Documentation for Redis Checkpoint

* Add Godoc style comments for each of the functions

redis_checkpoint.go

@@ -15,6 +15,8 @@ type RedisCheckpoint struct {
 	StreamName     string
 }
 
+// Check whether a checkpoint for a particular Shard exists. Typically used to determine whether we should
+// start processing the shard with TRIM_HORIZON or AFTER_SEQUENCE_NUMBER (if checkpoint exists).
 func (c *RedisCheckpoint) CheckpointExists(shardID string) bool {
 	val, _ := c.client.Get(c.key(shardID))
 
@@ -26,15 +28,19 @@ func (c *RedisCheckpoint) CheckpointExists(shardID string) bool {
 	}
 }
 
+// Get the current checkpoint stored for the specified shard.
 func (c *RedisCheckpoint) SequenceNumber() string {
 	return c.sequenceNumber
 }
 
+// Record a checkpoint for a shard (e.g. sequence number of last record processed by application).
+// Upon failover, record processing is resumed from this point.
 func (c *RedisCheckpoint) SetCheckpoint(shardID string, sequenceNumber string) {
 	c.client.Set(c.key(shardID), []byte(sequenceNumber))
 	c.sequenceNumber = sequenceNumber
 }
 
+// Generate a unique Redis key for storage of Checkpoint.
 func (c *RedisCheckpoint) key(shardID string) string {
 	return fmt.Sprintf("%v:checkpoint:%v:%v", c.AppName, c.StreamName, shardID)
 }