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My table is (device, type, value, timestamp), where (device,type,timestamp) makes a unique combination ( a candidate for composite key in non-DynamoDB DBMS).

My queries can range between any of these three attributes, such as

GET (value)s from (device) with (type) having (timestamp) greater than <some-timestamp>

I'm using dynamoosejs/dynamoose. And from most of the searches, I believe I'm supposed to use a combination of the three fields (as a single field ; device-type-timestamp) as id. However the set: function of Schema doesn't let me use the object properties (such as this.device) and due to some reasons, I cannot do it externally.

The closest I got (id:uuidv4:hashKey, device:string:GlobalSecIndex, type:string:LocalSecIndex, timestamp:Date:LocalSecIndex)

and

(id:uuidv4:rangeKey, device:string:hashKey, type:string:LocalSecIndex, timestamp:Date:LocalSecIndex)

and so on..

However, while using a Query, it becomes difficult to fetch results of particular device,type as the id, (hashKey or rangeKey) keeps missing from the scene.

So the question. How would you do it for such kind of table?

And point to be noted, this table is meant to gather content from IoT devices, which is generated every 5 mins by each device on an average.

Huzaib Shafi
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2 Answers2

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I'm curious why you are choosing DynamoDB for this task. Advanced queries like this seem to be much better suited for a SQL based database as opposed to a NoSQL database. Due to the advanced nature of SQL queries, this task in my experience is a lot easier in SQL databases. So I would encourage you to think about if DynamoDB is truly the right system for what you are trying to do here.

If you determine it is, you might have to restructure your data a little bit. You could do something like having a property that is device-type and that will be the device and type values combined. Then set that as an index, and query based on that and sort by the timestamp, and filter out the results that are not greater than the value you want.

You are correct that currently, Dynamoose does not pass in the entire object into the set function. This is something that personally I'm open to exploring. I'm a member on the GitHub project, and if you would like to submit a PR adding that feature I would be more than happy to help explore that option with you and get that into the codebase.

The other thing you might want to explore is having a DynamoDB stream, that will set that device-type property whenever it gets added to your DynamoDB table. That would abstract that logic out of DynamoDB and your application. I'm not sure if it's necessary for what you are doing to decouple it to that level, but it might be something you want to explore.

Finally, depending on your setup, you could figure out which item will be more unique, device or type, and setup an index on that property. Then just query based on that, and filter out the results of the other property that you don't want. I'm not sure if that is what you are looking for, it will of course work, but I'm not sure how many items you will have in your table, and there get to be questions about scalability at a certain level. One way to solve some of those scalability questions might be to set the TTL of your items if you know that you the timestamp you are querying for is constant, or predictable ahead of time.

Overall there are a lot of ways to achieve what you are looking to do. Without more detail about how many items, what exactly those properties will be doing, the amount of scalability you require, which of those properties will be most unique, etc. it's hard to give a good solution. I would highly encourage you to think about if NoSQL is truly the best way to go. That query you are looking to do seems a LOT more like a SQL query. Not saying it's impossible in DynamoDB, but it will require some thought about how you want to structure your data model, and such.

Charlie Fish
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Considering opinion of @charlie-fish, I decided to jump into Dynamoose and improvise the code to pass the model to the set function of the attribute. However, I discovered that the model is already being passed to default parameter of the attribute. So I changed my Schema to the following:

  id:hashKey;default: function(model){ return model.device + "" + model.type; }
  timestamp:rangeKey

For anyone landing here on this answer, please note that the default & set functions can access attribute options & schema instance using this . However both those functions should be regular functions, rather than arrow functions.

Keeping this here as an answer, but I won't accept it as an answer to my question for sometime, as I want to wait for someone else to hit out a better approach. I also want to make sure that if a value is passed for id field, it shouldn't be set. For this I can use set to ignore the actual incoming value, which I don't know how, as of yet.

Huzaib Shafi
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