Add basic architecture notes (#14)

book/src/SUMMARY.md

@@ -1,7 +1,8 @@
 # Summary
 
-* [Introduction](./intro.md)
-* [FAQs](./faq.md)
-* [Development Environment](./setup.md)
-* [Contributing](./contributing.md)
-* [Protocol Developers](./developers.md)
+- [Introduction](./intro.md)
+- [FAQs](./faq.md)
+- [Development Environment](./setup.md)
+- [Contributing](./contributing.md)
+- [Protocol Developers](./developers.md)
+  - [Architecture](./architecture.md)

book/src/architecture.md

@@ -0,0 +1,78 @@
+# Architectural Overview
+
+This section provides developers an overview of the architectural design of Anchor. The intent of this
+is to help gain an easy understanding of the client and associated code.
+
+## Thread Model
+
+Anchor is a multi-threaded client. There are a number of long standing tasks that are spawned when
+Anchor is initialised. This section lists these high-level tasks and describes their purpose along
+with how they are connected.
+
+### Task Overview
+
+The following diagram gives a basic overview of the core tasks inside Anchor.
+
+```mermaid
+graph
+
+  A(Core Client) <--> B(HTTP API)
+  A(Core Client) <--> C(Metrics)
+  A(Core Client) <--> E(Execution Service)
+  A(Core Client) <--> F(Duties Service)
+  F <--> G(Processor)
+  H(Network) <--> G
+  I(QBFT) <--> G
+```
+
+The boxes here represent stand alone tasks, with the arrows representing channels between these
+tasks. Memory is often shared between these tasks, but this is to give an overview of how the client
+is pieced together.
+
+The tasks are:
+
+- **HTTP API** - A HTTP endpoint to read data from the client, or modify specific components.
+- **Metrics** - Another HTTP endpoint designed to be scraped by a Prometheus instance. This provides
+    real-time metrics of the client.
+- **Execution Service** - A service used to sync SSV information from the execution layer nodes.
+- **Duties Service** - A service used to watch the beacon chain for validator duties for our known
+    SSV validator shares.
+- **Network** - The p2p network stack (libp2p) that sends/receives data on the SSV p2p network.
+- **Processor** - A middleware that handles CPU intensive tasks and prioritises the workload of the
+    client.
+- **QBFT** - Spawns a QBFT instance and drives it to completion in order to reach consensus in an
+    SSV committee.
+
+### General Event Flow
+
+Generally, tasks can operate independently from the core client. The main task that drives the
+Anchor client is the duties service. It specifies when and what kind of validator duty we must be
+doing at any given time.
+
+Once a specific duty is assigned, a message is sent to the processor to start one (or many) QBFT instances for
+this specific duty or duties. Simultaneously, we are awaiting messages on the network service. As
+messages are received they are routed to the processor, validation is performed and then they are
+routed to the appropriate QBFT instance.
+
+Once we have reached consensus, messages are sent (via the processor) to the network to sign our
+required duty.
+
+A summary of this process is:
+
+1. Await a duty from the duties service
+1. Send the duty to the processor
+1. The processor spins up a QBFT instance
+1. Receive messages until the QBFT instance completes
+1. Sign required consensus message
+1. Publish the message on the p2p network.
+
+An overview of how these threads are linked together is given below:
+
+```mermaid
+graph LR
+
+  A(Core Client) <--> B(Duties Service)
+  B <--> C(Processor)
+  C <--> D(Network)
+  C <--> E(QBFT)
+```