Design Overview

This document sketches the design for Zebra.

Desiderata

The following are general desiderata for Zebra:

  • [George's list..]

  • As much as reasonably possible, it and its dependencies should be implemented in Rust. While it may not make sense to require this in every case (for instance, it probably doesn't make sense to rewrite libsecp256k1 in Rust, instead of using the same upstream library as Bitcoin), we should generally aim for it.

  • As much as reasonably possible, Zebra should minimize trust in required dependencies. Note that "minimize number of dependencies" is usually a proxy for this desideratum, but is not exactly the same: for instance, a collection of crates like the tokio crates are all developed together and have one trust boundary.

  • Zebra should be well-factored internally into a collection of component libraries which can be used by other applications to perform Zcash-related tasks. Implementation details of each component should not leak into all other components.

  • Zebra should checkpoint on Canopy activation and drop all Sprout-related functionality not required post-Canopy.

Non-Goals

  • Zebra keeps a copy of the chain state, so it isn't intended for lightweight applications like light wallets. Those applications should use a light client protocol.

Notable Blog Posts

Service Dependencies

Note: dotted lines are for "getblocktemplate-rpcs" feature

services transaction_verifier transaction_verifier state state transaction_verifier->state mempool mempool mempool->transaction_verifier mempool->state peer_set peer_set mempool->peer_set inbound inbound inbound->state inbound->mempool block_verifier_router block_verifier_router inbound->block_verifier_router rpc_server rpc_server rpc_server->state rpc_server->mempool rpc_server->block_verifier_router checkpoint_verifier checkpoint_verifier block_verifier_router->checkpoint_verifier block_verifier block_verifier block_verifier_router->block_verifier checkpoint_verifier->state syncer syncer syncer->block_verifier_router syncer->peer_set block_verifier->transaction_verifier block_verifier->state

Architecture

Unlike zcashd, which originated as a Bitcoin Core fork and inherited its monolithic architecture, Zebra has a modular, library-first design, with the intent that each component can be independently reused outside of the zebrad full node. For instance, the zebra-network crate containing the network stack can also be used to implement anonymous transaction relay, network crawlers, or other functionality, without requiring a full node.

At a high level, the fullnode functionality required by zebrad is factored into several components:

  • zebra-chain, providing definitions of core data structures for Zcash, such as blocks, transactions, addresses, etc., and related functionality. It also contains the implementation of the consensus-critical serialization formats used in Zcash. The data structures in zebra-chain are defined to enforce structural validity by making invalid states unrepresentable. For instance, the Transaction enum has variants for each transaction version, and it's impossible to construct a transaction with, e.g., spend or output descriptions but no binding signature, or, e.g., a version 2 (Sprout) transaction with Sapling proofs. Currently, zebra-chain is oriented towards verifying transactions, but will be extended to support creating them in the future.

  • zebra-network, providing an asynchronous, multithreaded implementation of the Zcash network protocol inherited from Bitcoin. In contrast to zcashd, each peer connection has a separate state machine, and the crate translates the external network protocol into a stateless, request/response-oriented protocol for internal use. The crate provides two interfaces:

    • an auto-managed connection pool that load-balances local node requests over available peers, and sends peer requests to a local inbound service, and
    • a connect_isolated method that produces a peer connection completely isolated from all other node state. This can be used, for instance, to safely relay data over Tor, without revealing distinguishing information.

  • zebra-script provides script validation. Currently, this is implemented by linking to the C++ script verification code from zcashd, but in the future we may implement a pure-Rust script implementation.

  • zebra-consensus performs semantic validation of blocks and transactions: all consensus rules that can be checked independently of the chain state, such as verification of signatures, proofs, and scripts. Internally, the library uses tower-batch-control to perform automatic, transparent batch processing of contemporaneous verification requests.

  • zebra-state is responsible for storing, updating, and querying the chain state. The state service is responsible for contextual verification: all consensus rules that check whether a new block is a valid extension of an existing chain, such as updating the nullifier set or checking that transaction inputs remain unspent.

  • zebrad contains the full node, which connects these components together and implements logic to handle inbound requests from peers and the chain sync process.

All of these components can be reused as independent libraries, and all communication between stateful components is handled internally by internal asynchronous RPC abstraction ("microservices in one process").

zebra-chain

Internal Dependencies

None: these are the core data structure definitions.

Responsible for

  • definitions of commonly used data structures, e.g.,

    • Block,
    • Transaction,
    • Address,
    • KeyPair...

  • parsing bytes into these data structures

  • definitions of core traits, e.g.,

    • ZcashSerialize and ZcashDeserialize, which perform consensus-critical serialization logic.

Exported types

  • [...]

zebra-network

Internal Dependencies

  • zebra-chain

Responsible for

  • definition of a well structured, internal request/response protocol
  • provides an abstraction for "this node" and "the network" using the internal protocol
  • dynamic, backpressure-driven peer set management
  • per-peer state machine that translates the internal protocol to the Bitcoin/Zcash protocol
  • tokio codec for Bitcoin/Zcash message encoding.

