Introduction

In any SaaS application, storing customer’s data is a critical mission. We have to ensure the correctness, availability, safety, retention, cost efficiency, and much more. I am lucky to be part of the Event Storage Team of Datadog as a software engineer. Today, I would like to share my experience with data management, focusing on the deletion part: 8 lessons I learned from data deletion.

But… why should we care about data deletion? We want to care about it because deleting data is an important part of the lifecycle. By doing it well, we can improve the cost efficiency and make our business more profitable by cleaning the expired data on time; we can reduce the risk of sensitive data exposure by deleting the right data; etc. Also, thinking about the potential risk of we’re doing it wrong: we may lose customer data; it may lead to service interruption; in case of mistake, it can be difficult or impossible to recover if something goes wrong.

After reading this article, you will understand some thoughts before, during, and after the deletion process:

  • Data organization
  • Naming convention
  • Deletion checks
  • Multi-step deletion
  • Dangling resources
  • Observability
  • Technical constraints
  • Verification

Now, let’s get started!

Data Organization

The way we store the data impacts dramatically the efficiency of data lookup and deletion. Here are some data organization that I saw in the past and I recommend for you.

Grouping data per customer. Storing data per customer means that all data of a customer will be located in the same place. It provides a physical or virtual separation across customers, which makes a clear boundary for each of them. Some concrete examples of physical separation are: Amazon S3 buckets, Docker volumes, Elasticsearch clusters, etc. Some concrete examples of virtual separation are: different prefixes in the same Amazon S3 bucket, different directories in the same Docker volume, different repositories of the same Elasticsearch cluster, etc. When visiting data inside one location, you know that you are accessing data of which customer. A pseudo-code to represent such an organization is:

/{customerId}/{data}

Therefore, when deleting data, you have to specify the location by including the customer ID, which makes the deletion request explicit and without ambiguity.

Grouping data per type. You may need to store different types of data for each customer. Grouping data per type is also a good choice. It makes it easy to understand which type of data you are visiting. The type can be functional or technical. A concrete example of a functional type is products: each product may have their specific type of storage, different retention, different administrators, etc. Therefore, it makes sense to separate the storage in this case. When deleting data, you have to specify the type of data in your request, which clarifies the target type of data. A pseudo-code to represent such an organization is:

 /{customerId}/{type}/{data}

Grouping data per date. You may want to group the data per date or time as well. When a new date is started, the system creates a new location to store the new data. This is useful when your system continuously receives data from most of the customers every day and your deletion request is often related to a time range. When implementing this solution, I suggest using ISO 8601 to format the date-time objects. A pseudo-code to represent such an organization is:

 /{customerId}/{datetime}/{data}

There must be other ways to organize your storage. The key points of data organization are to make the important parameters visible, to avoid mistakesi, and facilitate the operations.

Naming Convention

Having a naming convention for the data makes your life easier. Instead of having a random string for each resource as an identifier, you may consider using something meaningful and human-readable. For example, include the customer ID, the creation time of the resource, the type of the product, the version, etc. Thanks to the naming convention, you can retrieve the key info without performing any additional requests to the database or other places.

My favorite way to do this is by using label trees. It consists of a sequence of multiple labels separated by dots, such as L1.L2.L3, representing a path from the root of a hierarchical tree to a particular node. Each label is a sequence of alphanumeric characters and underscores (A-Za-z0-9_). In PostgreSQL, this data structure is called ltree. Let’s see one concrete example of a label tree, where a resource is named using the customer ID, the type of product, and the creation time of the resource in epoch second:

{customer}.{type}.{creationTime}

Besides dot (.), other commonly used separators are dash (-), underscore (_). Some other characters are less used, such as slash / (used for path), dollar $ (used for variables). Another thing I like is to add a one-letter prefix for each label section so that it’s easier for humans to understand the type of information. For example, use “2020-w49” to represent the 49th week of the year 2020 rather than using “2020-49”.

In my opinion, the key points of having a good naming convention are:

  • Provide a unique ID for each resource (data)
  • Provide key information for humans to make decisions face to different operations, such as deletion, without the need for additional queries.
  • Have a common rule for all the resources.

Deletion Checks

Before deleting any data, it’s important to check that we can delete it. The first reason is that deletion is an irreversible action: we cannot go back if we choose to do it. The second reason is that there is usually a dependency between some resources, in which case we must delete them in a specific order. Doing it in the wrong order may result in unexpected errors and even service interruption.

But what should we check before deletion?

I would like to split this discussion into two types of deletion: deleting data vs deleting a data container. For example, deleting an index inside an Elasticsearch cluster vs deleting an Elasticsearch cluster itself.

There are several dimensions to think about when deleting data:

  • Retention. Ensure that the data is expired before any deletion.
  • Authorization. Ensure that the actor has sufficient permission to perform this action.
  • Dependency. Ensure that there is no dependent referencing this resource to avoid cascading errors.

There are several dimensions to think about when deleting a “data container”:

  • Storage. Ensure that the container is empty: it does not store any data. Or the existing data are expired thus it is safe to delete.
  • Upstream/downstream. Ensure that no services in the upstream or downstream are referencing this container. For example, before deleting an Elasticsearch cluster, ensure that none of the Elasticsearch clients will insert new documents to this cluster by changing the routing; ensure that none of the Elasticsearch clients will read from the cluster.
  • Routing. Ensure that nothing will be routed to this container in the future because it won’t exist as a destination.
  • Scheduling. Ensure that nothing will be scheduled for this container in the future because it won’t exist as a destination. The difference between this one and the previous one is that in this case, we are using the current routing but the event is scheduled to be executed in the future, such as some events being backed off.
  • Authorization. Ensure that the actor has sufficient permission to perform this action.

