Update one and just one document in DynamoDB

For me, I would perhaps use a secondary index for this. Lets imagine each of the items in the pool are a separate item and your table looks like this:

CallNo	Date	Other	isTaken	User
1234	2022-09-01T00:00	Data	No
9876	2022-09-02T16:00	Data		Jeff
5678	2022-09-01T10:00	Data	No
4567	2022-09-01T12:00	Data		Lee

No, each of the available calls have the attribute isTaken with a value of "No". So then we have a Global Secondary Index which is sparse in nature, and we decide the partition key for that is the isTaken column while the sort key is Date, in which only items from the table with a value for isTaken will be stored in the index, ordered by the date:

isTaken	Date	CallNo	Other
No	2022-09-01T00:00	1234	Data
No	2022-09-01T10:00	5678	Data

So now you can see the only items in the index are the calls which are in the Available queue. So when an operator wishes to retrieve an available call, they do a Query operation on the index defining a partition key of No and a Limit=1, in which they will retrieve the oldest item in the waiting queue:

aws dynamodb query \
--table-name test1 \
--index-name available \
--key-condition-expression "isTaken = :val" \
--expression-attribute-values  '{":val":{"S":"No"}}' \
--limit 1

isTaken	Date	CallNo	Other
No	2022-09-01T00:00	1234	Data

Once an operator receives their item, they must lock it to themselves by adding their User name to it and removing the isTaken value to remove it from the available queue:

aws dynamodb update-item \
--table-name test1 \
--key '{"CallNo": {"N": "1234"}}' \
--update-expression "SET #attr_one = :attr_val REMOVE #attr_two " \
--expression-attribute-names '{"#attr_one":"User", "#attr_two: "isTaken"}' \
--expression-attribute-values '{":attr_one":{"S": "Lee"}}' \
--condition-expression "attribute_exists(#attr_two)"

The update will remove the item from the GSI by removing the value for isTaken, it uses a condition expression only to update that item if isTaken already exists, if it evaluates to false, the update will fail which means another user has already taken that call and you have to poll the available queue for the next available call.

After this, your table will look like below:

CallNo	Date	Other	isTaken	User
1234	2022-09-01T00:00	Data		Lee
9876	2022-09-02T16:00	Data		Jeff
5678	2022-09-01T10:00	Data	No
4567	2022-09-01T12:00	Data		Lee

And the Available index:

isTaken	Date	CallNo	Other
No	2022-09-01T10:00	5678	Data

Hello - You can update (or perform general CRUD operations) on a single item by following this guide.

The TLDR is essentially:

DynamoDB provides four operations for basic create, read, update, and delete (CRUD) functionality. All these operations are atomic.

PutItem — Create an item.

GetItem — Read an item.

UpdateItem — Update an item.

DeleteItem — Delete an item.

Each of these operations requires that you specify the primary key of the item that you want to work with. For example, to read an item using GetItem, you must specify the partition key and sort key (if applicable) for that item.