Regarding the startup order of dynamodb streams and lambdas.

The other answers for this question seem to be slightly vague and do not allow you to fully understand the ordering of items in the stream. While some answers highlight key parts, they tend not to provide you with the big picture. Let me try to explain how streams order items:

First of all let's understand what a DynamoDB Partition is:

To write an item to the table, DynamoDB calculates the hash value of the partition key to determine which partition should contain the item. In that partition, several items could have the same partition key value. So DynamoDB stores the item among the others with the same partition key, in ascending order by sort key.

Next the relationship between partitions and shards:

Each partition on a DynamoDB maps 1:1 with a DynamoDB (active) Stream Shard. This means that items which share the same partition key will enter the same shard on the DynamoDB Stream (an important caveat coming later).

Finally the relationship between shards and Lambda consumers:

Each shard on your DynamoDB stream maps 1:1 with a Lambda container. This means that all items which enter a given shard will be processed by the same Lambda function, and they will maintain the same order in which they were written to the partition.

In summary of the above 3 points:

The relationship between partition:shard:lambda is 1:1:1. We now know that each item which is written to any given partition will enter a given shard in the order they were written and ultimately be consumed in Lambda maintaining the same ordering.

Now the caveat:

For the most part items which share the same partition key will share the same partition, this would mean that the ordering is maintained for an Item Collection (all items sharing the same partition key). However, it is not guaranteed that Item Collections will remain on the same partition, an Item Collection can be split across multiple partitions for many reasons:

1. The throughput for that Item Collection exceeds the hard limit of 1000WCU or 3000RCU and Adaptive Capacity has split the collection to provide you more throughput.
2. The Item Collection exceeded the 10GB storage size limit of a partition and the collection to provide you more storage.

It is for this reason that DynamoDB Streams guarantees ordering only at the item level, the item being an item which shares both the same partition and sort key.

DynamoDB Streams captures a time-ordered sequence of item-level modifications in any DynamoDB table and stores this information in a log for up to 24 hours. Applications can access this log and view the data items as they appeared before and after they were modified, in near-real time.

A DynamoDB stream is an ordered flow of information about changes to items in a DynamoDB table. When you enable a stream on a table, DynamoDB captures information about every modification to data items in the table.

Direct Answer:

Next, add the following data in order.

① groupId = 1, seq = 1
② groupId = 1, seq = 2
③ groupId = 1, seq = 3
④ groupId = 1, seq = 4

As these items share a different sort key, then the ordering is not guaranteed as mentioned above. While its not guaranteed, for the most part it will be in order but you should be aware of the aforementioned caveat and ensure you do not take a dependency on the items being in order.