All Peerplays nodes keep updating an internal database by applying the transactions as they arrive in incoming blocks. The difference between the node types lies in the amount of history they keep track of, and in the functionality they provide.
As the name implies, this is a node run by a Witness. Each Witness node validates all blocks and transactions it receives. The nodes of elected Witnesses take turns in bundling new transactions into blocks and broadcasting them to the network.
API nodes provide network services to client applications. They usually have account transaction histories accessible through API calls, but can vary in the amount of available history. These nodes have an open RPC port to expose the API.
A type of API node with a complete transaction history of all accounts.
Seed nodes accept incoming P2P connections. They are the first nodes contacted by a freshly started node; the entry point into the network. Once a node has entered the network it will receive additional node addresses from its peers, so all nodes can connect to each other. A seed node can also be an API node. Seed nodes are not mandatory, but highly recommended.
Bookie Oracle System nodes - BOS nodes are required to operate the Bookie Oracle System to ensure the accuracy and decentralization of the data fed into the BookiePro application. The BOS node must be run on a separate server to the Witness node.
Sidechain Operator Nodes - SONs facilitate the transfer of off-chain assets (like Bitcoin, Hive, or Ethereum tokens) between the Peerplays chain and the asset's native chain. These nodes often run the Peerplays node software and node software of other chains.
Can run together with
Elected by the community to produce blocks of validated transactions.
Provides an API gateway for apps to interact with the Peerplays chain. Full nodes offer the whole transaction history for all accounts.
Opening a P2P port allows new nodes to more readily perform the initial download of the Peerplays chain.
BOS nodes are whitelisted by Witnesses to feed data to the BookiePro app.
Elected by the community to facilitate asset transfers between the Peerplays chain and sidechains.
Likely (see note)
Every Witness is required to run nodes on both Public Mainnet (Alice) and Public Testnet (Beatrice) environments.
The minimum node requirements include a Witness node and BOS node for both Testnet and Mainnet. If you also run a Seed Node, API Node, and SON Node then the number of required servers could be as many as ten.