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Data modeling building blocks in DynamoDB

This section covers the building block layer to give you design patterns you can use in your application.

Composite sort key building block

When people think of NoSQL, they may also think of it as non-relational. Ultimately, there is no reason relationships cannot be placed into a DynamoDB schema, they just look different than relational databases and their foreign keys. One of the most critical patterns we can use to develop a logical hierarchy of our data in DynamoDB is a composite sort key. The most common style for designing one is with each layer of the hierarchy (parent layer > child layer > grandchild layer) separated by a hashtag. For example, PARENT#CHILD#GRANDCHILD#ETC.

While a partition key in DynamoDB always requires the exact value to query for data, we can apply a partial condition to the sort key from left to right similar to traversing a binary tree.

In the example above, we have an e-Commerce store with a Shopping Cart that needs to be maintained across user sessions. Whenever the user logs in, they may want to see the entire Shopping Cart including items saved for later. But when they enter the checkout, only items in the active cart should be loaded for purchase. Since both of these KeyConditions explicitly ask for CART sort keys, the additional wishlist data is simply ignored by DynamoDB at read time. While both saved and active items are a part of the same cart, we need to treat them differently in different parts of the application, so applying a KeyCondition to the prefix of the sort key is the most optimized way of retrieving only the data needed for each part of the application.

Key features of this building block

Multi-tenancy building block

Many customers use DynamoDB to host data for their multi-tenant applications. For these scenarios, we want to design the schema in a way that keeps all data from a single tenant in its own logical partition of the table. This leverages the concept of an Item Collection, which is a term for all items in a DynamoDB table with the same partition key. For more information on how DynamoDB approaches multitenancy, see Multitenancy on DynamoDB.

For this example, we are running a photo hosting site with potentially thousands of users. Each user will only upload photos to their own profile initially, but by default we will not allow a user to see the photos of any other user. An additional level of isolation would ideally be added to the authorization of each user's call to your API to ensure they are only requesting data from their own partition, but at the schema level, unique partition keys is adequate.

Key features of this building block

Sparse index building block

Sometimes an access pattern requires looking for items that match a rare item or an item that receives a status (which requires an escalated response). Rather than regularly query across the entire dataset for these items, we can leverage the fact that global secondary indexes (GSI) are sparsely loaded with data. This means that only items in the base table that have the attributes defined in the index will be replicated to the index.

In this example, we see an IOT use case where each device in the field is reporting back a status on a regular basis. For the majority of the reports we expect the device to report everything is okay, but on occasion there can be a fault and it must be escalated to a repair technician. For reports with an escalation, the attribute EscalatedTo is added the item, but is not present otherwise. The GSI in this example is partitioned by EscalatedTo and since the GSI brings over keys from the base table we can still see which DeviceID reported the fault and at what time.

While reads are cheaper than writes in DynamoDB, sparse indexes are a very powerful tool for use cases where instances of a specific type of item is rare but reads to find them are common.

Key features of this building block

Time to live building block

Most data have some duration of time for which it can be considered worth keeping in a primary datastore. To facilitate data aging out from DynamoDB, it has a feature called time to live (TTL). The TTL feature allows you to define a specific attribute at the table level that needs monitoring for items with an epoch timestamp (that's in the past). This allows you to delete expired records from the table for free.

In this example, we have an application designed to let a user create messages that are short-lived. When a message is created in DynamoDB, the TTL attribute is set to a date seven days in the future by the application code. In roughly seven days, DynamoDB will see that the epoch timestamp of these items is in the past and delete them.

Since the deletes done by TTL are free, it is strongly recommended to use this feature to remove historical data from the table. This will reduce the overall storage bill each month and will likely reduce the costs of user reads since there will be less data to be retrieved by their queries. While TTL is enabled at the table level, it is up to you which items or entities to create a TTL attribute for and how far into the future to set the epoch timestamp to.

Key features of this building block

Time to live for archival building block

While TTL is an effective tool for deleting older data from DynamoDB, many use cases require an archive of the data be kept for a longer period of time than the primary datastore. In this instance, we can leverage TTL's timed deletion of records to push expired records into a long-term datastore.

When a TTL delete is done by DynamoDB, it is still pushed into the DynamoDB Stream as a Delete event. When DynamoDB TTL is the one who performs the delete though, there is an attribute on the stream record of principal:dynamodb. Using a Lambda subscriber to the DynamoDB Stream, we can apply an event-filter for only the DynamoDB principal attribute and know that any records that match that filter are to be pushed to an archival store like S3 Glacier.

Key features of this building block

Vertical partitioning building block

Users familiar with a document model database will be familar with the idea of storing all related data within a single JSON document. While DynamoDB supports JSON data types, it does not support excuting KeyConditions on nested JSON. Since KeyConditions are what dictate how much data is read from disk and effectively how many RCUs a query consumes, this can result in inefficiencies at scale. To better optimize the writes and reads of DynamoDB, we recommend breaking apart the document's individual entities into individual DynamoDB items, also referred to as vertical partitioning.

Vertical partitoning, as shown above, is a key example of single table design in action but can also be implemented across multiple tables if desired. Since DynamoDB bills writes in 1KB increments, you should ideally partition the document in a way that results in items under 1KB.

Key features of this building block

Write sharding building block

One of the very few hard limits DynamoDB has in place is the restriction of how much throughput a single physical partition can maintain per second (not necessarily a single partition key). These limits are presently:

In the event requests against the table exceed either of these limits, an error is sent back to the client SDK of ThroughputExceededException, more commonly referred to as throttling. Use cases that require read operations beyond that limit will mostly be served best by placing a read cache in front of DynamoDB, but write operations require a schema level design known as write sharding.

To solve this problem, we'll append a random integer onto the end of the partition key for each contestant in the application's UpdateItem code. The range of the random integer generator will need to have an upper bound matching or exceeding the expected amount of writes per second for a given contestant divided by 1000. To support 20,000 votes per second, it would look like rand(0,19). Now that the data is stored under separate logical partitions, it must be combined back together at read time. Since vote totals doesn't need to be real time, a Lambda function scheduled to read all vote partitions every X minutes could perform occasional aggregation for each contestant and write it back to a single vote total record for live reads.

Key features of this building block