cosmos-sdk/docs/guide.md

Introduction

If you want to see some examples, take a look at the examples/basecoin directory.

Design Goals

The design of the Cosmos SDK is based on the principles of "capabilities systems".

Capabilities systems

Need for module isolation

Capability is implied permission

TODO Link to thesis

Tx & Msg

The SDK distinguishes between transactions (Tx) and messages (Msg). A Tx is a Msg wrapped with authentication and fee data.

Messages

Users can create messages containing arbitrary information by implementing the Msg interface:

type Msg interface {

	// Return the message type.
	// Must be alphanumeric or empty.
	Type() string

	// Get some property of the Msg.
	Get(key interface{}) (value interface{})

	// Get the canonical byte representation of the Msg.
	GetSignBytes() []byte

	// ValidateBasic does a simple validation check that
	// doesn't require access to any other information.
	ValidateBasic() error

	// Signers returns the addrs of signers that must sign.
	// CONTRACT: All signatures must be present to be valid.
	// CONTRACT: Returns addrs in some deterministic order.
	GetSigners() []Address
}

Messages must specify their type via the Type() method. The type should correspond to the messages handler, so there can be many messages with the same type.

Messages must also specify how they are to be authenticated. The GetSigners() method return a list of addresses that must sign the message, while the GetSignBytes() method returns the bytes that must be signed for a signature to be valid.

Addresses in the SDK are arbitrary byte arrays that are hex-encoded when displayed as a string or rendered in JSON.

Messages can specify basic self-consistency checks using the ValidateBasic() method to enforce that message contents are well formed before any actual logic begins.

Finally, messages can provide generic access to their contents via Get(key), but this is mostly for convenience and not type-safe.

For instance, the Basecoin message types are defined in x/bank/tx.go:

type SendMsg struct {
	Inputs  []Input  `json:"inputs"`
	Outputs []Output `json:"outputs"`
}

type IssueMsg struct {
	Banker  sdk.Address `json:"banker"`
	Outputs []Output       `json:"outputs"`
}

Each specifies the addresses that must sign the message:

func (msg SendMsg) GetSigners() []sdk.Address {
	addrs := make([]sdk.Address, len(msg.Inputs))
	for i, in := range msg.Inputs {
		addrs[i] = in.Address
	}
	return addrs
}

func (msg IssueMsg) GetSigners() []sdk.Address {
	return []sdk.Address{msg.Banker}
}

Transactions

A transaction is a message with additional information for authentication:

type Tx interface {

	GetMsg() Msg

	// Signatures returns the signature of signers who signed the Msg.
	// CONTRACT: Length returned is same as length of
	// pubkeys returned from MsgKeySigners, and the order
	// matches.
	// CONTRACT: If the signature is missing (ie the Msg is
	// invalid), then the corresponding signature is
	// .Empty().
	GetSignatures() []StdSignature
}

The tx.GetSignatures() method returns a list of signatures, which must match the list of addresses returned by tx.Msg.GetSigners(). The signatures come in a standard form:

type StdSignature struct {
	crypto.PubKey // optional
	crypto.Signature
	Sequence int64
}

It contains the signature itself, as well as the corresponding account's sequence number. The sequence number is expected to increment every time a message is signed by a given account. This prevents "replay attacks", where the same message could be executed over and over again.

The StdSignature can also optionally include the public key for verifying the signature. An application can store the public key for each address it knows about, making it optional to include the public key in the transaction. In the case of Basecoin, the public key only needs to be included in the first transaction send by a given account - after that, the public key is forever stored by the application and can be left out of transactions.

The address responsible for paying the transactions fee is the first address returned by msg.GetSigners(). The convenience function FeePayer(tx Tx) is provided to return this.

The standard way to create a transaction from a message is to use the StdTx:

type StdTx struct {
	Msg
	Signatures []StdSignature
}

Encoding and Decoding Transactions

Messages and transactions are designed to be generic enough for developers to specify their own encoding schemes. This enables the SDK to be used as the framwork for constructing already specified cryptocurrency state machines, for instance Ethereum.

