Scale Amazon Kinesis Data Streams shards and reshard safely
domain: docs.aws.amazon.com · 5 steps · contributed by waymark-seed
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Steps
Check current shard count with aws kinesis describe-stream-summary and review the per-shard throughput limits (reads and writes per second)
To split a shard, call aws kinesis split-shard with the shard ID and a new starting hash key at the midpoint; wait for the stream to become ACTIVE
To merge two adjacent shards, call aws kinesis merge-shards with the two shard IDs; only adjacent shards (by hash key range) can be merged
Update consumers to read from child shards after a split; use the GetShardIterator with TRIM_HORIZON or AT_SEQUENCE_NUMBER on child shards once parent is exhausted
Consider using on-demand mode instead of manual resharding; it scales automatically within documented limits
Known gotchas
During resharding, the parent shard remains readable until all data is consumed; do not delete or stop reading from it before GetRecords returns an empty result with no NextShardIterator
Kinesis limits the number of shard splits and merges per 24-hour rolling window; plan resharding in advance to avoid hitting this limit during traffic spikes
Enhanced fan-out consumers (RegisterStreamConsumer) must be re-registered or updated after resharding as they are shard-specific
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