DynamoDB: When to use what PK type?

Question

I am trying to read up on best practices on DynamoDB. I saw that DynamoDB has two PK types:

1. Hash Key
2. Hash and Range Key

From what I read, it appears the latter is like the former but supports sorting and indexing of a finite set of columns.

So my question is why ever use only a hash key without a range key? Is it a viable choice only when the table is not searched?

It'd also be great to have some general guidelines on when to use what key type. I've read several guides (including Amazon's own documentation on DynamoDB) but none of them appear to directly address this question.

Thanks

Answer 1

The choice of which key to use comes down to your Use Cases and Data Requirements for a particular scenario. For example, if you are storing User Session Data it might not make much sense using the Range Key since each record could be referenced by a GUID and accessed directly with no grouping requirements. In general terms once you know the Session Id you just get the specific item querying by the key. Another example could be storing User Account or Profile data, each user has his own and you most likely will access it directly (by User Id or something else).

However, if you are storing Order Items then the Range Key makes much more sense since you probably want to retrieve the items grouped by their Order.

In terms of the Data Model, the Hash Key allows you to uniquely identify a record from your table, and the Range Key can be optionally used to group and sort several records that are usually retrieved together. Example: If you are defining an Aggregate to store Order Items, the Order Id could be your Hash Key, and the OrderItemId the Range Key. Whenever you would like to search the Order Items from a particular Order, you just query by the Hash Key (Order Id), and you will get all your order items.

You can find below a formal definition for the use of these two keys:

"Composite Hash Key with Range Key allows the developer to create a primary key that is the composite of two attributes, a 'hash attribute' and a 'range attribute.' When querying against a composite key, the hash attribute needs to be uniquely matched but a range operation can be specified for the range attribute: e.g. all orders from Werner in the past 24 hours, or all games played by an individual player in the past 24 hours." [VOGELS]

So the Range Key adds a grouping capability to the Data Model, however, the use of these two keys also have an implication on the Storage Model:

"Dynamo uses consistent hashing to partition its key space across its replicas and to ensure uniform load distribution. A uniform key distribution can help us achieve uniform load distribution assuming the access distribution of keys is not highly skewed." [DDB-SOSP2007]

Not only the Hash Key allows to uniquely identify the record, but also is the mechanism to ensure load distribution. The Range Key (when used) helps to indicate the records that will be mostly retrieved together, therefore, the storage can also be optimized for such need.

Choosing the correct keys to represent your data is one of the most critical aspects during your design process, and it directly impacts how much your application will perform, scale and cost.

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Footnotes:

• The Data Model is the model through which we perceive and manipulate our data. It describes how we interact with the data in the database [FOWLER]. In other words, it is how you abstract your data model, the way you group your entities, the attributes that you choose as primary keys, etc

• The Storage Model describes how the database stores and manipulates the data internally [FOWLER]. Although you cannot control this directly, you can certainly optimize how the data is retrieved or written by knowing how the database works internally.