Randomly select DynamoDB entry

Question

I'm have a DynamoDB table called URLArray that contains a list of URL's (myURL) and a unique video name (myKey).

I need to do two things:

1. When a user clicks the next video button, a random entry needs to be selected from this URLArray. There could be potentially tens of thousands of rows.

The user is logged into the app. Everytime they finish watching a video, the video's unique video name is recorded. So....when the user has seen a video, its added to a list in a table called Users under the user's info row.

2. Soo...This random entry that gets selected when the user clicks the next video button in point 1, has to be compared to the list of videos they've already seen. To make sure that it doesn't randomly appear again for that particular user.

I do something woefully inefficient so far, that works, but it's not great:

By the way i'm using AppSync + GraphQL to interact with the DynamoDB table. I first get a local copy of the URLArray:

 //Gets a list of the Key/URL pairs in the UrlArrays table in GraphQL   ****IN CONSTRUCTOR, so we have this URLArray data when componentDidMount()****
  listUrlArrays = async () => {  
    try {

      URLData = await API.graphql(graphqlOperation(ListUrlArrays)); //GraphQL query
      //URLData[] is available in the entire class
     
      this.setState({urlArrayLength: apiData.data.listURLArrays.items.length}); //gets the length of URLArray (i.e. how many videos are in the database)
      }
   }

As an overview, when user clicks for the next video:

     //When clicking next video
      async nextVideo(){
        
        await this.logVideosSeen(); //add myKey to the list of videos in *Users* table the logged in user has now seen
    
        await this.getURL();  //get the NEXT upcoming video's details, for Video Player to play and make sure it's not been seen before
    
      }
    

      //This will update the 'listOfVideosSeen[]' in Users table with videos unique myKey, the logged in user has seen
      logVideosSeen = async () => {     
           .......
      }

    async getURL() {  
        var dbIndex = this.getUniqueRandomNumber(this.state.urlArrayLength);  //Choose a number between 0 and N number of videos in URLArray
        
        //the hasVideoBeenSeen() basically gets the list of videos a user has already seen from `Users` table with the GraphQL getusers command, and creates a local copy of this list (can get big). I use javascripts indexOf() to check whether myKey already exists in the list 
        while(await this.hasVideoBeenSeen(this.state.URLData[dbIndex].myKey))  //while true i.e. user has seen that video before
        {
          dbIndex = this.getUniqueRandomNumber(this.state.urlArrayLength);  //get another random number to fetch a new myKey
        }
        
        //If false, we'll exit the loop and know we've got a not seen before myKey, proceed to set to play...
        if(dbIndex != null){
          this.setState({ playURL: this.state.URLData[dbIndex].vidURL });   //Retrieve the URL from the local URLArray that we're going to play (i.e. the next video to come)
          
        }   
      }

I can share a little more code if needed, but essentially I wanted to know how to:

1. Let a Lambda function select a random number based on the current URLArray size (i may need to keep a local copy of URLArray anyway). But i think point 2 here is where it's really inefficient..

2. Let a Lambda function check (the while loop) against the Users table whether myKey has already been seen. Mainly to shift this computational burden to the cloud instead of the local device the app runs on.

AFTER A THINK................

Thanks for the suggestion Seth. I have been thinking about it for some time, and while the randomness requirement still holds true, I think there is some truth in what you’ve suggested. The reason I need randomness, is so that 2 users sat side by side for example, can’t predict which video is coming next. It shouldn’t be a predictable sequence of videos. I'm not sure I can use Scan function with AWS Amplify/GraphQL. So remember there’s 2 things going on here: (1) a video upload, recording it in the URLArray sensibly for future reference. (2) users viewing a previously unseen random video and then moving onto another unseen random video

*(1) I like your idea of using a number to index the URLArray, and it’s helped to make life a bit easier. So the first URL being at index 0, the next at 1 etc…

My thinking here (to avoid me doing a ListUrlArrays() and bringing the WHOLE array locally to the phone), is to create a GSI called VideoNumber for the URLArray table. This will be the unique VideoNumber column with a number 0-N. So imagine the diagram above having another column called VideoNumber. Row 1 having VideoNumber set to 0, Row 2 having VideoNumber set to 1 etc… THEN all I would need to do, is locally on the device, generate a random number between 0-N, call a getURLArrayIdbyVideoNumber() query specific for that GSI, with the number that we just generated, and it’ll unlock the information I need from the row. Voila! I think that shifts most of that heavy burden away now.

