Entity caching and computed fields

I have a custom entity that I've added some computed fields to, so the classes involved are:

• class MyEntity extends ContentEntityBase
• class ComputedFieldA extends FieldItemList
• class ComputedFieldB extends FieldItemList

My issue is that I want the computed fields to either not be cached at all, or to be cached but with a different invalidation strategy.

Is what I want possible? Or do I just need to invalidate cache for the whole entity?

Edit: The custom field itself is basically a count of the rows in another database table. I want that count to be cached separately then invalidated if new rows are inserted.

The output is being consumed via the default jsonapi endpoint for the entity.

Edit 2: I guess this is more complex than I thought so I'll describe the entities in more detail:

Entity A:

• Has a 1:1 relationship with nodes based on UUID
• Has a count of Entity B entities that are related to the node (computed)
• Has a calculation based on that count (computed)

Entity B:

• Has an N:1 relationship with nodes based on NID

So, if a new Entity B is created for node X then I want the cache invalidated on the two computed fields of Entity A for node XXXX-XXXX-XXXX-XXXX so that the jsonapi endpoint shows up to date computed data.

If I can add a cache tag of node:X to each instance of the computed field I expect that would achieve what I want.

I doesn't matter whether the field is computed. Caching depends on how the field is rendered in the field formatter. Cache metadata added to the render array bubbles up and is merged with that of the other fields and the entity.

Fields by default are not cached on their own, but you can change this. If the field needs a different caching strategy, add cache keys, and the rendered field can be shared between fields in other entity instances rendering with the same cache keys. If the field is highly dynamic then add a lazy builder and make sure it gets placeholdered, either automatically or by setting #create_placeholder.

Edit 2:

The entity class could add a cache list tag for the entity b:

class MyEntity extends ContentEntityBase {

  /**
   * {@inheritdoc}
   */
  public function getCacheTags() {
    $cache_tags = parent::getCacheTags();
    $cache_tags[] = 'entity_b_list';
    return $cache_tags;
  }