Monday, September 23, 2013

Putting it all together

A ChangeSafe repository is implemented as a transient wrapper object around a persistent object. The wrapper object caches some immutable metadata. You'd hate to have to run a transaction in the middle of the print function in order to print the repository name. The wrapper also contains metadata associated with the backing store that the repository is using.

Oh yeah, there is something interesting going on in the wrapper, We keep track of the ongoing transactions by mapping the user-id to a list of transaction contexts (every nested transaction by a user "pushes" a new txn-context).

Anyway, it's the repository-persistent-information that has the interesting stuff:
(defclass repository-persistent-information ()
  (
   (type  :initarg :type
          :initform (error "Required initarg :type omitted.")
          :reader repository-persistent-information/type
          :type repository-type)

   ;; Database parent is the root extent for an extent database, or the master database for a satellite.
   ;; Root extents or master repositories won't have a parent
   (parent-repository :initarg :parent-repository
                      :initform nil
                      :reader repository-persistent-information/parent
                      :type (optional relative-pathname))

   ;; Satellite repositories is non-nil only for master repositories.
   (satellite-repositories :initform nil
                           :initarg :satellite-repositories
                           :accessor repository-persistent-information/satellite-repositories)

   (canonical-class-dictionary :initform (make-instance 'canonical-class-dictionary)
                               :reader repository-persistent-information/canonical-class-dictionary)
   (cid-master-table :initform (make-instance 'cid-master-table)
                     :reader repository-persistent-information/cid-master-table)
   (root-mapper  :initarg :root-mapper
                 :initform (error "Required initarg :root-mapper omitted.")
                 :reader repository-persistent-information/root-mapper)
   (cid-mapper   :initarg :cid-mapper
                 :initform (error "Required initarg :cid-mapper omitted.")
                 :reader repository-persistent-information/cid-mapper)
   (local-mapper :initarg :local-mapper
                 :initform (error "Required initarg :local-mapper omitted.")
                 :reader repository-persistent-information/local-mapper)
   (locally-named-roots :initarg :locally-named-roots
                        :initform (error "Required initarg :locally-named-roots omitted.")
                        :reader repository-persistent-information/locally-named-roots)
   (anonymous-user :initarg :anonymous-user
                   :initform nil
                   :reader repository-persistent-information/anonymous-user))
  (:default-initargs :node-id +object-id-of-root+)  ;; force this to always be the root object.
  (:documentation "Persistent information describing a repositiory, and stored in the repository")
  (:metaclass persistent-standard-class)
  (:schema-version 0))

The repository-type is just a keyword:
(defconstant *repository-types* '(:basic :master :satellite :transport :extent :workspace)
  "Type of repositories.  Note that all but :EXTENT types of repositories
   serve as root extents for databases which have multiple extents, and therefore imply extent.")
The parent-repository and the
satellite-repositories are for juggling multiple "satellite" repositories for holding particular subsets of changes (for, say, geographically distributing the servers for different product groups).

The canonical-class-dictionary is an intern table for objects.

The cid-master-table is (logically) the collection of audit-records. A CID (after change id) is represented as an integer index into the master table.

The root-mapper is a mapping table from distributed identifiers to objects.

The cid-mapper is a mapping table from the distributed identifier that represents the CID to the integer index of that CID in the master table. It is a subtable of the local mapper.

The local-mapper is submapping of the root-mapping, but a supermapping of the cid-mapper.

The locally-named-rootsis a hash table for storing the root objects of the repository.

Finally, there is the anonymous-user slot, which is the user id assigned for bootstrapping.

And all this crap is in support of this procedure:
(defun call-with-repository-transaction (&key repository 
                                              transaction-type
                                              user-id-specifier
                                              reason

                                              ;; generally, you only want to specify these two
                                              meta-cid-set-specifier
                                              cid-set-specifier 
                                              ;; but if you are doing a comparison,
                                              ;; specify these as well
                                              aux-meta-cid-set-specifier
                                              aux-cid-set-specifier 

                                              receiver)
  (check-type user-id-specifier (or keyword distributed-identifier))
  (check-type transaction-type repository-transaction-type)
  (check-type reason string)
  ;; implementation omitted for brevity, ha ha
  )

Naturally we need to specify the :repository, the :transaction-type is one of
(defconstant *repository-transaction-types* '(:read-only
                                              :read-write
                                              :read-cons
                                              :read-only-compare
                                              :read-cons-nonversioned
                                              :read-only-nonversioned
                                              :read-write-nonversioned))
The :user-id-specifier should be a distributed-identifier of a core-user instance.

The :reason is a human readable string describing the transaction.

The :meta-cid-set-specifier is mumble, mumble... just a sec...

The :cid-set-specifier is how you specify which CIDs will form the basis view for the transaction. We allow this to be a procedure that returns a cid-set object, and we will call this procedure as we are setting up the transaction and use the :meta-cid-set-specifier to specify the CIDs to form the versioned view the procedure will see.

The :meta-cid-set-specifier can be the symbol :latest-metaversion, a timestamp, or a cid-set. :latest-metaversion means to use all CIDS while resolving the :cid-set-specifier, a timestamp is useful for rewinding the world, and the main use for using an explicit cid-set is for synchronizing views between master and satellite repositories.

The :receiver is invoked within the dynamic extent of a transaction. It is passed a core-txn object that contains the metadata associated with the transaction.

The ChangeSafe core components are the repository that holds changes and associated meta-information, and simple versioned CLOS objects. It is only useful as a foundation layer, though.

Next up, another level of abstraction...
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