Page History

...

EVE has to deal with the intended (also called desired) state on one side, which in this case is EdgeDevConfig received from the controller, and with the actual (also known as current) state on the other side - the configuration currently applied on the device. The intended state is defined (using protobuf) to be more high-level, describing the intent but not the implementation. EVE first has to map it to low-level configuration primitives that implement the desired functionality. Next, it has to determine the difference between the currently running configuration and the new intended state. Finally, it perform performs all changes necessary as in the form of Create/Modify/Delete operations to transition from the current state to the new intended state. Ordering of these operations matters and it has to respect any dependencies that exist between configuration items. For example, an application virtual interface (VIF) cannot be inserted into a bridge if that bridge has not yet been yet created. This is further complicated by the fact that the configuration space is split between microservices, sending any updates between each other using pubsub.

...

The task no. 1 is specific to each microservice. It may also depend on the hardware (e.g. which virtualization technology to configure). Note that making this part of the code separated using interfaces/structs, would allow us to have the configuration implementation replacablereplaceable. For example, it might be possible to easily switch from Linux network stack to some vswitch, like OVS.

However, the topic of this proposal is the tasks 2. and 3., which can be solved generically using a dependency graph. Every configuration primitive rendered from the step 1. can be represented by a single graph node. Dependencies between items are modeled using directed edges. For the start, we will need to support dependency with the semantics "must exists". For example, if a route depends on a particular network interface to be configured first, there would be an edge originating at the route's graph node and pointing to the node of the network interface. A visual example with dependency graph representing network configuration is shown below:

...

For the step 3.c, the graph needs to have an access to handlers of configuration items. For the graph this can be a structure that implements an interface with Create/Modify/Delete. For every distinct configuration item type (like "Linux route", "container", "Linux bridge", "dnsmasq"), there will be a separate handler registered with the graph. For the graph, these handlers (in the implementation called "Configurators") are essentially backends or drivers, that the graph calls as needed to synchronize the current state with the latest desired state.

Dependency graph will not only allow to solve a common problem in one place, therefore shortening the code size and the complexity of microservices that will use it, but it will also enforce a much more readable and sustainable programming style. Consider the following comparison between the current and the new programming style (note that the code here is only symbolic, not actually taken from EVE):

whenNetworkConfigChanges() {
	determineObsoleteVlans()
	removeObsoleteVlans()
	determineObsoleteBonds()
	removeObsoleteBonds()
	changeInterfaceIPsIfNeeded()
	ifSomethingChangedRestartDnsmasq()
	addNewBonds()
	addNewVlans()
	...
}

whenNetworkConfigChanges() {
	newConfig := []ConfigItem{
		interface(params),
		arpEntry(params),
		arpEntry(params),
		route(params),
		route(params),
		bridge(params),
		dnsmasq(params),
		// A comment explaining why this config item is here…
		iptablesChain(params),
		iptablesChain(params),
	    ...
	}
	graph.Cluster(<network-instance-name>).Put(newConfig)
	err := graph.Sync()
    ...
}

Note that the example also presents the concept of clustering (subgraphs), that the depency graph will support and which was borrowed from graphviz. Having support for graph clustering will allow us to group items which are in some way related to each other. For example, all components of the same application (domain, volume, VIFs) could be grouped under one cluster. This will be mostly done to simplify modifications to the intended state. As shown in the example, intended state of a single specific network instance will be replaces with one call: graph.Cluster(<network-instance-name>).Put(newConfig)

Note that the new approach is not only easier for the developer and therefore less bug-prone, but also allows to explicitly express the intent (= newConfig), while steps (sequence of configuration changes) needed to take to get there are implicit. Compare that with the current approach, where the steps are explicit, but the programmer's intent is implicit. To determine what the program is trying to configure, one must study the code thoroughly and build a mental image of the intended state. If the programmer made a mistake in that complex code, one might get a wrong idea of what the intended state is.