September 3, 2020 by Jatinder Khabra

Lossless soft-deletes with Hasura

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Soft deletes provide the ability to delete records from a database, without actually losing data. It’s like putting data in a recycle bin, so that it is practically deleted, but can still be restored if needed.

Most important benefit soft-deletes provide is perhaps, the restore capability. They are also important for keeping a healthy data warehouse. We don’t want references in our data warehouse to suddenly start pointing to non-existent data.

Traditional soft delete

Traditionally, we simply add a bit column, e.g is_deleted to mark a row as deleted. Instead of deleting rows, we flip this bit column. It is a rather popular method, especially in smaller applications. Even Hasura itself recommends this method to handle soft deletes.

It is however a double-edged sword. Our biggest beef with this method is that we lose a lot of database’s power. Soft-deleted records cause constraint violations, which usually means not using any constraints at database level at all. Designing database indexes also become very hard.

Alternatives

Problems with the traditional soft-delete are serious enough. Two common approaches used to circumvent them are:

  1. Using an event sourcing database, which keeps an immutable record of all events
  2. Exporting deleted records to an outside storage, and re-importing for restore

They both have their pros and cons as well. Event sourced systems tend to grow very complex, and need a lot of care for growing. Exporting and importing itself becomes a pain to maintain for developers.

Goals

When writing Sos App, we had a clear picture of what we wanted to achieve from our soft-delete implementation:

  1. Full utilization of database’s abilities

    Fully functional database constraints is a must-have for clean boundaries. Easy and efficient indexes are also critical.

  2. Developer productivity

    Soft delete shouldn’t become a burden for developers. SoS Inc is a small development shop. We try to optimize for developer productivity as much as we can.

Our Approach

Soft Deletes with Hasura

We went for a sort of a hybrid of event sourcing and exporting deleted records. This approach we laid out was:

  1. Actually delete rows from database, so that deleted data don’t interfere with our everyday use
  2. Export deleted rows to a dedicated deleted_entities table, in form of “Deleted” events

Trick is to use Hasura’s trigger mechanism for making the whole process really cheap to develop. Once the hook is set, zero developer intervention is needed moving forward. Developers can keep using the database without needing to pay any special care to soft deletes.

The whole process goes like:

  1. Client deletes a record
  2. Hasura actually deletes the record from database, but raises an event to be processed by a sidecar application
  3. Sidecar prepares a “delete event” with additional information
    • Who deleted the row?
    • Which table has the row been deleted from?
    • The actual data that was deleted
  4. Sidecar inserts the “delete event” to deleted_entities

Wins

  • Allows full utilization of database capabilities, e.g constraint violations
  • No future intervention from developers needed
  • Queries do not need to be manually adjusted to account for soft deletion logic
  • Keep track who deleted the data and when
  • Does not become a problem as application grows

LET’S WORK TOGETHER