Conceptual Data Tree


Data Tree:

An instantiated logical tree that represents configuration or operational state data of a modeled problem domain (for example, a controller or a network)

Data Tree Consumer:

A component acting on data, after this data are introduced into one or more particular subtrees of a Data Tree.

Data Tree Identifier:

A unique identifier for a particular subtree of a Data Tree. It is composed of the logical data store type and the instance identifier of the subtree’s root node. It is represented by a DOMDataTreeIdentifier.

Data Tree Producer:

A component responsible for providing data for one or more particular subtrees of a Data Tree.

Data Tree Shard:

A component responsible for providing storage or access to a particular subtree of a Data Tree.

Shard Layout:

A longest-prefix mapping between Data Tree Identifiers and Data Tree Shards responsible for providing access to a data subtree.

Basic Concepts

Data Tree is a Namespace

The concept of a data tree comes from RFC6020. It is is vaguely split into two instances, configuration and operational. The implicit assumption is that config implies oper, i.e. any configuration data is also a valid operational data. Further interactions between the two are left undefined and the YANG language is not strictly extensible in the number and semantics of these instances, leaving a lot to implementation details. An outline of data tree use, which is consistent with the current MD-SAL design, is described in draft-kwatsen-netmod-opstate.

The OpenDaylight MD-SAL design makes no inherent assumptions about the relationship between the configuration and operational data tree instances. They are treated as separate entities and they are both fully addressable via the DOMDataTreeIdentifier objects. It is up to MD-SAL plugins (e.g. protocol plugins or applications) to maintain this relationship. This reflects the asynchronous nature of applying configuration and also the fact that the intended configuration data may be subject to translation (such as template configuration instantiation).

Both the configuration and operational namespaces (data trees) are instances of the Conceptual Data Tree. Any data item in the conceptual data tree is addressed via a YangInstanceIdentifier object, which is a unique, hierarchical, content-based identifier. All applications use the identifier objects to identify data to MD-SAL services, which in turn are expected to perform proper namespace management such that logical operation connectivity is maintained.

Identifiers versus Locators

It is important to note that when we talk about Identifiers and Locators, we do not mean URIs and URLs, but rather URNs and URLs as strictly separate entities. MD-SAL plugins do not have access to locators and it is the job of MD-SAL services to provide location independence.

The details of how a particular MD-SAL service achieves location independence is currently left up to the service’s implementation, which leads to the problem of having MD-SAL services cooperate, such as storing data in different backends (in-memory, SQL, NoSQL, etc.) and providing unified access to all available data. Note that data availability is subject to capabilities of a particular storage engine and its operational state, which leads to the design decision that a YangInstanceIdentifier lookup needs to be performed in two steps:

  1. A longest-prefix match is performed to locate the storage backend instance for that identifier

  2. Masked path elements are resolved by the storage engine

Data Tree Shard

A process similar to the first step above is performed today by the Distributed Data Store implementation to split data into Shards. The concept of a Shard as currently implemented is limited to specifying namespaces, and it does not allow for pluggable storage engines.

In context of the Conceptual Data Tree, the concept of a Shard is generalized as the shorthand equivalent of a storage backend instance. A Shard can be attached at any (even wildcard) YangInstanceIdentifier. This contract is exposed via the DOMShardedDataTree, which is an MD-SAL SPI class that implements an YangInstanceIdentifier -> Shard registry service. This is an omnipresent MD-SAL service, Shard Registry, whose visibility scope is a single OpenDaylight instance (i.e. a cluster member). Shard Layout refers to the mapping information contained in this service.

Federation, Replication and High Availability

Support for various multi-node scenarios is a concern outside of core MD-SAL. If a particular scenario requires the shard layout to be replicated (either fully or partially), it is up to Shard providers to maintain an omnipresent service on each node, which in turn is responsible for dynamically registering DOMDataTreeShard instances with the Shard Registry service.

Since the Shard Layout is strictly local to a particular OpenDaylight instance, an OpenDaylight cluster is not strictly consistent in its mapping of YangInstanceIdentifier to data. When a query for the entire data tree is executed, the returned result will vary between member instances based on the differences of their Shard Layouts. This allows each node to project its local operational details, as well as the partitioning of the data set being worked on based on workload and node availability.

Partial symmetry of the conceptual data tree can still be maintained to the extent that a particular deployment requires. For example the Shard containing the OpenFlow topology can be configured to be registered on all cluster members, leading to queries into that topology returning consistent results.


Data Tree Listener

A Data Tree Listener is a data consumer, for example a process that wants to act on data after it has been introduced to the Conceptual Data Tree.

