Syncing

Scenarios

• 1-way: On setup, Server A's packages are copied to Server B. On sync, changes to packages on Server A are transferred to Server B.

• 2-way: On setup, packages from each server is copied to the other. On sync, changes on each are transferred to the other.

We will focus now on 1-way, leaving 2-way for the future.

Requirements

• Merging of changes from both machines. If there is a conflict then it is logged and a result is chosen.

• Use of Server A and Server B continues undisturbed during sync.

Issues

• Clashes of package/tag/group names.

• Sync between CKAN instances of different versions of ckan & vdm.

• Unversioned objects - make versioned? User, Group, Authz, Rating.

• How to test system.

Use cases

• First sync - all packages and revisions are copied from Server A to Server B.

• Subsequent sync after package changes on A and/or B.

• Sync after package purged on A. (Package also purged on B.)

• Sync after package purged on B. (Package not recreated on B.)

• Server B syncs at different times from a third server.

• Package/Tag/Group? name on Server A clashes with an existing one on Server B. Log all of them. Merge tag and group. Not sure about package.

• Objects on Server A with restricted authz are by default editable on Server B.

Operation

First sync - 10am

Server B asks "Give me all your revisions and unrevisioned objects."

Server A replies "Rev1 and associated revisions Pkg1Rev1, Pkg2Rev1, !PkgTagRev1, !PkgResource1; Rev2 with Pkg1Rev2; Tag1; User1; !PackageGroup1; Group1; ratings"

Server B creates Rev1, Pkg1Rev1, Pkg2Rev1, Pkg1Rev2, Pkg1, Pkg2, !PkgTagRev1, !PkgResource1, Tag1, User1, Auth, !PackageGroup1, Group1. UUIDs are the same as Server A.

Server B updates search vectors for Pkg1 and Pkg2.

Server records the time of the sync - 10am.

Meanwhile - 10.20am

On Server A, user updates Pkg1 twice, creating Rev2/Pkg1Rev3 and Rev3/Pkg1Rev4PkgTagRev2. User1 updates his name.

Meanwhile - 10.40am

On Server B, user updates Pkg1 once, creating Pkg1Rev5.

Sesequent sync

Server B asks "Give me revisions and diffs since 10am."

Server A replies Rev2/Pkg1Rev3, Rev3/Pkg1Rev4 and gives diff of Pkg1Rev2 -> Pkg1Rev4

Server B looks at its own revisions since 10am and sees Pkg1 now has two heads. It calculates diff of Pkg1Rev2 -> Pkg1Rev5.

Server B takes Pkg1Rev2 and applies the two diffs in the order of priority, logging any conflicts, calling the result Pkg1Rev6.

Tickets

• Sync set-up stored in config file (server URI). Last sync status stored in local db.

• Repository method 'all_revs_since'. It returns all revisions since a time/revision (or since the beginning).

• Object method 'diff'. It returns a Diff object which is the diff of two ObjectResources. Already exists for Package, but need for PackageTag, PackageExtra

• Revision method 'serialize'.

• Diff method 'serialize'.

• API access to revisions: /api/rest/revision?since=ab49f348-fd23-ae3c

• API access to diffs?: /api/rest/package?diff-from&diff-to=ab49f348-fd23-ae3c

• API access to unrevisioned objects?

• Repository method 'import_revisions'. It takes serialised revisions and diffs and creates revision objects exactly matching spec.

• Object method 'merge_diffs'. It takes an original object and two diffs that apply to it and applies them both in a new revision.