Replace serial entity IDs with UUIDs in URLs, or even globally?

Background

• The idea was mentioned a couple of times in various issues:

  #1637370-14: Add UUID support to core entity types → and #55+
  #1553358-15: Figure out a new implementation for url aliases →

Problem

• Serial IDs are only useful within the scope of a single site, but lose their entire meaning in staging scenarios.
• Menu links as well as reference fields (e.g., taxonomy term, file, image, ...) store the serial ID as value.
• Links within content contain serial IDs and thus may point to a wrong target after being staged.
• Serial IDs present collision problems with migration: #2748609: [meta] Preserving auto-increment IDs on migration is fragile →

Goal

• Make entity references compatible with content staging.

Proposed solution

1. Replace serial IDs with UUIDs in reference field values.
2. Replace serial IDs with UUIDs in URLs.

Details

• Storage lookups for integer/serial IDs vs. string UUIDs should be identical performance-wise.
• URL paths, such as /node/1637370, are not really any nicer than /node/4bd1c937-f530-456f-a218-241d7bfd40b8.
• For backwards-compatibility, there could be URL aliases for the original serial IDs.

Related issues

• ✨ Pathauto in Core Needs work
• #1269742: Make path lookup code into a pluggable class →
• #1508872-30: Image effects duplicate on edit →
• #1720784: Add method for getting an entity's UUID →

Notes

• Upgrade path will become quite complex, especially for in-text/content links.
• http://drupal.org/project/uuid_redirect is a contrib attempt to use UUIDs in URLs.
• Consider to remove the $entity_type argument from entity_load_by_uuid(), and introduce a global mapping table for UUID » entity_type mappings (required since entities live in different storages), so entities can be loaded through the UUID only.