+
 ##### Failover | Non-partitioned topics
 
 - If there is one non-partitioned topic. The broker picks consumers in the order they subscribe to non-partitioned topics. 
@@ -568,9 +581,9 @@ A consumer is selected by running a module operation `mod (partition index, cons
 
  For example, in the diagram below, there are 4 non-partitioned topics and 2 consumers. 
  
- - The non-partitioned topic 1 and non-partitioned topic 4 are assigned to consumer B. 
+ - The non-partitioned topic 1 and non-partitioned topic 4 are assigned to consumer B. Consumer A is their stand-by consumer.
  
- - The non-partitioned topic 2 and non-partitioned topic 3 are assigned to consumer A.
+ - The non-partitioned topic 2 and non-partitioned topic 3 are assigned to consumer A. Consumer B is their stand-by consumer.
 
  ![Failover subscriptions](/assets/pulsar-failover-subscriptions-3.svg)
 
@@ -594,6 +607,18 @@ In the *Key_Shared* type, multiple consumers can attach to the same subscription
 
 ![Key_Shared subscriptions](/assets/pulsar-key-shared-subscriptions.svg)
 
+:::note
+
+If there is a newly switched over active consumer, it will start reading messages from the position where messages are acked by the old inactive consumer.
+
+For example, if P0 is assigned to Consumer A. Consumer A is the active consumer and Consumer B is the stand-by consumer. 
+
+- If Consumer A gets disconnected without reading any messages from P0, when Consumer C is added and becomes the new active consumer, then Consumer C will start reading messages directly from P0.
+
+- If Consumer A gets disconnected after reading messages (0,1,2,3) from P0, when Consumer C is added and becomes the active consumer, then Consumer C will start reading messages (4,5,6,7) from P0.
+
+:::
+
 There are three types of mapping algorithms dictating how to select a consumer for a given message key (or ordering key): Sticky, Auto-split Hash Range, and Auto-split Consistent Hashing. The steps for all algorithms are:
 1. The message key (or ordering key) is passed to a hash function (e.g., Murmur3 32-bit), yielding a 32-bit integer hash.
 2. That hash number is fed to the algorithm to select a consumer from the existing connected consumers.
@@ -652,7 +677,7 @@ C1 is disconnected:
 |------- C3 ------|-------------------------- C2 -----------------------|
 ```
 
-The advantages of this algorithm is that it affects only a single existing consumer upon add/delete consumer, at the expense of regions not evenly sized. Thi means some consumers gets more keys that others. The next algorithm does the other way around.
+The advantages of this algorithm is that it affects only a single existing consumer upon add/delete consumer, at the expense of regions not evenly sized. This means some consumers gets more keys that others. The next algorithm does the other way around.
 
 ##### Auto-split Consistent Hashing