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SnapshotCache

SnapshotCache is a snapshot-based caching mechanism that maintains a versioned config snapshot per node. See the original README scope section for more detailed explanations on the individual cache systems.

NOTE: SnapshotCache can operate as a REST or bi-di streaming xDS server

Create a Snapshot Cache

To create a snapshot cache, we simply call the provided constructor:

// Create a cache
cache := cache.NewSnapshotCache(false, cache.IDHash{}, nil)

This cache object holds a fully compliant SnapshotCache with the necessary methods to manage a configuration cache lifecycle.

NOTE: The cache needs a high access level inside the management server as it's the source of truth for xDS resources.

Snapshots

Snapshots are groupings of resources at a given point in time for a node cluster. In other words Envoy's and consuming xDS clients registering as node abc all share a snapshot of config. This snapshot is the singular source of truth in the cache that represents config for any of those consumers.

NOTE: Snapshots can be partial, e.g., only including RDS or EDS resources.

snap, err := cache.NewSnapshot("v0", map[resource.Type][]types.Resource{
    resource.EndpointType:        endpoints,
    resource.ClusterType:         clusters,
    resource.RouteType:           routes,
    resource.ScopedRouteType:     scopedRoutes,
    resource.ListenerType:        listeners,
    resource.RuntimeType:         runtimes,
    resource.SecretType:          secrets,
    resource.ExtensionConfigType: extensions,
})

For a more in-depth example of how to genereate a snapshot, explore our example found here.

We recommend verifying that your new snapshot is consistent within itself meaning that the dependent resources are exactly listed in the snapshot:

if err := snapshot.Consistent(); err != nil {
   l.Errorf("snapshot inconsistency: %+v\n%+v", snapshot, err)
   os.Exit(1)
}
  • all EDS resources are listed by name in CDS resources
  • all SRDS/RDS resources are listed by name in LDS resources
  • all RDS resources are listed by name in SRDS resources

NOTE: Clusters and Listeners are requested without name references, so Envoy will accept the snapshot list of clusters as-is even if it does not match all references found in xDS.

Setting a snapshot is as simple as:

// Add the snapshot to the cache
if err := cache.SetSnapshot("envoy-node-id", snapshot); err != nil {
    l.Errorf("snapshot error %q for %+v", err, snapshot)
    os.Exit(1)
}

This will trigger all open watches internal to the caching config watchers and anything listening for changes will received updates and responses from the new snapshot.

Note: that a node ID must be provided along with the snapshot object. Internally a mapping of the two is kept so each node can receive the latest version of its configuration.