Keep service hints¶

Objective¶

Clients can use the Keep API to retrieve data that is stored on remote servers. As with local data, permission tokens and data hashes are provided by the API server in a manifest.

Background¶

When a client reads a data block referenced by a manifest, it requests a list of "available keep services" from the API server and (if there is more than one "disk" service on the list) uses the rendezvous hashing algorithm to select one.

Alternatives¶

Client libraries could communicate directly with non-Keep services.

• It would be impossible to use Arvados permission controls.
• An N×M array of code would have to be maintained in order to support N backing services from M SDK languages.
• The API server would have to maintain the mapping of hashes to remote data objects (and permissions for this map).

Each keepstore server could know how to communicate with each non-Keep service in use.

• Simpler client code.
• Artificial link between keep disk services and gateway services (they couldn't be independently scaled or shut down for maintenance).
• External clients couldn't be given direct access to the third-party gateway services without also giving them direct access to the disk services.
• Either the keepstore servers would have to keep their hash-to-remote-object mappings synchronized -- or the map of hash to remote service would be distributed across various servers. Either way introduces an unsuitable level of complexity: unlike in a native keepstore system, the underlying data is expected to change over time.
• When encountering an error (notably 404), client code would make many redundant attempts to read from various gateway services, based on the mistaken assumption that the various services have different sets of available data blocks.

High level design¶

Tools (TBD) can create manifests with +Kuuid hints, referencing data in remote storage services by indicating the UUID of a storage gateway capable of accessing it. In future work, Arvados can manage these hints actively: for example, data manager could tag blocks with S3 bucket names, and API server could load-balance S3 gateways by selecting one of several available gateway UUIDs for a given block.

Each client library knows that when it sees +Kuuid it should connect to the keep service with the given UUID (instead of using the usual rendezvous hashing algorithm to select a service).

Specifics¶

A block locator provided by the API server in a manifest might have a hint of the form +Kuuid where uuid is the UUID of a keep service. In order to retrieve the block data, the client should look up the keep service with the given UUID, and perform an HTTP GET request at the appropriate host and port.

• Given acbd18db4cc2f85cedef654fccc4a4d8+3+K1h9kt-bi6l4-20fty0xbp8l9wwe,
  • Retrieve https://1h9kt.arvadosapi.com/arvados/v1/keep_services/1h9kt-bi6l4-20fty0xbp8l9wwe to determine scheme, host, port
  • Retrieve data from {scheme}://{host}{port}/acbd18db4cc2f85cedef654fccc4a4d8+3+K1h9kt-bi6l4-20fty0xbp8l9wwe

As before, if a hint of the form +K{prefix} is given (where {prefix} is a string of five characters in [0-9a-z]), the client should perform a GET request at https://keep.{prefix}.arvadosapi.com/locator.

• Given acbd18db4cc2f85cedef654fccc4a4d8+3+K1h9kt,
  • Retrieve data from https://keep.1h9kt.arvadosapi.com/acbd18db4cc2f85cedef654fccc4a4d8+3+K1h9kt

Future work¶

Data manager can update manifests to reflect additional locations where data blocks can be retrieved: for example, +Kuuid1+Kuuid2 to signify that multiple remote gateways can retrieve the data, or +K+Kuuid1 to signify that the data is available locally and via a remote gateway.