DynamoDB: Have sequencing within Items - sequence

Question

I am developing forums on DynamoDB.
There is a table posts which contains all the posts in a thread.
I need to have a notion of sequence in the posts, i.e. I need to know which post came first and which came later.
My service would be running in a distributed env.
I am not sure if using Timestamp is the best solution for deciding the sequence, as the hosts might have slightly different times and might be off my milliseconds/ seconds.
Is there another way to do this?
Can I get DynamoDB to populate the date so it is consistent?
Or is there a sequence generator that I can use in a distributed env?

Answer 1

You can't use DynamoDB to auto-populate dates. You can use other services to provide you with auto-generating numbers or use DynamoDB's atomic increment to create your own UUID.
This can become a bottleneck if your forum is very successful (needs lots of numbers per second). I think you should start with timestamp and later on add complexity to your id generating (concatenate timestamp+uuid or timstamp+atomiccounter)
It is always a best practice to sync your servers clock (ntpd)

Answer 2

Use a dedicated sequence table. If you have only one sequence (say, PostId), then there's going to be only one row with two attributes in the table.
Yes, there's extra cost and effort of managing another table, but this is the best solution I know by far and haven't seen any one else mentioning it.
The table should have a key attribute as primary partition key, and a numeric value attribute with initial value of 1 (or whatever you want the initial value to be).
Every time you want to get the next available key, you tell DynamoDB to do this:
Increment the value where key = PostId by 1, and return the value before incrementing.
Note that this is one single atomic operation. DynamoDB handles the auto-incrementing, so there's no concurrency issues.
In code, there're more than one ways of implementing this. Here's one example:
Map<String,AttributeValue> key = new HashMap<>();
key.put("key", new AttributeValue("PostId"));
Map<String, AttributeValueUpdate> item = new HashMap<String, AttributeValueUpdate>();
item.put("value",
new AttributeValueUpdate()
.withAction(AttributeAction.ADD)
.withValue(1));
UpdateItemRequest request = new UpdateItemRequest("Sequences", key, item).withReturnValues(ReturnValue.ALL_OLD);
UpdateItemResult result = dynamoDBClient.updateItem(request);
Integer postId = Integer.parseInt(result.getAttributes().get("value").getN()); // <- this is the sequential ID you want to set to your post

Answer 3

Another variation of Chen's suggestion is to have strict ordering of posts within a given Forum Thread, as opposed to globally across all Threads. One way to do this is to have a Reply table with the Hash key of ThreadId, and a range key of ReplyId. The ReplyId would be a Number type attribute starting at 0. Every time someone replies, your app does a Query on the Reply table for the one most recent reply on that thread (ScanIndexForward: false, Limit: 1, ThreadId: ). To insert your new reply use the ReplyId of the one returned in the Query, + 1. Then use PutItem, using a Conditional Write, so that if someone else replies at the same time, an error will be returned, and your app can start again with the query.
If you want the simplest initial solution possible, then the timestamp+uuid concatenation Chen suggests is the simplest approach. A global atomic counter item will be a scaling bottleneck, as Chen mentions, and based on what you've described, a global sequence number isn't required for your app.