# Application Development Guide


This page is undergoing deprecation. All content is being moved to the new home of the ABCI specification (opens new window).

# ABCI Design

The purpose of ABCI is to provide a clean interface between state transition machines on one computer and the mechanics of their replication across multiple computers. The former we call 'application logic' and the latter the 'consensus engine'. Application logic validates transactions and optionally executes transactions against some persistent state. A consensus engine ensures all transactions are replicated in the same order on every machine. We call each machine in a consensus engine a 'validator', and each validator runs the same transactions through the same application logic. In particular, we are interested in blockchain-style consensus engines, where transactions are committed in hash-linked blocks.

The ABCI design has a few distinct components:

  • message protocol
    • pairs of request and response messages
    • consensus makes requests, application responds
    • defined using protobuf
  • server/client
    • consensus engine runs the client
    • application runs the server
    • two implementations:
      • async raw bytes
      • grpc
  • blockchain protocol
    • abci is connection oriented
    • Tendermint Core maintains three connections:
      • mempool connection: for checking if transactions should be relayed before they are committed; only uses CheckTx
      • consensus connection: for executing transactions that have been committed. Message sequence is -for every block -BeginBlock, [DeliverTx, ...], EndBlock, Commit
      • query connection: for querying the application state; only uses Query and Info

The mempool and consensus logic act as clients, and each maintains an open ABCI connection with the application, which hosts an ABCI server. Shown are the request and response types sent on each connection.

Most of the examples below are from kvstore application (opens new window), which is a part of the abci repo. persistent_kvstore application (opens new window) is used to show BeginBlock, EndBlock and InitChain example implementations.

# Blockchain Protocol

In ABCI, a transaction is simply an arbitrary length byte-array. It is the application's responsibility to define the transaction codec as they please, and to use it for both CheckTx and DeliverTx.

Note that there are two distinct means for running transactions, corresponding to stages of 'awareness' of the transaction in the network. The first stage is when a transaction is received by a validator from a client into the so-called mempool or transaction pool -this is where we use CheckTx. The second is when the transaction is successfully committed on more than 2/3 of validators - where we use DeliverTx. In the former case, it may not be necessary to run all the state transitions associated with the transaction, as the transaction may not ultimately be committed until some much later time, when the result of its execution will be different. For instance, an Ethereum ABCI app would check signatures and amounts in CheckTx, but would not actually execute any contract code until the DeliverTx, so as to avoid executing state transitions that have not been finalized.

To formalize the distinction further, two explicit ABCI connections are made between Tendermint Core and the application: the mempool connection and the consensus connection. We also make a third connection, the query connection, to query the local state of the app.

# Mempool Connection

The mempool connection is used only for CheckTx requests. Transactions are run using CheckTx in the same order they were received by the validator. If the CheckTx returns OK, the transaction is kept in memory and relayed to other peers in the same order it was received. Otherwise, it is discarded.

CheckTx requests run concurrently with block processing; so they should run against a copy of the main application state which is reset after every block. This copy is necessary to track transitions made by a sequence of CheckTx requests before they are included in a block. When a block is committed, the application must ensure to reset the mempool state to the latest committed state. Tendermint Core will then filter through all transactions in the mempool, removing any that were included in the block, and re-run the rest using CheckTx against the post-Commit mempool state (this behaviour can be turned off with [mempool] recheck = false).

In go:

Copy func (app *KVStoreApplication) CheckTx(req types.RequestCheckTx) types.ResponseCheckTx { return types.ResponseCheckTx{Code: code.CodeTypeOK, GasWanted: 1} }

In Java:

Copy ResponseCheckTx requestCheckTx(RequestCheckTx req) { byte[] transaction = req.getTx().toByteArray(); // validate transaction if (notValid) { return ResponseCheckTx.newBuilder().setCode(CodeType.BadNonce).setLog("invalid tx").build(); } else { return ResponseCheckTx.newBuilder().setCode(CodeType.OK).build(); } }

# Replay Protection

To prevent old transactions from being replayed, CheckTx must implement replay protection.

Tendermint provides the first defence layer by keeping a lightweight in-memory cache of 100k ([mempool] cache_size) last transactions in the mempool. If Tendermint is just started or the clients sent more than 100k transactions, old transactions may be sent to the application. So it is important CheckTx implements some logic to handle them.

If there are cases in your application where a transaction may become invalid in some future state, you probably want to disable Tendermint's cache. You can do that by setting [mempool] cache_size = 0 in the config.

# Consensus Connection

The consensus connection is used only when a new block is committed, and communicates all information from the block in a series of requests: BeginBlock, [DeliverTx, ...], EndBlock, Commit. That is, when a block is committed in the consensus, we send a list of DeliverTx requests (one for each transaction) sandwiched by BeginBlock and EndBlock requests, and followed by a Commit.

# DeliverTx

DeliverTx is the workhorse of the blockchain. Tendermint sends the DeliverTx requests asynchronously but in order, and relies on the underlying socket protocol (ie. TCP) to ensure they are received by the app in order. They have already been ordered in the global consensus by the Tendermint protocol.

