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Nodes
Summary
A node is JVM run-time with a unique network identity running the Corda software
The node has two interfaces with the outside world:
- A network layer, for interacting with other nodes
- RPC, for interacting with the node's owner
The node's functionality is extended by installing CorDapps in the plugin registry
Node architecture
A Corda node is a JVM run-time environment with a unique identity on the network that hosts Corda services and CorDapps.
We can visualize the node's internal architecture as follows:
The core elements of the architecture are:
- A persistence layer for storing data
- A network interface for interacting with other nodes
- An RPC interface for interacting with the node's owner
- A service hub for allowing the node's flows to call upon the node's other services
- A cordapp interface and provider for extending the node by installing CorDapps
Persistence layer
The persistence layer has two parts:
- The vault, where the node stores any relevant current and historic states
- The storage service, where it stores transactions, attachments and flow checkpoints
The node's owner can query the node's storage using the RPC interface (see below).
Network interface
All communication with other nodes on the network is handled by the node itself, as part of running a flow. The node's owner does not interact with other network nodes directly.
RPC interface
The node's owner interacts with the node via remote procedure calls (RPC). The key RPC operations the node exposes are documented in api-rpc.
The service hub
Internally, the node has access to a rich set of services that are used during flow execution to coordinate ledger updates. The key services provided are:
- Information on other nodes on the network and the services they offer
- Access to the contents of the vault and the storage service
- Access to, and generation of, the node's public-private keypairs
- Information about the node itself
- The current time, as tracked by the node
The CorDapp provider
The CorDapp provider is where new CorDapps are installed to extend the behavior of the node.
The node also has several CorDapps installed by default to handle common tasks such as:
- Retrieving transactions and attachments from counterparties
- Upgrading contracts
- Broadcasting agreed ledger updates for recording by counterparties
Draining mode
In order to operate a clean shutdown of a node, it is important than no flows are in-flight, meaning no checkpoints should be persisted. The node is able to be put in a Flows Draining Mode, during which:
- Commands requiring to start new flows through RPC will be rejected.
- Scheduled flows due will be ignored.
- Initial P2P session messages will not be processed, meaning peers will not be able to initiate new flows involving the node.
- All other activities will proceed as usual, ensuring that the number of in-flight flows will strictly diminish.
As their number - which can be monitored through RPC - reaches zero, it is safe to shut the node down. This property is durable, meaning that restarting the node will not reset it to its default value and that a RPC command is required.