When initializing an application, a developer must specify a TxDecoder function which determines how an arbitrary byte array should be unmarshalled into a Tx:

type TxDecoder func(txBytes []byte) (Tx, error)

In Basecoin, we use the Tendermint wire format and the go-wire library for encoding and decoding all message types. The go-wire library has the nice property that it can unmarshal into interface types, but it requires the relevant types to be registered ahead of type. Registration happens on a Codec object, so as not to taint the global name space.

For instance, in Basecoin, we wish to register the SendMsg and IssueMsg types:

cdc.RegisterInterface((*sdk.Msg)(nil), nil)
cdc.RegisterConcrete(bank.SendMsg{}, "cosmos-sdk/SendMsg", nil)
cdc.RegisterConcrete(bank.IssueMsg{}, "cosmos-sdk/IssueMsg", nil)

Note how each concrete type is given a name - these name determine the type's unique "prefix bytes" during encoding. A registered type will always use the same prefix-bytes, regardless of what interface it is satisfying. For more details, see the go-wire documentation.

MultiStore

MultiStore is like a filesystem

Mounting an IAVLStore

TODO:
- IAVLStore: Fast balanced dynamic Merkle store.
  - supports iteration.
- MultiStore: multiple Merkle tree backends in a single store 
  - allows using Ethereum Patricia Trie and Tendermint IAVL in same app
- Provide caching for intermediate state during execution of blocks and transactions (including for iteration)
- Historical state pruning and snapshotting.
- Query proofs (existence, absence, range, etc.) on current and retained historical state.

Context

The SDK uses a Context to propogate common information across functions. The Context is modeled after the Golang context.Context object, which has become ubiquitous in networking middleware and routing applications as a means to easily propogate request context through handler functions.

The main information stored in the Context includes the application MultiStore (see below), the last block header, and the transaction bytes. Effectively, the context contains all data that may be necessary for processing a transaction.

Many methods on SDK objects receive a context as the first argument.

Handler

Transaction processing in the SDK is defined through Handler functions:

type Handler func(ctx Context, tx Tx) Result

A handler takes a context and a transaction and returns a result. All information necessary for processing a transaction should be available in the context.

While the context holds the entire application state (all referenced from the root MultiStore), a particular handler only needs a particular kind of access to a particular store (or two or more). Access to stores is managed using capabilities keys and mappers. When a handler is initialized, it is passed a key or mapper that gives it access to the relevant stores.

// File: cosmos-sdk/examples/basecoin/app/init_stores.go
app.BaseApp.MountStore(app.capKeyMainStore, sdk.StoreTypeIAVL)
app.accountMapper = auth.NewAccountMapper(
	app.capKeyMainStore, // target store
	&types.AppAccount{}, // prototype
)

// File: cosmos-sdk/examples/basecoin/app/init_handlers.go
app.router.AddRoute("bank", bank.NewHandler(app.accountMapper))

// File: cosmos-sdk/x/bank/handler.go
// NOTE: Technically, NewHandler only needs a CoinMapper
func NewHandler(am sdk.AccountMapper) sdk.Handler {
	return func(ctx sdk.Context, msg sdk.Msg) sdk.Result {
		cm := CoinMapper{am}
		...
	}
}

AnteHandler

Handling Fee payment

Handling Authentication

Accounts and x/auth

sdk.Account

auth.BaseAccount

auth.AccountMapper

Wire codec

Why another codec?

vs encoding/json

vs protobuf

KVStore example

Basecoin example

The quintessential SDK application is Basecoin - a simple multi-asset cryptocurrency. Basecoin consists of a set of accounts stored in a Merkle tree, where each account may have many coins. There are two message types: SendMsg and IssueMsg. SendMsg allows coins to be sent around, while IssueMsg allows a set of predefined users to issue new coins.

Conclusion