doc(cmd/gossamer): README for cmd package

README.md

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cmd/gossamer/README.md

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+# Gossamer `cmd` Package
+
+This package encapsulates the entry point to Gossamer - it uses the popular
+[`cli` package from `urfave`](https://github.com/urfave/cli/blob/master/docs/v1/manual.md) to expose a command-line
+interface (CLI). The Gossamer CLI accepts several subcommands, each of which is associated with an "action"; these
+subcommands and their corresponding actions are defined in [`main.go`](main.go). When the Gossamer CLI is executed
+without a subcommand, the `gossamerAction` is invoked. What follows is a list of the Gossamer subcommands, as well as an
+overview of some of the flags/parameters they accept. The flags/parameters that the Gossamer CLI supports are defined in
+[`flags.go`](flags.go). For an exhaustive reference of the Gossamer CLI capabilities, follow
+[the installation instructions](../../README.md#installation) and execute `./bin/gossamer --help`.
+
+- `gossamerAction` - This the default Gossamer execution action - it will launch a Gossamer blockchain client. The
+  details of how Gossamer orchestrates a blockchain client are [described below in the Client Components section](#client-components).
+  - `chain` - specifies the [chain configuration](../../chain) that the
+    [Gossamer host node](https://chainsafe.github.io/gossamer/getting-started/overview/host-architecture/) should load
+  - `log` - supports levels `crit` (silent), `error`, `warn`, `info` and `trce` (detailed), default is `info`
+  - `name` - node name, as it will appear in, e.g., [telemetry](https://telemetry.polkadot.io/)
+- `account` - The `account` subcommand provides the user with capabilities related to generating and using `ed25519`,
+  `secp256k1`, and `sr25519` [account keys](https://wiki.polkadot.network/docs/learn-keys), and managing the keys
+  present in the [Gossamer keystore](#keystore). The action for this subcommand is defined in [account.go](account.go);
+  it is an interface to the capabilities defined in the [`lib/crypto`](../../lib/crypto) and
+  [`lib/keystore`](../../lib/keystore) packages. This subcommand provides capabilities that are similar to
+  [Parity's Subkey utility](https://docs.substrate.io/v3/tools/subkey).
+  - `generate` - creates a new key pair; specify `--ed25519`, `--secp256k1`, or `--sr25519` (default)
+  - `list` - lists the keys in the Gossamer keystore
+  - `password` - allows the user to provide a password to either encrypt a generated key or unlock the Gossamer
+    keystore
+- `import-runtime` - This subcommand takes a [Wasm runtime binary](https://wiki.polkadot.network/docs/learn-wasm) and
+  uses it to generate a [genesis](https://wiki.polkadot.network/docs/glossary#genesis) configuration file; it does not
+  require any flags, but expects the path to a Wasm file to be provided as a command-line parameter (example:
+  `./bin/gossamer import-runtime runtime.wasm > genesis.json`).
+- `build-spec` - This subcommand allows the user to "compile" a human-readable Gossamer genesis configuration file into
+  a format that the Gossamer node can consume. If the `--genesis` parameter is not provided, the generated genesis
+  configuration will represent the Gossamer default configuration.
+  - `genesis` - path to the human-readable configuration file that should be compiled into a format that Gossamer can
+    consume
+- `import-state` - The `import-state` subcommand allows a user to seed [Gossamer storage](../../dot/state) with
+  key-value pairs in the form of a JSON file. The input for this subcommand can be retrieved from
+  [the `state_getPairs` RPC endpoint](https://github.com/w3f/PSPs/blob/master/PSPs/drafts/psp-6.md#1114-state_getpairs).
+  - `first-slot` - the first [BABE](https://wiki.polkadot.network/docs/learn-consensus#block-production-babe) slot,
+    which is the timestamp of the first block divided by the slot duration
+  - `header` - path to a JSON file that describes the block header corresponding to the given state
+  - `state` - path to a JSON file that contains the key-value pairs with which to seed Gossamer storage
+- `init` - This subcommand accepts a genesis configuration file and uses it to initialise the Gossamer node and its
+  state.
+  - `genesis` - path to the "compiled" genesis configuration file that should be used to initialise the Gossamer node
+    and its state
+- `export` - The `export` subcommand transforms a genesis configuration and Gossamer state into a TOML configuration
+  file.
+  - `config` - path to a TOML configuration file (e.g. [Polkadot](../../chain/polkadot/config.toml))
+  - `basepath` - path to the Gossamer data directory that defines the state to export
+
+## Client Components
+
+In its default method of execution, Gossamer orchestrates a number of modular services that run
+[concurrently as goroutines](https://www.golang-book.com/books/intro/10) and work together to implement the protocols of
+a blockchain network. Alongside these services, Gossamer manages [a keystore](#keystore), [a runtime](#runtime), and
+[monitoring utilities](#monitoring), all of which are described in greater detail below. The entry point to the Gossamer
+blockchain client capabilities is the `gossamerAction` function that is defined in [main.go](main.go), which in turn
+invokes the `NewNode` function in [dot/node.go](../../dot/node.go). `NewNode` calls into functions that are defined in
+[dot/services.go](../../dot/services.go) and starts the services that power a Gossamer node. What follows is a list that
