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Exonum Architecture

This document describes the high-level architecture of Exonum. Its target audience is developers of Exonum core. To learn how to use Exonum, see documentation.

This document is provided on the best effort basis — it is not guaranteed to be up-to-date and complete. The purpose of this document is to highlight the most important types, modules and functions in the code, to allow new contributors to dive into Exonum quickly. It is assumed that the reader is familiar with core concepts of Exonum, such as services and transactions.

Code organization

The repository is a single Cargo workspace of several related packages:

  • exonum, most of the code lives here,

  • sandbox, where integration tests live,

  • testkit, a framework for testing Exonum services.

This document currently describes only the exonum crate.

Storage

Storage module can be found at components/merkledb. It provides an abstraction over an embedded key-value store: the Database trait. The keys and values can be represented as slices of bytes: &[u8].

To transform raw bytes into native Rust data structures and vice versa, the BinaryKey, BinaryValue and ProtobufConvert traits are used.

On top of raw storage, collections like sets, lists and maps are provided: see various *Index structs. Of particular interest are components/merkledb/src/proof_map_index and components/merkledb/src/proof_list_index modules, which provide Merkelized collections, capable of providing compact proofs for search queries.

Note that indexes are used both by the Exonum core and by the services. The core uses indexes to store the internal blockchain state, which is described in the next section. The services define and use indexes to store arbitrary service-specific data.

Blockchain

The format of the block of the Exonum blockchain is described in exonum/src/blockchain/block.rs. Blocks are stored in the Database. The schema at exonum/src/blockchain/schema.rs describes the indices used for blocks, transactions and some other data used by Exonum core.

The heart of the Blockchain is Blockchain::create_patch method, which, given a list of transactions, executes them and produces a new Block.

Node

The entities responsible for bringing all the pieces together and connecting blockchain and services to the external world are Node and NodeHandler. Note that there are several impl blocks for NodeHandler.

The entry point is Node::run, and the event dispatch loop is started in Node::run_handler.

The event dispatch itself happens in exonum/src/node/basic.rs module.

Events specific to the consensus protocol messages are handled in the exonum/src/node/consensus.rs module. The messages themselves are defined in the exonum/src/messages/protocol.rs module.

Node is also responsible for starting an HTTP API server. The API is assembled from the built-in part, specified in exonum/src/api and parts, provided by each service. The entry point of API construction is a create_app function of exonum/src/api/backend/actix.rs module.

Configuration

The code responsible for assembling a node with a particular set of services and configuration lives in exonum/src/helpers/fabric. NodeBuilder::run is the "fn main" of Exonum.