Exported types

  • Request, an enum representing all possible requests in the internal protocol;
  • Response, an enum representing all possible responses in the internal protocol;
  • AddressBook, a data structure for storing peer addresses;
  • Config, a configuration object for all networking-related parameters;
  • init<S: Service>(Config, S) -> (impl Service, Arc<Mutex<AddressBook>>), the main entry-point.

The init entrypoint constructs a dynamically-sized pool of peers sending inbound requests to the provided S: tower::Service representing "this node", and returns a Service that can be used to send requests to "the network", together with an AddressBook updated with liveness information from the peer pool. The AddressBook can be used to respond to inbound requests for peers.

All peerset management (finding new peers, creating new outbound connections, etc) is completely encapsulated, as is responsibility for routing outbound requests to appropriate peers.

zebra-state

Internal Dependencies

  • zebra-chain for data structure definitions.

Responsible for

  • block storage API
    • operates on parsed block structs
      • these structs can be converted from and into raw bytes
    • primarily aimed at network replication, not at processing
    • can be used to rebuild the database below
  • maintaining a database of tx, address, etc data
    • this database can be blown away and rebuilt from the blocks, which are otherwise unused.
    • threadsafe, typed lookup API that completely encapsulates the database logic
    • handles stuff like "transactions are reference counted by outputs" etc.
  • providing tower::Service interfaces for all of the above to support backpressure.

Exported types

  • Request, an enum representing all possible requests in the internal protocol;
    • blocks can be accessed via their chain height or hash
    • confirmed transactions can be accessed via their block, or directly via their hash
  • Response, an enum representing all possible responses in the internal protocol;
  • init() -> impl Service, the main entry-point.

The init entrypoint returns a Service that can be used to send requests for the chain state.

All state management (adding blocks, getting blocks by index or hash) is completely encapsulated.

zebra-script

Internal Dependencies

  • ??? depends on how it's implemented internally

Responsible for

  • the minimal Bitcoin script implementation required for Zcash
  • script parsing
  • context-free script validation

Notes

This can wrap an existing script implementation at the beginning.

If this existed in a "good" way, we could use it to implement tooling for Zcash script inspection, debugging, etc.

Questions

  • How does this interact with NU4 script changes?

Exported types

  • [...]

zebra-consensus

Internal Dependencies

  • zebra-chain for data structures and parsing.
  • zebra-state to read and update the state database.
  • zebra-script for script parsing and validation.

Responsible for

  • consensus-specific parameters (network magics, genesis block, pow parameters, etc) that determine the network consensus
  • consensus logic to decide which block is the current block
  • block and transaction verification
    • context-free validation, e.g., signature, proof verification, etc.
    • context-dependent validation, e.g., determining whether a transaction is accepted in a particular chain state context.
    • verifying mempool (unconfirmed) transactions
  • block checkpoints
    • mandatory checkpoints (genesis block, canopy activation)
    • optional regular checkpoints (every Nth block)
  • modifying the chain state
    • adding new blocks to ZebraState, including chain reorganisation
    • adding new transactions to ZebraMempoolState
  • storing the transaction mempool state
    • mempool transactions can be accessed via their hash
  • providing tower::Service interfaces for all of the above to support backpressure and batch validation.

Exported types

  • block::init() -> impl Service, the main entry-point for block verification.
  • ZebraMempoolState
    • all state management (adding transactions, getting transactions by hash) is completely encapsulated.
  • mempool::init() -> impl Service, the main entry-point for mempool transaction verification.

The init entrypoints return Services that can be used to verify blocks or transactions, and add them to the relevant state.

zebra-rpc

Internal Dependencies

  • zebra-chain for data structure definitions
  • zebra-node-services for shared request type definitions
  • zebra-utils for developer and power user tools

Responsible for

  • rpc interface

Exported types

  • [...]

zebra-client

Internal Dependencies

  • zebra-chain for structure definitions
  • zebra-state for transaction queries and client/wallet state storage
  • zebra-script possibly? for constructing transactions

Responsible for

  • implementation of some event a user might trigger
  • would be used to implement a full wallet
  • create transactions, monitors shielded wallet state, etc.

Notes

Communication between the client code and the rest of the node should be done by a tower service interface. Since the Service trait can abstract from a function call to RPC, this means that it will be possible for us to isolate all client code to a subprocess.

Exported types

  • [...]

zebrad

Abscissa-based application which loads configs, all application components, and connects them to each other.

Responsible for

  • actually running the server
  • connecting functionality in dependencies

Internal Dependencies

  • zebra-chain
  • zebra-network
  • zebra-state
  • zebra-consensus
  • zebra-client
  • zebra-rpc

Unassigned functionality

Responsibility for this functionality needs to be assigned to one of the modules above (subject to discussion):

  • [ ... add to this list ... ]