How to implement pagination when using amazon Dynamo DB in rails

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I want to use amazon Dynamo DB with rails.But I have not found a way to implement pagination.

I will use AWS::Record::HashModel as ORM.

This ORM supports limits like this:

People.limit(10).each {|person| ... } 

But I could not figured out how to implement following MySql query in Dynamo DB.

SELECT * 
  FROM  `People` 
 LIMIT 1 , 30

You issue queries using LIMIT. If the subset returned does not contain the full table, a LastEvaluatedKey value is returned. You use this value as the ExclusiveStartKey in the next query. And so on...

From the DynamoDB Developer Guide.

Querying and Pagination with DynamoDB - via @codeship, Create a users table in DynamoDB with the above structure if you haven't already . found resp.items.try(:first) rescue Aws::DynamoDB::Errors::ServiceError with query logic that fetches items from database to Rails models. I currently use a set of hyperlinks in a separate pagination area which looks like: Pagination Area. What pagination support does DynamoDB provide? Amazon DynamoDB documentation says that DynamoDB paginates the results from scan/query operations. With pagination, the scan results are divided into “pages” of data that are 1 MB in size (or less).

You can provide 'page-size' in you query to set the result set size. The response of DynamoDB contains 'LastEvaluatedKey' which will indicate the last key as per the page size. If response does't contain 'LastEvaluatedKey' it means there are no results left to fetch. Use the 'LastEvaluatedKey' as 'ExclusiveStartKey' while fetching next time.

I hope this helps.

DynamoDB Pagination

Class: Aws::DynamoDB::Client — AWS SDK for Ruby V2, Each time you create an on-demand backup, the entire table data is backed up. Associate a set of tags with an Amazon DynamoDB resource. The output from ListTables is paginated, with each page returning a maximum of 100 table� Querying in DynamoDB comes in two flavors: query operation and scan operation. Both operations have different use cases. Query Operation. The query operation in DynamoDB is different from how queries are performed in relational databases due to its structure. You can query only Primary Key and Secondary Key attributes from a table in DynamoDB.

I faced a similar problem.

The generic pagination approach is, use "start index" or "start page" and the "page length". 

The "ExclusiveStartKey" and "LastEvaluatedKey" based approach is very DynamoDB specific.

I feel this DynamoDB specific implementation of pagination should be hidden from the API client/UI.

Also in case, the application is serverless, using service like Lambda, it will be not be possible to maintain the state on the server. The other side is the client implementation will become very complex.

I came with a different approach, which I think is generic ( and not specific to DynamoDB)

When the API client specifies the start index, fetch all the keys from the table and store it into an array.

Find out the key for the start index from the array, which is specified by the client.

Make use of the ExclusiveStartKey and fetch the number of records, as specified in the page length.

If the start index parameter is not present, the above steps are not needed, we don't need to specify the ExclusiveStartKey in the scan operation.

This solution has some drawbacks -

We will need to fetch all the keys when the user needs pagination with start index.

We will need additional memory to store the Ids and the indexes. Additional database scan operations ( one or multiple to fetch the keys )

But I feel this will be very easy approach for the clients, which are using our APIs. The backward scan will work seamlessly. If the user wants to see "nth" page, this will be possible.

Paginating Table Query Results, Learn to use pagination when querying Amazon DynamoDB tables. Use paginating when you need the ability to easily page through a query's result set in an� Ruby on Rails developer at Cabforward, LLC. amazon-dynamodb. Score 34. Posts 1. nosql. Score 34. 34 How to implement pagination when using amazon Dynamo DB in

Dynamoid/dynamoid: Ruby ORM for Amazon's DynamoDB., Dynamoid is an ORM for Amazon's DynamoDB for Ruby applications. It provides This way it can be used for instance to implement efficiently pagination in� In this tutorial I will show you how to save data in a NoSQL database like Amazon DynamoDB, using Ruby on Rails framework. To make this example as useful as possible I've created the project using the following steps: created a new rails application and a NoSQL database in Amazon DynamoDB; included the aws-sdk gem

Method: Aws::DynamoDB::Client#scan — Documentation for aws , If LastEvaluatedKey is present in the response, you need to paginate the result set. For more information, see Parallel Scan in the Amazon DynamoDB Developer Guide. Note that if you use the IndexName parameter, you must also provide TableName Generated on Tue Aug 4 20:13:17 2020 by yard 0.9. 25 (ruby-2.7.0). DynamoDB with Rails. A typical Ruby on Rails application comes with ActiveRecord, which makes using RDBMS (such as MySQL, PostgreSQL etc) cake walk. There are times, when we want to leverage power of NoSQL with our Rails application. There are many NoSQL database available such as Cassandra, MongoDB, DynamoDB etc. In this tutorial we’ll

Implementing Pagination WIth Dynamo DB, What pagination support does DynamoDB provide? Examine the Scan result: Construct a new Scan request, with the same parameters as the previous one—but this time, take the LastEvaluatedKey value from step 1 and use it as the ExclusiveStartKey parameter in the new Scan request. Run the new Scan request. Go to step 1. It’s easy to start filling an Amazon DynamoDB table with data. However, without forethought about organizing your data, you can limit your data-retrieval options later. Data organization and planning for data retrieval are critical steps when designing a table. Without proper data organization, the only options for retrieving data are retrieval by partition key or […]

Comments
  • this answer is not taking into account that DynamoDB just provide pagination in one direction with mentioned LastEvaluatedKey and ExclusiveStartKey(going forward). Standard pagination with, at least, previous and next options is not possible out of the box with DynamoDB
  • @Srle, You provided yourself the way to have NEXT operation by using LastEvaluatedKey and ExclusiveStartKey. You can reverse the order of query and get PREVIOUS operation. That is available in DynamoDB out of the box.
  • @nickolay.laptev can you provide example, gist or something with supporting both directions out of the box?
  • @Srle, "To reverse the order, set the ScanIndexForward parameter to false." from docs.aws.amazon.com/amazondynamodb/latest/developerguide/…
  • This does not appear to be very scale-able, since it requires a table scan at the beginning. If you have millions of records, for instance, you'll still need to scan the entire data-set, and then store all the keys in memory, losing a lot of usefulness that having a database gives you in the first place. Not sure there's a good way to get the nth page in dynamodb at scale.