Limit number of documents in a partition for cosmosdb - azure-cosmosdb

Question

I have a cosmosdb collection with each partition containing a set of documents.I would like to maintain the collection such that a logical partition('id' in this case) does not go over the limit of 5 documents. In the sample below, when a sixth entry(say for 8/11/2020) is added, I want to delete the document created on 7/13/2020 since that was updated the earliest.
Basically, I want to make sure for item with id 12345, there are only 5 latest entries and no more. This is to reduce the data in the db and thus avoid querying more data than what's needed.
{
"id": 12345,
"lastUpdated": 8/10/2020
},
{
"id": 12345,
"lastUpdated": 8/3/2020
},
{
"id": 12345,
"lastUpdated": 7/27/2020
},
{
"id": 12345,
"lastUpdated": 7/20/2020
},
{
"id": 12345,
"lastUpdated": 7/13/2020
}
I could do something like this:
Get all documents for id 12345
If count of documents is >=5, get the last document (with instance 5) and delete it.
Insert new document
However, that is a running 3 queries to insert a single document.
Is there a more elegant way to do this?
Thanks!

Answer 1

You can use OFFSET 1 LIMIT 5 to get 5 latest entries. For more details, you can read offical document about OFFSET LIMIT clause in Azure Cosmos DB.
You can get the count(assume 100) of data and set ttl, or delete directly. We can query like below.
SELECT f.id, f.lastUpdated FROM yourcosmosdb f ORDER BY f.lastUpdated OFFSET 6 LIMIT 100
Foreach
List<Task> concurrentDeleteTasks = new List<Task>();
while (feedIterator.HasMoreResults)
{
FeedResponse<response> res = await feedIterator.ReadNextAsync();
foreach (var item in res)
{
concurrentDeleteTasks.Add(container.DeleteItemAsync<response>(item.id, new PartitionKey(item.deviceid)));
}
}
await Task.WhenAll(concurrentDeleteTasks.Take(3));
You also can foreach the collection and set ttl=10, these data will be deleted 10s later.
You can get the latest 5 data：
SELECT f.id, f.lastUpdated FROM yourcosmosdb f ORDER BY f.lastUpdated OFFSET 1 LIMIT 5