I read you can have multiple consumer apps per kinesis stream.
http://docs.aws.amazon.com/kinesis/latest/dev/developing-consumers-with-kcl.html
however, I heard you can only have one consumer per shard. Is this true? I don't find any documentation to support this, and can't imagine how that could be if multiple consumers are reading from the same stream. Certainly, it doesn't mean the producer needs to repeat content in different shards for different consumers.
Kinesis Client Library starts threads in the background, each listens to 1 shard in the stream. You cannot connect to a shard over multiple threads, that is by-design.
http://docs.aws.amazon.com/kinesis/latest/dev/kinesis-record-processor-scaling.html
For example, if your application is running on one EC2 instance, and is processing one Amazon Kinesis stream that has four shards. This one instance has one KCL worker and four record processors (one record processor for every shard). These four record processors run in parallel within the same process.
In the explanation above, the term "KCL worker" refers to a Kinesis consumer application. Not the threads.
But below, the same "KCL worker" term refers to a "Worker" thread in the application; which is a runnable.
Typically, when you use the KCL, you should ensure that the number of instances does not exceed the number of shards (except for failure standby purposes). Each shard is processed by exactly one KCL worker and has exactly one corresponding record processor, so you never need multiple instances to process one shard.
See the Worker.java class in KCL source.