There may be more checks that are not covered in this section. Also, not all the checks are necessary for you. It’s up to you to decide which checks are the most relevant for you. My suggestion is to write a simple framework to support deletion checks, then add more checks when the business grows, when the deletion happens more often, or when regression (incident) is detected.

Multi-Step Deletion

Sometimes, deleting a resource cannot be done in one step because the resource is stored in multiple locations. For example, some data in the database and some files in the filesystem. There are several ways to handle this situation:

  • Delete data from the source of truth and consider data from other locations as dangling resources.
  • Introduce a status for deletion-in-progress, such as “DELETING”.

Let’s dig deeper into the first approach: deleting data from the source of truth. Let’s say you have a database and you consider that data inside this database is the source of truth. In this case, any other locations outside of the database are not the source of truth, such as the filesystem, Elasticsearch clusters, Amazon S3, etc. Therefore, when deleting data, we can do it in multiple steps: the first step is to delete the data from the source of truth (database), and the remaining steps are to delete data from other locations. In case of failure in the database, you just have to do it again. In case of failure in other locations, they are considered as unreferenced objects. You can implement a job to clean them up periodically.

Let’s dig deeper into the second approach: introduce a status “DELETING”. Let’s say you have a database and you consider that data inside this database is the source of truth. When deleting a resource, we change its status into DELETING in the database and perform deletion on other locations first. Also, prevent access to this resource from the read path just like it’s deleted. Once the resource is deleted in every location, remove the entry from the database, and consider the deletion process is complete.

Maybe you have other ideas. Please let me know your thoughts :)

Dangling Resources

Dangling resources are unreferenced resources. They are incomplete and unusable from the user’s point of view. Therefore, it’s important to reduce the lifetime and the volume of dangling resources.

Safety. Not all the dangling resources are intended to be deleted. Some of them may be introduced by an undesired operation. For example, we have a file in the filesystem and a record in the database pointing to this file. If someone deleted the record from the database mistakenly, the file in the filesystem becomes dangling. However, we may not want to delete the file in this case to avoid worsening the situation. In such a situation, we want to recover from this situation by restoring the record in the database. Therefore, it’s important to design a deletion logic that does not delete dangling resources automatically. Also, we should design the system so that we have sufficient information about the file itself when such a situation happens. The way we organize the resource and naming the resource plays an important role here. For example, having the custom ID in the resource name will make the recovery easier comparing to having a random generated ID, because we know immediately which customer is impacted.

Decision making. We should provide enough information about the resource so that an administrator can understand the situation, estimate the importance, and decide what to do to this resource. The information can be related to dates (creation, expiry), customer, size (bytes), quantity, references from other locations, etc.

Other things that are important to be considered such as how to reduce the dangling resources in the first place and how to automate the clean-up. But we don’t have enough time to dig into these topics.

Observability

It’s important to improve the observability of the system. Let’s take logging and metrics as examples.

For logging, it’s important to add a log mentioning which resource had been deleted for audit purposes. Sometimes it’s also worth adding two logs: one when the deletion is requested and another one when the deletion is complete. This is because some deletions can fail.

For metrics, here are some examples I have in mind. You can record metrics about the actions, record metrics about data in different states, and use them to compute additional metrics

Metric Name Description
Number of deletion requests Useful to measure how often the deletion happens.
Number of successful deletions  
Success rate  
Failure rate  
Number of failed deletions Useful to detect the anormaly
Number of dangling resources Useful as volume lag and dangling resource ratio
The age of the oldest dangling resource Useful as the time lag
Dangling resource ratio Storage efficiency and trend

Technical Constraints

There are also some technical constraints to be considered when deleting resources.

Concurrent actions. For example, in the older version of Elasticsearch, you cannot perform multiple snapshot actions at the same time. You can either create, restore, or delete a snapshot, but you cannot do multiple actions simultaneously. So you have to design your workflow to make everything works as expected.

Rate limit. When dealing with an Amazon S3 bucket, you cannot send more than 3,500 PUT/COPY/POST/DELETE or 5,500 GET/HEAD requests per second per prefix in an S3 bucket. So it’s important to design the storage structure inside the bucket to avoid hitting the rate-limit problem. See How can I increase Amazon S3 request limits to avoid throttling on my Amazon S3 bucket?

Verification

How to verify the data are deleted?

There are multiple ways to verify that the data are deleted.

Testing. You can write unit tests or integration tests to ensure the deletion logic is working as expected.

Verify storage. If your data well organized, you can just visit the storage location and verify that the data for the target customer or target dates don’t exist anymore. This can be true for filesystem, databases, cloud storage, and much more.

End to end verification. You can verify the deletion by asking the customer to check this manually or if possible, to substitute the customer to check the deletion by launching a query or visiting the target location. This is important, especially for sensitive data deletion.

Conclusion

In this article, I shared some lessons I learned about data deletion (before, during, and after the deletion process): data organization, naming convention, deletion checks, multi-step deletion, dangling resources, observability, technical constraints, and verification. Interested to know more? You can subscribe to the feed of my blog, follow me on Twitter or GitHub. Hope you enjoy this article, see you the next time!