DeliverTx returns a abci.Result, which includes a Code, Data, and Log. The code may be non-zero (non-OK), meaning the corresponding transaction should have been rejected by the mempool, but may have been included in a block by a Byzantine proposer.

The block header will be updated (TODO) to include some commitment to the results of DeliverTx, be it a bitarray of non-OK transactions, or a merkle root of the data returned by the DeliverTx requests, or both.

In go:

Copy // tx is either "key=value" or just arbitrary bytes func (app *KVStoreApplication) DeliverTx(req types.RequestDeliverTx) types.ResponseDeliverTx { var key, value []byte parts := bytes.Split(req.Tx, []byte("=")) if len(parts) == 2 { key, value = parts[0], parts[1] } else { key, value = req.Tx, req.Tx } app.state.db.Set(prefixKey(key), value) app.state.Size += 1 events := []types.Event{ { Type: "app", Attributes: []kv.Pair{ {Key: []byte("creator"), Value: []byte("Cosmoshi Netowoko")}, {Key: []byte("key"), Value: key}, }, }, } return types.ResponseDeliverTx{Code: code.CodeTypeOK, Events: events} }

In Java:

Copy /** * Using Protobuf types from the protoc compiler, we always start with a byte[] */ ResponseDeliverTx deliverTx(RequestDeliverTx request) { byte[] transaction = request.getTx().toByteArray(); // validate your transaction if (notValid) { return ResponseDeliverTx.newBuilder().setCode(CodeType.BadNonce).setLog("transaction was invalid").build(); } else { ResponseDeliverTx.newBuilder().setCode(CodeType.OK).build(); } }

# Commit

Once all processing of the block is complete, Tendermint sends the Commit request and blocks waiting for a response. While the mempool may run concurrently with block processing (the BeginBlock, DeliverTxs, and EndBlock), it is locked for the Commit request so that its state can be safely updated during Commit. This means the app MUST NOT do any blocking communication with the mempool (ie. broadcast_tx) during Commit, or there will be deadlock. Note also that all remaining transactions in the mempool are replayed on the mempool connection (CheckTx) following a commit.

The app should respond to the Commit request with a byte array, which is the deterministic state root of the application. It is included in the header of the next block. It can be used to provide easily verified Merkle-proofs of the state of the application.

It is expected that the app will persist state to disk on Commit. The option to have all transactions replayed from some previous block is the job of the Handshake.

In go:

Copy func (app *KVStoreApplication) Commit() types.ResponseCommit { // Using a memdb - just return the big endian size of the db appHash := make([]byte, 8) binary.PutVarint(appHash, app.state.Size) app.state.AppHash = appHash app.state.Height += 1 saveState(app.state) return types.ResponseCommit{Data: appHash} }

In Java:

Copy ResponseCommit requestCommit(RequestCommit requestCommit) { // update the internal app-state byte[] newAppState = calculateAppState(); // and return it to the node return ResponseCommit.newBuilder().setCode(CodeType.OK).setData(ByteString.copyFrom(newAppState)).build(); }

# BeginBlock

The BeginBlock request can be used to run some code at the beginning of every block. It also allows Tendermint to send the current block hash and header to the application, before it sends any of the transactions.

The app should remember the latest height and header (ie. from which it has run a successful Commit) so that it can tell Tendermint where to pick up from when it restarts. See information on the Handshake, below.

In go:

Copy // Track the block hash and header information func (app *PersistentKVStoreApplication) BeginBlock(req types.RequestBeginBlock) types.ResponseBeginBlock { // reset valset changes app.ValUpdates = make([]types.ValidatorUpdate, 0) return types.ResponseBeginBlock{} }

In Java:

Copy /* * all types come from protobuf definition */ ResponseBeginBlock requestBeginBlock(RequestBeginBlock req) { Header header = req.getHeader(); byte[] prevAppHash = header.getAppHash().toByteArray(); long prevHeight = header.getHeight(); // run your pre-block logic. Maybe prepare a state snapshot, message components, etc return ResponseBeginBlock.newBuilder().build(); }

# EndBlock

The EndBlock request can be used to run some code at the end of every block. Additionally, the response may contain a list of validators, which can be used to update the validator set. To add a new validator or update an existing one, simply include them in the list returned in the EndBlock response. To remove one, include it in the list with a power equal to 0. Validator's address field can be left empty. Tendermint core will take care of updating the validator set. Note the change in voting power must be strictly less than 1/3 per block if you want a light client to be able to prove the transition externally. See the light client docs (opens new window) for details on how it tracks validators.

