What is cache tag in Drupal 8

Cache tags provide a declarative way to track which cache items depend on some data managed by Drupal. This is essential for a content management system/framework like Drupal because the same content can be reused in many ways. In any of the places where the content is used, it may be cached.

A cache tag is a string.

Cache tags are passed around in sets (order doesn't matter) of strings, so they are typehinted to string[]. They're sets because a single cache item can depend on (be invalidated by) many cache tags.

Syntax

By convention, they are of the form thing:identifier — and when there's no concept of multiple instances of a thing, it is of the form thing. The only rule is that it cannot contain spaces.

There is no strict syntax.

Examples:

node:5 — cache tag for Node entity 5 (invalidated whenever it changes)

user:3 — cache tag for User entity 3 (invalidated whenever it changes)

node_list — list cache tag for Node entities (invalidated whenever any Node entity is updated, deleted or created, i.e., when a listing of nodes may need to change). Applicable to any entity type in following format: {entity_type}_list.

node_list:article — list cache tag for the article bundle (content type). Applicable to any entity + bundle type in following format: {entity_type}_list:{bundle}.

config:node_type_list — list cache tag for Node type entities (invalidated whenever any content types are updated, deleted or created). Applicable to any entity type in the following format: config:{entity_bundle_type}_list.

config:system.performance — cache tag for the system.performance configuration

library_info — cache tag for asset libraries

Common cache tags

The data that Drupal manages fall in 3 categories:

entities — these have cache tags of the form <entity type ID>:<entity ID> as well as <entity type ID>_list and <entity type ID>_list:<bundle> to invalidate lists of entities. Config entity types use the cache tag of the underlying configuration object.

configuration — these have cache tags of the form config:<configuration name>

custom (for example library_info)

Drupal provides cache tags for entities & configuration automatically — see the Entity base class and the ConfigBase base class. (All specific entity types and configuration objects inherit from those.)

Although many entity types follow a predictable cache tag format of <entity type ID>:<entity ID>, third-party code shouldn't rely on this. Instead, it should retrieve cache tags to invalidate for a single entity using its::getCacheTags() method, e.g., $node->getCacheTags(), $user->getCacheTags(), $view->getCacheTags() etc.