19.4. When the model ecosystem is broken

Obviously, some models are essential for MyContexts to function correctly. These are:

model://perspectives.domains#System
model://perspectives.domains#Couchdb
model://perspectives.domains#Serialise
model://perspectives.domains#Sensor
model://perspectives.domains#Utilities

However, these models themselves have been created using MyContexts, particularly their machine-readable versions (DomeinFiles). To be able to do so we need yet more models:

model://perspectives.domains#CouchdbManagement
model://perspectives.domains#BodiesWithAccounts
model://perspectives.domains#Parsing

Using CouchdbManagement, we can describe

  • the CouchdbServer that represents the actual server with a CouchdbServer running on it

  • the Repository perspectives.domains for the namespace model://perspectives.domains/

  • the Manifests describing the models listed above

  • the Versions that hold the source codes and can compile the DomeinFiles.

There are several ways in which the ecosystem can break down:

  1. the MyContexts installation from which these descriptions have been published, has been lost, subdivided as

    1. the models and public instances are still available (on the actual server)

    2. just the models are still available

    3. no remote resources are left

  2. the MyContexts installation is intact but the remote data is all lost.

19.4.1. MyContexts lost, models and public instances available

First, observe that a fresh MyContexts installation can be created without problem. We can also install CouchdbManagement and it will come complete with the CouchdbManagementApp context instance. We just need to re-create the local versions of the published resources, starting with the CouchdbServer itself.

Here we will assume it is possible to restore local resources from public ones, as described in Restoring from public resources. We can trigger the restoration process by bringing an instance of the CouchdbManagementApp$CouchdbServers role in a state by providing the property PublicFiller with the value that identifies the external role of the public version of the CouchdbServer instance. On entering that state, we execute a function CopyPublicResource, that will retrieve the external role, followed by the context and its roles, recursively until the entire remotely available structure has been restored locally.

19.4.2. MyContexts lost, just models are available

Restore the ecosystem as follows:

  • make models available in the models_perspectives_domains database without version numbers. This is because the function retrieveDomeinFile will not find any manifests and thus will load versionless models.

  • It is now possible to create a fresh MyContexts installation, but it will have no BaseRepository and thus no list of models.

  • However, it should be possible to add CouchdbManagement to the installation. To this end, we’ve created a state of PerspectivesSystem that provides an action to User to do so.

  • Once CouchdbManagement is available, create the CouchdbServer, the Repository, the Manifests and their Versions and add the source code of the models.

19.4.3. Everything is lost

Assuming there is a copy of the essential DomeinFiles,

  • recreate the databases models_perspectives_domains and models_perspectives_domains_write (make sure they have no members, so are publicly accessible);

  • replicate from the write database to the read database;

  • replicate from the copy to the write database;

  • follow the procedure given in the previous paragraph.

19.4.4. Public databases have been lost but the MyContexts installation is intact

We have as yet no procedure for this case. It should be rare; it is entirely possible to protect against this situation by creating regular backups at the system level of the Couchdb databases on https://perspectives.domains.

19.4.5. Precautions to be taken

In order to prepare for the eventualities given above, it is imperative to

  • maintain a copy of the basic models in a safe place, ideally a CouchdbServer instance from which it is possible to restore them to the server at https://perspectives.domains.

  • keep a copy the source code of the basic models, lest they be lost forever with the Versions;

  • create backups at the system level of https://perspectives.domains, at the very least of the Couchdb installation.

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1. If no authoring role is provided by the API caller, we take it to be the System User.