This is a simple service.

It receives a request to compute a fixity for a file to which it has access, described as a relative path from a fixed route, or a key into S3 storage. The remote requester presumably also has access to that file and can compute the relative path, though the absolute path may differ. The advantages of having this server compute it instead of the remote server are a) the work is offloaded and b) this server is ideally has better access to the content. I.e. in our case the remote server has an NFS mount and this server ideally has a direct GPFS mount. Also, this should be easily modifiable also to compute fixities of resources specified by urls.

Configuration

config/medusa_fixity_server.yaml contains the amqp configuration and information about the medusa_storage roots, including a default root if desired. See the template for available parameters.

Running

The medusa_fixity.sh script can be used to start and stop the server. There is also a toggle-halt command that will let the server finish the request it is working on and then halt (or if used again go cancel this behavior). This works by sending USR2 to the server, so you can do that manually as well.

Requests

A request is a JSON object with three fields:

• action: The action being requested. Currently only 'file_fixity' is supported.
• parameters: Parameters needed for the action.
• pass_through: A JSON object that the server will pass back to the client with its response. The intended use is for the client to be able to know what is being responded to.

A response is a JSON object with fields:

• pass_through: Whatever the client originally passed in this field, or nil if absent.
• status: Either 'success' or 'failure'.
• error_message: If the status is failure then this may be returned to give information.
• action: The originally requested action
• parameters: A JSON object with parameters appropriate to the request. For the fixity request would have the hash type(s) and value(s).

Note that for certain errors (e.g. if the request isn't parseable as JSON) it may not be possible to return some of these things.

file_fixity action:

• Incoming parameters:

  • path - this is the file path/key relative to the root.
  • algorithms - an array of names of fixity algorithms to be computed. Possible values include "md5" and "sha1". Unknown values will be ignored. If there are no valid values then the md5 only will be computed and returned.
  • root (optional) - which storage root the content lies under. Uses the defined default root if not present.

And:

• Outgoing parameters:

  • found - a boolean telling if the file was found. If not then checksums will be empty, but the server interaction is still considered to be a success.
  • checksums - an object with keys the hash types and values the computed checksums