This document is deprecated as of Ganeti Web Manager version 0.11.
Ganeti Web Manager uses a cache system that stores information about ganeti clusters in the database. This allows the following:
- Permissions are stored in the database and are associated to the cached objects
- The cached data can be searched and or filtered
- Limits the amount of traffic between the webserver and ganeti cluster.
The cache system is transparent and will load cached data automatically when the object is initialized.
Lazy Cache Refresh¶
Cached objects will refresh themselves transparently when they are out of date. This happens transparently when objects are queried from the ORM. Lazy cache refreshing is inefficient, it will cause multiple calls to the ganeti RAPI to fetch information. For this reason the lazy refresh mechanism is intended to only be used for testing, and as a backup to ensure that objects will always be refreshed.
The functionality for lazy caching is built into an abstract model, CachedClusterObject. Extending this model will enable caching for the object. It requires that _refresh() be implemented with an object specific method for querying fresh info from ganeti. Currently only Cluster and VirtualMachine are cached, but this may extend to Node and Job objects in the future.
parse_persistent_info() can be overridden to parse object specific properties that should be stored in the database. This allows properties to be used as query filters, without requiring the entire object to be loaded.
Bypassing The Cache Refresh¶
It is not currently possible to bypass the automatic cache refresh in a simple way since it is part of the models *init*. Currently the only way to bypass the cache is to query the object with a values or values_list query, and copy the values into a new object.
values = VirtualMachine.objects.get(id=id) vm = VirtualMachine() for k, v in values.items(): setattr(vm, k , v)
RAPI Client Cache¶
Ganeti remote API clients are also cached. This reduces the number of database calls to retrieve a client capable of connecting to a cluster. This is a deterministic cache based off connection credentials. The keys are a hash of hostname, port, user, and password. This allows changes in settings to be easily detected. Cached objects should store the hash as part of its model and use it to look up existing clients without querying the cluster for the full set of connection credentials.