API Version: 2013-12-02

This operation merges two adjacent shards in a stream and combines them into a single shard to reduce the stream's capacity to ingest and transport data. Two shards are considered adjacent if the union of the hash key ranges for the two shards form a contiguous set with no gaps. For example, if you have two shards, one with a hash key range of 276...381 and the other with a hash key range of 382...454, then you could merge these two shards into a single shard that would have a hash key range of 276...454. After the merge, the single child shard receives data for all hash key values covered by the two parent shards.

MergeShards is called when there is a need to reduce the overall capacity of a stream because of excess capacity that is not being used. The operation requires that you specify the shard to be merged and the adjacent shard for a given stream. For more information about merging shards, see the Amazon Kinesis Developer Guide.

If the stream is in the ACTIVE state, you can call MergeShards. If a stream is in CREATING or UPDATING or DELETING states, then Amazon Kinesis returns a ResourceInUseException. If the specified stream does not exist, Amazon Kinesis returns a ResourceNotFoundException.

You can use the DescribeStream operation to check the state of the stream, which is returned in StreamStatus.

MergeShards is an asynchronous operation. Upon receiving a MergeShards request, Amazon Kinesis immediately returns a response and sets the StreamStatus to UPDATING. After the operation is completed, Amazon Kinesis sets the StreamStatus to ACTIVE. Read and write operations continue to work while the stream is in the UPDATING state.

You use the DescribeStream operation to determine the shard IDs that are specified in the MergeShards request.

If you try to operate on too many streams in parallel using CreateStream, DeleteStream, MergeShards or SplitShard, you will receive a LimitExceededException.

MergeShards has limit of 5 transactions per second per account.

http://docs.aws.amazon.com/kinesis/2013-12-02/APIReference/API_MergeShards.html