Arvados

h1. Multi-cluster user database

It is sometimes desirable to share a single user database across multiple Arvados clusters. For example:

* Clusters aaaaa, bbbbb, ccccc, ddddd, eeeee are on different continents, but they use the same upstream authentication providers (ldap/google).

* A down/unreachable cluster should not prevent any user from using _other_ clusters in the group -- even if the down/unreachable cluster is the one where the user's account was initially created.

This requires some changes to login and token validation. (Currently, any given user account has a single "home cluster" that can issue or validate tokens for it.)

h2. Logging in

Each user should be able to log in to their account using any cluster, regardless of where/whether they have logged in previously.

To achieve this (without depending real-time communication between clusters) the participating clusters need to agree on a mapping of upstream authentication results to Arvados user UUIDs. For example, if the upstream authentication result is @"ldap://ldap.example foo@bar.example"@ ("ldap://ldap.example assures us this user is foo@bar.example"):

# Generate a UUID "eeeee-tpzed-${sha1part(upstream)}" (where eeeee is a common prefix used by all participating clusters and sha1part() is the first 15 chars of base-36-encoded sha1())

# If it doesn't already exist, add a row to the users table with this UUID

# If another row exists in the users table with the same upstream (or same identity_url) but a different UUID, [offer to] merge the old account's data/objects/permissions into the new account (it isn't possible to log into the old account any more, but we know it belongs to the same person as the new account).

Notes

* the "upstream" field is similar to identity_url as initially conceived. Since #4601, identity_url has been an opaque SSO-generated UUID, with no info about upstream -- so we will rely on it to detect "same upstream as old account that needs to be migrated" but we can't use it to generate the same user UUID as other clusters, hence the need for a new "upstream" field

* "remote" accounts (the kind that we already have in the users table with foreign UUIDs) have a null identity_url field, and will also have a null upstream field

|uuid                        |upstream                            |identity_url                |significance               |

|eeeee-tpzed-012340123401234 |ldap://ldap.example foo@bar.example |login-tpzed-aaaaaaaaaaaaaaa |Newly created user account |

|aaaaa-tpzed-aaaaaaaaaaaaaaa |NULL                                |login-tpzed-aaaaaaaaaaaaaaa |Old user account (can't log in to this any more - contents should be migrated to eeeee-*) |

|ooooo-tpzed-ooooooooooooooo |NULL                                |NULL                        |Remote user from cluster ooooo (not part of our multi-cluster group) |

h2. Configuration

Each cluster needs to know

* the uuid prefix to use when creating a new account, e.g., "eeeee"

* additional user uuid prefixes that remote clusters are trusted to validate

<pre><code class="yaml">

Clusters:

  aaaaa:

    Login:

      AssignUUIDPrefix: eeeee

    RemoteClusters:

      bbbbb:

        Proxy: true

        Authenticate:

          bbbbb: {} # (implied)

          eeeee: {} # accept tokens issued by bbbbb for users with uuid eeeee-*

</code></pre>

Example: aaaaa needs to validate a token issued by bbbbb.

* Do a callback to bbbbb (or check JWT signature) to confirm bbbbb really issued this token and get the relevant user UUID (result: yes, user uuid is eeeee-tpzed-012340123401234)

* Fetch eeeee's config; if RemoteClusters.bbbbb.Authenticate.eeeee is present, accept the token

* Otherwise, reject the token

h2. Validating tokens

(...even when the issuing cluster is unreachable)

Each cluster should be able to validate a token that was issued by a different, currently unreachable, cluster. This contrasts with the current setup, where aaaaa validates tokens issued by bbbbb by doing a callback to bbbbb.

This seems easy enough: instead of random strings, tokens can be [like] "JSON Web Tokens":https://jwt.io/, signed by a private key whose public part is known by all clusters. (This would also be more efficient than callbacks, benefiting the mutually-untrusted cluster scenario too.)

h2. User preferences and alternate emails

The authoritative place to store/load per-user information (preferences, and the "this email is just an alternate way to log in to a different account" marker) is:

* ...for callers outside the "eeeee" group of clusters: the "eeeee" cluster

* ...for callers inside the "eeeee" group of clusters, for now: a manually designated "master" cluster (probably "eeeee")

* ...for callers inside the "eeeee" group of clusters, in future: a group-wide distributed database whose default/initial "master" is eeeee

Until a distributed database is implemented, each non-master cluster can update its cached user record (if stale at login or token validation time) from the master cluster, and proxy update requests to the master.