+describes the services and capabilities that inform a Gossamer blockchain client:
+
+- State - This service is a wrapper around an instance of [`chaindb`](https://github.com/ChainSafe/chaindb), a key-value
+  database that is built on top of [BadgerDB](https://github.com/dgraph-io/badger) from [Dgraph](https://dgraph.io/).
+  The state service provides storage capabilities for the other Gossamer services - each service is assigned a prefix
+  that is added to its storage keys. The state service is defined in [dot/state/service.go](../../dot/state/service.go).
+- Network - The network service, which is defined in [dot/network/service.go](../../dot/network/service.go), is built on
+  top of [the Go implementation](https://github.com/libp2p/go-libp2p) of [the `libp2p` protocol](https://libp2p.io/).
+  This service manages a `libp2p` "host", a peer-to-peer networking term for a network participant that is providing
+  both client _and_ server capabilities to a peer-to-peer network. Gossamer's network service manages the discovery of
+  other hosts as well as the connections with these hosts that allow Gossamer to communicate with its network peers.
+- Digest Handler - The digest handler ([dot/digest/digest.go](../../dot/digest/digest.go)) manages the verification of
+  the [cryptographic digests](https://docs.substrate.io/v3/getting-started/glossary/#digest) that are present in block
+  headers.
+- Consensus - The BABE and GRANDPA services work together to provide Gossamer with its
+  [hybrid consensus](https://wiki.polkadot.network/docs/learn-consensus#hybrid-consensus) capabilities. The term "hybrid
+  consensus" refers to the fact that block _production_ is decoupled from block _finalisation_. Block production is
+  handled by the BABE service, which is defined in [lib/babe/babe.go](../../lib/babe/babe.go); block finalisation is
+  handled by the GRANDPA service, which is defined in [lib/grandpa/grandpa.go](../../lib/grandpa/grandpa.go).
+- Sync - This service is concerned with keeping Gossamer in sync with a blockchain - it implements a "bootstrap" mode,
+  to download and verify blocks that are part of an existing chain's history, and a "tip-syncing" mode that manages the
+  multiple candidate forks that may exist at the head of a live chain. The sync service makes use of
+  [a block verification utility](../../lib/babe/verify.go) that implements BABE logic and is used by Gossamer to verify
+  blocks that were produced by other other nodes in the network. The sync service is defined
+  [dot/sync/syncer.go](../../dot/sync/syncer.go).
+- RPC - This service, which is defined in [dot/rpc/service.go](../../dot/rpc/service.go), exposes a JSON-RPC interface
+  that is used by client applications like [Polkadot JS Apps UI](https://polkadot.js.org/apps/). The RPC interface is
+  used to interact with Gossamer to perform tasks such as key management, as well as for interacting with the "inner"
+  runtime by querying storage and submitting transactions.
+- System - The system service is defined in [dot/system/service.go](../../dot/system/service.go) and exposes metadata
+  about the Gossamer system, such as the names and versions of the protocols that it implements.
+- Core - As its name implies, the core service ([dot/core/service.go](../../dot/core/service.go)) encapsulates a range
+  of capabilities that are central to the functioning of a Gossamer node. In general, the core service is a type of
+  dispatcher that coordinates interactions between services, e.g. writing blocks to the database, reloading
+  [the runtime](#runtime) when its definition is updated, etc.
+
+### Keystore
+
+The Gossamer keystore ([lib/keystore](../../lib/keystore)) is used for managing the public/private cryptographic key
+pairs that are used for participating in a blockchain network. Public keys are used to identify network participants;
+network participants use their private keys to sign messages in order to authorise privileged actions. In addition to
+informing the Gossamer blockchain client capabilities, the Gossamer keystore is accessible by way of the `account`
+subcommand. The Gossamer keystore manages a number of key types, some of which are listed below:
+
+- `babe` - This key is used for signing messages related to the BABE block production algorithm.
+- `gran` - The GRANDPA key is used for participating in GRANDPA block finalisation.
+- `imon` - The name of this key is a reference to "ImOnline", which is an
+  [online message](https://wiki.polkadot.network/docs/glossary#online-message) that Gossamer nodes use to report
+  liveliness.
+
+### Runtime
+
+In addition to the above-described services, Gossamer hosts a Wasm execution environment that is used to manage an
+upgradeable blockchain runtime. The runtime must be implemented in Wasm, and must expose an interface that is specified
+in [lib/runtime/interface.go](../../lib/runtime/interface.go). The runtime defines the blockchain's state transition
+function, and the various Gossamer services consume this capability in order to author blocks, as well as to verify
+blocks that were authored by network peers.
+
+### Monitoring
+
+Gossamer publishes telemetry data and also includes an embedded Prometheus server that reports metrics. The metrics
+capabilities are defined in the [dot/metrics](../../dot/metrics) package and build on
+[the metrics library that is included with Go Ethereum](https://github.com/ethereum/go-ethereum/blob/master/metrics/README.md).
+The Gossamer telemetry server publishes telemetry data that is compatible with
+[Polkadot Telemetry](https://github.com/paritytech/substrate-telemetry) and
+[its helpful UI](https://telemetry.polkadot.io/).