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CosmosDB - Cleanup items automatically

I'm currently attending NDC Oslo. During one of the sessions, this one to be exact, the speaker shared a nice CosmosDB feature; Time to Live.

Some of the typical questions, you ask yourself when building applications are:

  • Should I use hard or soft deletes?
  • Are there any regulations on how long I can keep this data(e.g. GDPR)?
  • How can I keep the amount of data under control?

In the talk, the example they mention is weather data that is collected through various services. This data captured and used to feed an artificial intelligence algorithm to predict the most fuel-efficient way to operate a vessel in the North and Baltic sea.

After a prediction is done, the weather data is no longer necessary and can safely be deleted. Thanks to the Time to Live(TTL) feature in CosmosDB implementing this requirement is really easy.

You have to 2 ways to control the TTL value:

  • At the container level
  • At the item level

Control TTL at the container level

  1. Open the Data Explorer pane for your Azure CosmosDB account.

  2. Select an existing container, expand the Settings tab and modify the following values:

    • Under Setting find, Time to Live.

    • Change the TTL value to On with a value specified in seconds.

    • Select Save to save the changes.

If you prefer to do it through code:

 

Control TTL at the item level

To enable this at the item, we need to introduce an extra ‘ttl’ field on the item:

Is this for free?

One question you probably ask yourself, does this feature comes with a cost? Unfortunately the answer is yes. Deletion of expired items is a background task that consumes left-over Request Units, that is Request Units that haven't been consumed by user requests. Even after the TTL has expired, if the container is overloaded with requests and if there aren't enough RU's available, the data deletion is delayed (although the data will no longer be returned by any queries).

If you want to learn more about this feature, have a look at the documentation here.

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