In go:

Copy // Update the validator set func (app *PersistentKVStoreApplication) EndBlock(req types.RequestEndBlock) types.ResponseEndBlock { return types.ResponseEndBlock{ValidatorUpdates: app.ValUpdates} }

In Java:

Copy /* * Assume that one validator changes. The new validator has a power of 10 */ ResponseEndBlock requestEndBlock(RequestEndBlock req) { final long currentHeight = req.getHeight(); final byte[] validatorPubKey = getValPubKey(); ResponseEndBlock.Builder builder = ResponseEndBlock.newBuilder(); builder.addDiffs(1, Types.Validator.newBuilder().setPower(10L).setPubKey(ByteString.copyFrom(validatorPubKey)).build()); return builder.build(); }

# Query Connection

This connection is used to query the application without engaging consensus. It's exposed over the tendermint core rpc, so clients can query the app without exposing a server on the app itself, but they must serialize each query as a single byte array. Additionally, certain "standardized" queries may be used to inform local decisions, for instance about which peers to connect to.

Tendermint Core currently uses the Query connection to filter peers upon connecting, according to IP address or node ID. For instance, returning non-OK ABCI response to either of the following queries will cause Tendermint to not connect to the corresponding peer:

  • p2p/filter/addr/<ip addr>, where <ip addr> is an IP address.
  • p2p/filter/id/<id>, where <is> is the hex-encoded node ID (the hash of the node's p2p pubkey).

Note: these query formats are subject to change!

In go:

Copy func (app *KVStoreApplication) Query(reqQuery types.RequestQuery) (resQuery types.ResponseQuery) { if reqQuery.Prove { value := app.state.db.Get(prefixKey(reqQuery.Data)) resQuery.Index = -1 // TODO make Proof return index resQuery.Key = reqQuery.Data resQuery.Value = value if value != nil { resQuery.Log = "exists" } else { resQuery.Log = "does not exist" } return } else { resQuery.Key = reqQuery.Data value := app.state.db.Get(prefixKey(reqQuery.Data)) resQuery.Value = value if value != nil { resQuery.Log = "exists" } else { resQuery.Log = "does not exist" } return } }

In Java:

Copy ResponseQuery requestQuery(RequestQuery req) { final boolean isProveQuery = req.getProve(); final ResponseQuery.Builder responseBuilder = ResponseQuery.newBuilder(); byte[] queryData = req.getData().toByteArray(); if (isProveQuery) { com.app.example.QueryResultWithProof result = generateQueryResultWithProof(queryData); responseBuilder.setIndex(result.getLeftIndex()); responseBuilder.setKey(req.getData()); responseBuilder.setValue(result.getValueOrNull(0)); responseBuilder.setHeight(result.getHeight()); responseBuilder.setProof(result.getProof()); responseBuilder.setLog(result.getLogValue()); } else { com.app.example.QueryResult result = generateQueryResult(queryData); responseBuilder.setIndex(result.getIndex()); responseBuilder.setValue(result.getValue()); responseBuilder.setLog(result.getLogValue()); } responseBuilder.setIndex(result.getIndex()); responseBuilder.setValue(ByteString.copyFrom(result.getValue())); responseBuilder.setLog(result.getLogValue()); } return responseBuilder.build(); }

# Handshake

When the app or tendermint restarts, they need to sync to a common height. When an ABCI connection is first established, Tendermint will call Info on the Query connection. The response should contain the LastBlockHeight and LastBlockAppHash - the former is the last block for which the app ran Commit successfully, the latter is the response from that Commit.

Using this information, Tendermint will determine what needs to be replayed, if anything, against the app, to ensure both Tendermint and the app are synced to the latest block height.

If the app returns a LastBlockHeight of 0, Tendermint will just replay all blocks.

In go:

Copy func (app *KVStoreApplication) Info(req types.RequestInfo) (resInfo types.ResponseInfo) { return types.ResponseInfo{ Data: fmt.Sprintf("{\"size\":%v}", app.state.Size), Version: version.ABCIVersion, AppVersion: ProtocolVersion.Uint64(), } }

In Java:

Copy ResponseInfo requestInfo(RequestInfo req) { final byte[] lastAppHash = getLastAppHash(); final long lastHeight = getLastHeight(); return ResponseInfo.newBuilder().setLastBlockAppHash(ByteString.copyFrom(lastAppHash)).setLastBlockHeight(lastHeight).build(); }

# Genesis

InitChain will be called once upon the genesis. params includes the initial validator set. Later on, it may be extended to take parts of the consensus params.

In go:

Copy // Save the validators in the merkle tree func (app *PersistentKVStoreApplication) InitChain(req types.RequestInitChain) types.ResponseInitChain { for _, v := range req.Validators { r := app.updateValidator(v) if r.IsErr() { app.logger.Error("Error updating validators", "r", r) } } return types.ResponseInitChain{} }

In Java:

Copy /* * all types come from protobuf definition */ ResponseInitChain requestInitChain(RequestInitChain req) { final int validatorsCount = req.getValidatorsCount(); final List<Types.Validator> validatorsList = req.getValidatorsList(); validatorsList.forEach((validator) -> { long power = validator.getPower(); byte[] validatorPubKey = validator.getPubKey().toByteArray(); // do somehing for validator setup in app }); return ResponseInitChain.newBuilder().build(); }