Flow of a Single Blockchain
The Metal Blockchain network consists of 3 built-in blockchains: X-Chain, C-Chain, and P-Chain. The X-Chain is used to manage assets and uses the Avalanche consensus protocol. The C-Chain is used to create and interact with smart contracts and uses the Snowman consensus protocol. The P-Chain is used to coordinate validators and stake and also uses the Snowman consensus protocol. A set of validators makes up a Subnet. Subnets can validate 1 or more chains. It is a common misconception that 1 Subnet = 1 chain and this is shown by the primary Subnet of Metal which is made up of the X-Chain, C-Chain, and P-Chain.
A node in the Metal network can either be a validator or a non-validator. A validator stakes METAL tokens and participates in consensus to earn rewards. A non-validator does not participate in consensus or have any METAL staked but can be used as an API server. Both validators and non-validators need to have their own copy of the chain and need to know the current state of the network.
Each blockchain on Metal has several components: the virtual machine, database, consensus engine, sender, and handler. These components help the chain run smoothly. Blockchains also interact with the P2P layer and the chain router to send and receive messages.
specifies methods that build messages of type
communicate to other nodes by sending
All messaging functions in
OutboundMsgBuilder can be categorized as follows:
- Nodes need to be on a certain version before they can be accepted into the network.
- State Sync
- A new node can ask other nodes for the current state of the network. It only syncs the required state for a specific block.
- Nodes can ask other nodes for blocks to build their own copy of the chain. A node can fetch all blocks from the locally last accepted block to the current last accepted block in the network.
- Once a node is up to tip they can participate in consensus! During consensus, a node conducts a poll to several different small random samples of the validator set. They can communicate decisions on whether or not they have accepted/rejected a block.
- VMs communicate application-specific messages to other nodes through app messages. A common example is mempool gossiping.
Currently, MetalGo implements its own message serialization to communicate. In the future, MetalGo will use protocol buffers to communicate.
is shared across all chains. It implements functions from the
interface. The two functions it implements are
a message of type
OutboundMessage to a specific set of nodes (specified by an
Gossip sends a message of type
OutboundMessage to a
random group of nodes in a Subnet (can be a validator or a non-validator).
Gossiping is used to push transactions across the network. The networking
protocol uses TLS to pass messages between peers.
Along with sending and gossiping, the networking library is also responsible for making connections and maintaining connections. Any node, either a validator or non-validator, will attempt to connect to the primary network.
routes all incoming messages to its respective blockchain using
does this by pushing all the messages onto the respective Chain handler’s queue.
ChainRouter references all existing chains on the network such as the
X-chain, C-chain, P-chain and possibly any other chain. The
handles timeouts as well. When sending messages on the P2P layer, timeouts are
registered on the sender and cleared on the
ChainRouter side when a response
is received. If no response is received, then it triggers a timeout. Because
timeouts are handled on the
ChainRouter side, the handler is reliable.
Timeouts are triggered when peers do not respond and the
still notify the handler of failure cases. The timeout manager within
ChainRouter is also adaptive. If the network is experiencing long latencies,
timeouts will then be adjusted as well.
The main function of the
is to pass messages from the network to the consensus engine. It receives these
messages from the
ChainRouter. It passes messages by pushing them onto a sync
or Async queue (depends on message type). Messages are then popped from the
queue, parsed, and routed to the correct function in consensus engine. This can
be one of the following.
- State sync message (sync queue)
- Bootstrapping message (sync queue)
- Consensus message (sync queue)
- App message (Async queue)
The main role of the
is to build and send outbound messages. It is actually a very thin wrapper
around the normal networking code. The main difference here is that sender
registers timeouts and tells the router to expect a response message. The timer
starts on the sender side. If there is no response, sender will send a failed
response to the router. If a node is repeatedly unresponsive, that node will get
benched and the sender will immediately start marking those messages as failed.
If a sufficient amount of network deems the node benched, it might not get
rewards (as a validator).
Consensus is defined as getting a group of distributed systems to agree on an
outcome. In the case of the Metal network, consensus is achieved when
validators are in agreement with the state of the blockchain. The novel
consensus algorithm is documented here.
There are two main consensus algorithms: Avalanche and
The engine is responsible for adding proposing a new block to consensus,
repeatedly polling the network for decisions (accept/reject), and communicating
that decision to the
is what kick-starts everything in regards to blockchain creation, starting with
the P-Chain. Once the P-Chain finishes bootstrapping, it will kickstart C-Chain
and X-Chain and any other chains. The
Manager’s job is not done yet, if a
create-chain transaction is seen by a validator, a whole new process to create a
chain will be started by the
Manager. This can happen dynamically, long after
the 3 chains in the Primary Network have been created and bootstrapped.