zebra/README.md

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### 🚧 UNDER CONSTRUCTION 🚧

[Zebra](https://zebra.zfnd.org/) is the Zcash Foundation's independent,
consensus-compatible implementation of the Zcash protocol, currently under
development.  Please [join us on Discord](https://discord.gg/na6QZNd) if you'd
like to find out more or get involved!

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`
fullnode.  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.

Our first goal is to be able to participate in the network and replicate the
Zcash chain state, and we intend to ship an alpha before the end of 2020 with
this functionality.  In 2021, we intend to add RPC support and wallet
integration.  This phased approach allows us to test the independent
implementation of the consensus rules before asking users to entrust it with
their funds.

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

- [`zebra-chain`](https://doc.zebra.zfnd.org/zebra_chain/index.html), 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*](https://zebra.zfnd.org/dev/rfcs/0002-parallel-verification.html#verification-stages) 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`](https://doc.zebra.zfnd.org/zebra_network/index.html),
  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 requests over available
  peers, 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`](https://doc.zebra.zfnd.org/zebra_script/index.html) 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`](https://doc.zebra.zfnd.org/zebra_consensus/index.html)
  performs [*semantic validation*](https://zebra.zfnd.org/dev/rfcs/0002-parallel-verification.html#verification-stages) 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`](https://doc.zebra.zfnd.org/tower_batch/index.html) to
  perform automatic, transparent batch processing of contemporaneous
  verification requests.

- [`zebra-state`](https://doc.zebra.zfnd.org/zebra_state/index.html) is
  responsible for storing, updating, and querying the chain state.  The state
  service is responsible for [*contextual verification*](https://zebra.zfnd.org/dev/rfcs/0002-parallel-verification.html#verification-stages): 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`](https://doc.zebra.zfnd.org/zebrad/index.html) contains the full
  node, which connects these components together and implements logic to handle
  inbound requests from peers and the chain sync process.

- `zebra-rpc` and `zebra-client` will eventually contain the RPC and wallet
  functionality, but as mentioned above, our goal is to implement replication
  of chain state first before asking users to entrust Zebra with their funds.

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

The [Zebra website](https://zebra.zfnd.org/) contains user documentation, such
as how to run or configure Zebra, set up metrics integrations, etc., as well as
developer documentation, such as design documents.  We also render [API
documentation](https://doc.zebra.zfnd.org) for the external API of our crates,
as well as [internal documentation](https://doc-internal.zebra.zfnd.org) for
private APIs.

## License

Zebra is distributed under the terms of both the MIT license
and the Apache License (Version 2.0).

See [LICENSE-APACHE](LICENSE-APACHE) and [LICENSE-MIT](LICENSE-MIT).