Set up a Spring Boot service to read and output data from a Dynamo DB.
Follow the KotlinLang guide for creating a Spring Boot Rest Service.
If done correctly, you should be able to run gradle bootRun and access your service at http://localhost:8080/greeting?name=kotlin
To connect to DynamoDB, we need to start by adding new dependencies to gradle:
- Jackson (for JSON handling)
- Reflect (for Spring Boot classes)
- AWS Java SDK Dynamo (for Dynamo annotations and functions)
- Spring Data DynamoDB (for Entity scanning and easier data retrieval)
Next, create a class to represent your table based on the example from Spring Data DynamoDB. This should point to the table you'll be using, and specify some object attributes to look for. For now, we only need a Hash Key. Importantly, default values must be provided to the parameters in the arguments of the data class. The AWS Dynamo SDK depends on the parameter-less constructor, and this constructor is created when all arguments of the class have default values. See the NOTE section of Kotlin's Secondary Constructors.
Create a class for the Repository, which will retrieve the data specified in your class from the relevant table. (The User class specifies what to retrieve, and the Repository handles the retrieval. Spring Data DynamoDB acts as a JPA (Java Persistence API) and handles the boilerplate of querying the table.)
Create a configuration to connect the REST service with a Dynamo DB. For the purposes of this example, I'll set it up for a Docker image running a DynamoDB. The environment, endpoint, and some table configuration are specified in the application.yml file. A Spring @Configuration class specifies where the Table/Repository classes are, and establishes the connection to the database. If no endpoint is provided, the AWS CLI configuration is used.
Finally, add the Repository created earlier to your Controller (letting Spring's dependency injection create the instance), create a route to query your table, and test that your Application spins up. For easy testing of data retrieval, spin up an aws dynamo-local Docker image and add some data with the AWS CLI. (If you're feeling ambitious, you can populate the table with dummy data using a Gradle task (see createLocalDynamoDBData) and some Dynamo DML)
If you see errors from missing classes, you may be missing a dependency. If you see ResourceNotFoundException: Cannot do operations on a non-existent table, you haven't created your table. You can create the table by using the AWS CLI, or by going to application.yml and ensuring that entity2ddl>auto is set to create-only. If you see java.lang.NoSuchMethodException: com.shoprunner.kotlinspringdynamoexample.dynamo.User.<init>(), the arguments for your table's data class need defaults
When using both a hash and sort key, they are treated as a composite key, where the table's "id" is both of them rather than one or the other.
Create a new class to represent the id, and annotate it to read the hash and sort keys. Then, make that class the primary constructor argument for your table's data class, and write getter/setter methods to interact with the hash and sort key through the composite key class.
When data contains nested maps, they can be read in a few ways. One is to use a Map as the type for the attribute, but this depends on all items in the Map being of the same type, and with a schema-less database like Dynamo, that's not guaranteed. Instead, create a data class with the @DynamoDBDocument annotation and read the attributes into that class.