Arvados

h1. Federated collections

* Fetch collection record by uuid

** use federated record retrieval strategy, already developed.

* Fetch collection record by PDH

** No location hint.  Must send out request to all federated clusters.

** Read-only, only need to support GET operation

* Can cache result by PDH.

Record will have a manifest with signed blocks.  However these blocks will be signed for the origin cluster.

Client needs to be able to fetch blocks from remote cluster.

arvados-controller could add block hints, using existing feature in the Python and Go SDK:

* Blocks in a manifest can include a hint in the form "+K@zzzzz".  Python SDK will attempt to fetch the block from "https://keep.zzzzz.arvadosapi.com/"

** Must conform to a particular naming DNS scheme.

** Could be generalized by looking up in "remote_hosts" and using the "keep_services.accessible" API.

** Every block will be requested from remote every time, because client is contacting remote server directly, limited opportunity for edge caching.

* Hint can also be a uuid of a "local gateway service".  This is instructs client to use a specific service from the keep_services table (indicated as "service_type" of "gateway:")

** Direct requests through a specific service

** Does not encode which remote cluster to pull a block from.

** Gateway service could search for blocks by sending request to every federated cluster

** Gateway service can cache blocks so they don't need to be re-fetched from remote.