Dynamodb update multiple items in one transaction - amazon-web-services

Question

In my dynamodb table, I have a scenario where when I add a new item to my table, I need to also increment a counter for another entry in my table.
For instance, when USER#1 follows USER#2, I would like to increment followers count for USER#2.
HashKey
RangeKey
counter
USER1
USER2
USER3
USER2
USER2
USER2
2
I do not want to use auto-increment as I want to control how the increment happens.
Naturally, everything works as expected if I make two update calls to dynamodb. One to create the relationship between users and another to update the count for the other user.
The question is, if it is a good approach to make two such calls or would a transactWrite be a better alternative.
If so how could I make an increment using transactwrite api.
I can add items using the following approach. But I am not sure how I can increment
"TransactItems": [
{
"Put": {
"TableName": "Table",
"Item": {
"hashKey": {"S":"USER1"},
"rangeKey": {"S":"USER2"}
}
}
},
{
"Update": {
"TableName": "TABLE",
"Key": {
"hashKey": {"S":"USER2"},
"rangeKey": {"S":"USER2"}
},
"ConditionExpression": "#cefe0 = :cefe0",
"ExpressionAttributeNames": {"#cefe0":"counter"},
"ExpressionAttributeValues": ?? how do I increment here
}
}
]

Answer 1

Transactions would defintely be the best way to approach it, you can increment using SET in the UpdateExpression:
"TransactItems": [
{
"Put": {
"TableName": "Table",
"Item": {
"hashKey": {"S":"USER1"},
"rangeKey": {"S":"USER2"}
}
}
},
{
"Update": {
"TableName": "TABLE",
"Key": {
"hashKey": {"S":"USER2"},
"rangeKey": {"S":"USER2"}
},
"UpdateExpression": "SET #cefe0 = #cefe0 + :cefe0",
"ExpressionAttributeNames": {"#cefe0":"counter"},
"ExpressionAttributeValues": {"cefe0": {"N": "1"}}
}
}
]