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Glossary

This glossary defines many terms used throughout documentation of Quark. If there is every a concept that seems unclear, check here. This is mainly to provide a background and general understanding of the different words and concepts that are used. Other documents will explain in more detail how to combine these concepts to build a particular application.

Transaction (tx)

A transaction is a packet of binary data that contains all information to validate and perform an action on the blockchain. The only other data that it interacts with is the current state of the chain (key-value store), and it must have a deterministic action. The tx is the main piece of one request.

We currently make heavy use of go-wire and data to provide binary and json encodings and decodings for struct or interfaceobjects. Here, encoding and decoding operations are designed to operate with interfaces nested any amount times (like an onion!). There is one publicTxMapper` in the basecoin root package, and all modules can register their own transaction types there. This allows us to deserialize the entire tx in one location (even with types defined in other repos), to easily embed an arbitrary tx inside another without specifying the type, and provide an automatic json representation to provide to users (or apps) to inspect the chain.

Note how we can wrap any other transaction, add a fee level, and not worry about the encoding in our module any more?

type Fee struct {
  Fee   coin.Coin      `json:"fee"`
  Payer basecoin.Actor `json:"payer"` // the address who pays the fee
  Tx    basecoin.Tx    `json:"tx"`
}

Context (ctx)

As a request passes through the system, it may pick up information such as the authorization it has received from another middleware, or the block height the request runs at. In order to carry this information between modules it is saved to the context. further, it all information must be deterministic from the context in which the request runs (based on the tx and the block it was included in) and can be used to validate the tx.

Data Store

To be able to provide proofs to Tendermint, we keep all data in one key-value (kv) store which is indexed with a merkle tree. This allows for the easy generation of a root hash and proofs for queries without requiring complex logic inside each module. Standardization of this process also allows powerful light-client tooling as any store data may be verified on the fly.

The largest limitation of the current implemenation of the kv-store is that interface that the application must use can only Get and Set single data points. This said, there are some data structures like queues and range queries that are available in state package. These provide higher-level functionality in a standard format, but have not yet been integrated into the kv-store interface.

Isolation

One of the main arguments for blockchain is security. So while we encourage the use of third-party modules, all developers must be vigilant against security holes. If you use the stack package, it will provide two different types of compartmentalization security.

The first is to limit the working kv-store space of each module. When DeliverTx is called for a module, it is never given the entire data store, but rather only its own prefixed subset of the store. This is achieved by prefixing all keys transparently with <module name> + 0x0, using the null byte as a separator. Since the module name must be a string, no malicious naming scheme can ever lead to a collision. Inside a module, we can write using any key value we desire without the possibility that we have modified data belonging to separate module.

The second is to add permissions to the transaction context. The tx context can specify that the tx has been signed by one or multiple specific actors. A tx will only be executed if the permission requirements have been fulfilled. For example the sender of funds must have signed, or 2 out of 3 multi-signature actors must have signed a joint account. To prevent the forgery of account signatures from unintended modules each permission is associated with the module that granted it (in this case auth), and if a module tries to add a permission for another module, it will panic. There is also protection if a module creates a brand new fake context to trick the downstream modules. Each context enforces the rules on how to make child contexts, and the stack middleware builder enforces that the context passed from one level to the next is a valid child of the original one.

These security measures ensure that modules can confidently write to their local section of the database and trust the permissions associated with the context, without concern of interference from other modules. (Okay, if you see a bunch of C-code in the module traversing through all the memory space of the application, then get worried....)

Handler

The ABCI interface is handled by app, which translates these data structures into an internal format that is more convenient, but unable to travel over the wire. The basic interface for any code that modifies state is the Handler interface, which provides four methods:

  Name() string
  CheckTx(ctx Context, store state.KVStore, tx Tx) (Result, error)
  DeliverTx(ctx Context, store state.KVStore, tx Tx) (Result, error)
  SetOption(l log.Logger, store state.KVStore, module, key, value string) (string, error)

Note the Context, KVStore, and Tx as principal carriers of information. And that Result is always success, and we have a second error return for errors (which is much more standard golang that res.IsErr())

The Handler interface is designed to be the basis for all modules that execute transactions, and this can provide a large degree of code interoperability, much like http.Handler does in golang web development.

Middleware

Middleware is a series of processing steps that any request must travel through before (and after) executing the registered Handler. Some examples are a logger (that records the time before executing the tx, then outputs info - including duration - after the execution), of a signature checker (which unwraps the tx by one layer, verifies signatures, and adds the permissions to the Context before passing the request along).

In keeping with the standardization of http.Handler and inspired by the super minimal negroni package, we just provide one more Middleware interface, which has an extra next parameter, and a Stack that can wire all the levels together (which also gives us a place to perform isolation of each step).

  Name() string
  CheckTx(ctx Context, store state.KVStore, tx Tx, next Checker) (Result, error)
  DeliverTx(ctx Context, store state.KVStore, tx Tx, next Deliver) (Result, error)
  SetOption(l log.Logger, store state.KVStore, module, key, value string, next Optioner) (string, error)

Modules

A module is a set of functionality which should be typically designed as self-sufficient. Common elements of a module are:

transaction types (either end transactions, or transaction wrappers)
custom error codes
data models (to persist in the kv-store)
handler (to handle any end transactions)
middleware (to handler any wrapper transactions)

To enable a module, you must add the appropriate middleware (if any) to the stack in main.go for the client application (Quark default: basecli/main.go), as well as adding the handler (if any) to the dispatcher (Quark default: app/app.go). Once the stack is compiled into a Handler, then each tx is handled by the appropriate module.