A Data Tree Listener implements the DOMDataTreeListener interface and registers itself using DOMDataTreeService.

A Data Tree Listener may register for multiple subtrees. Each time it is invoked it will be provided with the current state of all subtrees that it is registered for.

.DOMDataTreeListener interface signature

public interface DOMDataTreeListener extends EventListener {

    void onDataTreeChanged(Collection<DataTreeCandidate> changes, // (1)
        Map<DOMDataTreeIdentifier, NormalizedNode<?, ?>> subtrees);

    void onDataTreeFailed(Collection<DOMDataTreeListeningException> causes); // (2)
  1. Invoked when the data tree to which the Data Tree Listener is subscribed to changed. changes contains the collection of changes, subtrees contains the current state of all subtrees to which the listener is registered.

  2. Invoked when a subtree listening failure occurs. For example, a failure can be triggered when a connection to an external subtree source is broken.

Data Tree Producer

A Data Tree Producer represents source of data in system. Data TreeProducer implementations are not required to implement a specific interface, but use a DOMDataTreeProducer instance to publish data (i.e. to modify the Conceptual Data Tree).

A Data Tree Producer is exclusively bound to one or more subtrees of the Conceptual Data Tree, i.e. binding a Data Tree Producer to a subtree prevents other Data Tree Producers from modifying the subtree.

  • A failed Data Tree Producer still holds a claim to the namespace to which it is bound (i.e. the exclusive lock of the subtree) until it is closed.

DOMDataTreeProducer represents a Data Tree Producer context

  • allows transactions to be submitted to subtrees specified at creation time

  • at any given time there may be a single transaction open.

  • once a transaction is submitted, it will proceed to be committed asynchronously.

.DOMDataTreeProducer interface signature

public interface DOMDataTreeProducer extends DOMDataTreeProducerFactory, AutoCloseable {
    DOMDataWriteTransaction createTransaction(boolean isolated); // (1)
    DOMDataTreeProducer createProducer(Collection<DOMDataTreeIdentifier> subtrees); // (2)
  1. Allocates a new transaction. All previously allocated transactions must have been either submitted or canceled. Setting isolated to true disables state compression for this transaction.

  2. Creates a sub-producer for the provided subtrees. The parent producer loses the ability to access the specified paths until the resulting child producer is closed.

Data Tree Shard

  • A Data Tree Shard is always bound to either the OPERATIONAL, or the CONFIG space, never to both at the same time.

  • Data Tree Shards may be nested, the parent shard must be aware of sub-shards and execute every request in context of a self-consistent view of sub-shards liveness. Data requests passing through it must be multiplexed with sub-shard creations/deletions. In other words, if an application creates a transaction rooted at the parent Shard and attempts to access data residing in a sub-shard, the parent Shard implementation must coordinate with the sub-shard implementation to provide the illusion that the data resides in the parent shard. In the case of a transaction running concurrently with sub-shard creation or deletion, these operations need to execute atomically with respect to each other, which is to say that the transactions must completely execute as if the sub-shard creation/deletion occurred before the transaction started or as if the transaction completed before the sub-shard creation/deletion request was executed. This requirement can also be satisfied by the Shard implementation preventing transactions from completing. A Shard implementation may choose to abort any open transactions prior to executing a sub-shard operation.

  • Shard Layout is local to an OpenDaylight instance.

  • Shard Layout is modified by agents (registering / unregistering Data Tree Shards) in order to make the Data Tree Shard and the underlaying data available to plugins and applications executing on that particular OpenDaylight instance.

Registering a Shard with the Conceptual Data Tree


Namespace in this context means a Data Tree Identifier prefix.

  1. Claim a namespace - An agent that is registering a shard must prove that it has sufficient rights to modify the subtree where the shard is going to be attached. A namespace for the shard is claimed by binding a Data Tree Producer instance to same subtree where the shard will be bound. The Data Tree Producer must not have any open child producers, and it should not have any outstanding transactions.

  2. Create a shard instance - Once a namespace is claimed, the agent creates a shard instance.

  3. Attach shard - The agent registers the created shard instance and provides in the reigstration the Data Tree Producer instance to verify the namespace claim. The newly created Shard is checked for its ability to cooperate with its parent shard. If the check is successful, the newly created Shard is attached to its parent shard and recorded in the Shard layout.

  4. Remove namespace claim (optional) - If the Shard is providing storage for applications, the agent should close the Data Tree Producer instance to make the subtree available to applications.


Steps 1, 2 and 3 may fail, and the recovery strategy depends on which step failed and on the failure reason.