Question: Before each video is uploaded, how do I easily get the current total number of rows N in the table (or row count)? I would then increment it by one.

The other thing I can do is save this current count number in another DynamoDB table that I use for persisted data, read the number from there before upload, and write an N+1 after upload to increment it (2 DynamoDB operations per upload). It’s not ideal.

*(2) When a user has finished watching a video, I can log in a list (under the users information in DynamoDB), which video’s they’ve already seen. So for example this could now be a seen list: [3,12,73,108,57] for the 5 videos they’ve seen so far. When the user clicks nextVideo() we’ll generate a random newNumber, and straight away compare that with any number in the seen list. I use seenlist.indexOf(newNumber) and it will, either go again or stop if the newNumber doesn’t exist in the list. THEN I can go through the GSI query, and retrieve the relevant information to display the video from URLArray.

I think that this indexOf() is the biggest computational burden on the device, and obviously gets a little slower as the seenList increases. But it should be quicker with pure integer numbers then an alphanumeric myKey as I was using before. Any other suggestion would be welcome :)

I’ve yet to try it, but it was just an idea, as I need to keep the random element. But first, do you know how I can easily find the number of rows or table count of URLArray?

Answer 1

I think you'll have an easier time coming up with a solution to this problem if you drop the randomness requirement. It sounds like the more important requirement is presenting the user with a video they haven't seen before.

If that's correct, it sounds like your access pattern could be stated as

Fetch previously unseen video for user

which is an easier problem to solve.

Unlike SQL databases, there are often many ways to implement a given access pattern in DynamoDB. My answer here is just one way.

Imagine your URLArray table as a giant array. The first URL is at index 0, the next URL is at index 1, the second URL at index 2, and so on. Each user of your application would start by watching the video at URL index 0, then URL index 1, etc. This would ensure the user never sees the same video twice. You would not need to store a list of all the videos they've seen. Instead, you could store the index of the last video they saw.

Your application could grab the first n videos from the table to present to your users. Once that list was exhausted, it could go grab the next n videos. And so on...

What I've described here is essentially how pagination is implemented in DynamoDB. To bring this abstraction back to the world of DynamoDB, your algorithm could look something like this:

• Scan the URLArray table for the first "page" of URLs (a scan operation with no filter criteria)
• Along with the results, DynamoDB will respond with a LastEvaluatedKey, which will allow you to retrieve the next page of results starting from this position
• Present your user with each video you pulled back from the scan operation, making sure to record the id (the Primary Key) the the last video they saw.
• When you exhaust the URLs from step 1, execute another scan operation with the ExclusiveStartKey set to the LastEvaluatedKey returned from step 2.
• When users return to your application, query for the next page from the URLArray table with ExclusiveStartKey set to the id of the last video they viewed.

This effectively uses the scan operation to search through your URLArray table one page at a time. Your application would effectively be searching the table from top to bottom, keeping track of where each user is at any given time. When a user revisits your application, just start where they left off.

In response to your edit:

If your use case requires the next video to be unpredictable (e.g. no 2 users can predict what video is next), you have a few problems to solve at the same time:

1. Selecting an item in an unpredictable/random manner
2. Tracking what a user has already seen

Putting those two requirements together makes for a tricky access pattern. Let's say you have N videos in your table, and the user has viewed N-1 of these videos leaving only one video unseen. If you are fetching your next video randomly and need to ensure it has not yet been seen, how will you find the last unseen video? How many times would you need to guess before you came across the only unseen video? What query/scan operation could you perform that does this in a single request to DDB? I'm not saying it's impossible, it's just...complicated.

I think it's better to generate a strategy that is unpredictable to the user, but predictable to you when it comes to select the next unseen video.

For example, you could pre-calculate a random order of indexes from 1..N ahead of time, which would represent the order you present the videos for a given user. You could go through that list sequentially, keeping track of the last seen index. That way, you'd always know which video was next and that the video hadn't previously been seen by this user. Fetching that video would be a simple query operation to DDB.

You also asked how to find the number of items in DynamoDB. Unfortunately, there is no DynamoDB equivalent of the SQL count operation. The answer to this question is not straightforward. For the benefit of the community (and to get a diverse set of answers), I'd suggest you make a separate question on Stackoverflow regarding the number of items in a DDB table.