Dispatcher

We usually will want to have multiple modules working together, and need to make sure the correct transactions get to the correct module. So we have have coin sending money, roles creating multi-sig accounts, and ibc following other chains all working together without interference.

After the chain of middleware, we can register a Dispatcher, which also implements the Handler interface. We then register a list of modules with the dispatcher. Every module has a unique Name(), which is used for isolating its state space. We use this same name for routing tx. Each tx implementation must be registed with go-wire via TxMapper, so we just look at the registered name of this tx, which should be of the form <module name>/xxx. The dispatcher grabs the appropriate module name from the tx name and routes it if the module is present.

This all seems a bit of magic, but really just making use of the other magic (go-wire) that we are already using, rather than add another layer. The only thing you need to remember is to use the following pattern, then all the tx will be properly routed:

const (
  NameCoin = "coin"
  TypeSend = NameCoin + "/send"
)

Inter-Plugin Communication (IPC)

But wait, there's more... since we have isolated all the modules from each other, we need to allow some way for them to interact in a controlled fashion. One example is the fee middleware, which wants to deduct coins from the calling account and can accomplished most easilty with the coin module.

If we want to make a call from the middleware, this is relatively simple. The middleware already has a handle to the next Handler, which will execute the rest of the stack. It can simple create a new SendTx and pass it down the stack. If it returns success, then do the rest of the processing (and send the original tx down the stack), otherwise abort.

However, if one Handler inside the Dispatcher wants to do this, it becomes more complex. The solution is that the Dispatcher accepts not a Handler, but a Dispatchable, which looks like a middleware, except that the next argument is a callback to the dispatcher to execute a sub-transaction. If a module doesn't want to use this functionality, it can just implement Handler and call stack.WrapHandler(h) to convert it to a Dispatchable that never uses the callback.

One example of this is the counter app, which can optionally accept a payment. If the tx contains a payment, it must create a SendTx and pass this to the dispatcher to deduct the amount from the proper account. Take a look at counter plugin for a better idea.

Permissions

IPC requires a more complex permissioning system to allow the modules to have limited access to each other. Also to allow more types of permissions than simple public key signatures. So, rather than just use an address to identify who is performing an action, we can use a more complex structure:

type Actor struct {
  ChainID string     `json:"chain"` // this is empty unless it comes from a different chain
  App     string     `json:"app"`   // the app that the actor belongs to
  Address data.Bytes `json:"addr"`  // arbitrary app-specific unique id
}

Here, the Actor abstracts any address that can authorize actions, hold funds, or initiate any sort of transaction. It doesn't just have to be a pubkey on this chain, it could stem from another app (such as multi-sig account), or even another chain (via IBC)

ChainID is to be used for IBC, which is discussed below, but right now focus on App and Address. For a signature, the App is auth, and any modules can check to see if a specific public key address signed like this ctx.HasPermission(auth.SigPerm(addr)). However, we can also authorize a tx with roles, which handles multi-sig accounts, it checks if there were enough signatures by checking as above, then it can add the role permission like ctx = ctx.WithPermissions(NewPerm(assume.Role))

In addition to permissioning, the Actors are addresses just like public key addresses. So one can create a mulit-sig role, then send coin there, which can only be moved upon meeting the authorization requirements from that module. coin doesn't even know the existence of roles and one could build any other sort of module to provide permissions (like bind the outcome of an election to move coins or to modify the accounts on a role).

One idea (not implemented) is to provide scopes on the permissions. Right now, if I sign a tx to one module, it can pass it on to any other module over IPC with the same permissions. It could move coins, vote in an election, or anything else. Ideally, when signing, one could also specify the scope(s) that this signature authorizes. The oauth protocol also has to deal with a similar problem, and maybe could provide some inspiration.

Replay Protection

In order to prevent replay attacks a multi account nonce system has been constructed as a module, which can be found in modules/nonce. By adding the nonce module to the stack, each transaction is verified for authenticity against replay attacks. This is achieved by requiring that a new signed copy of the sequence number which must be exactly 1 greater than the sequence number of the previous transaction. A distinct sequence number is assigned per chain-id, application, and group of signers. Each sequence number is tracked as a nonce-store entry where the key is the marshaled list of actors after having been sorted by chain, app, and address.

// Tx - Nonce transaction structure, contains list of signers and current sequence number
type Tx struct {
	Sequence uint32           `json:"sequence"`
	Signers  []basecoin.Actor `json:"signers"`
	Tx       basecoin.Tx      `json:"tx"`
}

By distinguishing sequence numbers across groups of Signers, multi-signature Actors need not lock up use of their Address while waiting for all the members of a multi-sig transaction to occur. Instead only the multi-sig account will be locked, while other accounts belonging to that signer can be used and signed with other sequence numbers.

By abstracting out the nonce module in the stack, entire series of transactions can occur without needing to verify the nonce for each member of the series. An common example is a stack which will send coins and charge a fee. Within Quark this can be achieved using separate modules in a stack, one to send the coins and the other to charge the fee, however both modules do not need to check the nonce. This can occur as a separate module earlier in the stack.

IBC (Inter-Blockchain Communication)

Wow, this is a big topic. Also a WIP. Add more here...

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