# Applications

Please ensure you've first read the spec for ABCI Methods and Types

Here we cover the following components of ABCI applications:

  • Connection State - the interplay between ABCI connections and application state and the differences between CheckTx and DeliverTx.
  • Transaction Results - rules around transaction results and validity
  • Validator Set Updates - how validator sets are changed during InitChain and EndBlock
  • Query - standards for using the Query method and proofs about the application state
  • Crash Recovery - handshake protocol to synchronize Tendermint and the application on startup.

# State

Since Tendermint maintains three concurrent ABCI connections, it is typical for an application to maintain a distinct state for each, and for the states to be synchronized during Commit.

# Commit

Application state should only be persisted to disk during Commit.

Before Commit is called, Tendermint locks and flushes the mempool so that no new messages will be received on the mempool connection. This provides an opportunity to safely update all three states to the latest committed state at once.

When Commit completes, it unlocks the mempool.

WARNING: if the ABCI app logic processing the Commit message sends a /broadcast_tx_sync or /broadcast_tx_commit and waits for the response before proceeding, it will deadlock. Executing those broadcast_tx calls involves acquiring a lock that is held during the Commit call, so it's not possible. If you make the call to the broadcast_tx endpoints concurrently, that's no problem, it just can't be part of the sequential logic of the Commit function.

# Consensus Connection

The Consensus Connection should maintain a DeliverTxState - the working state for block execution. It should be updated by the calls to BeginBlock, DeliverTx, and EndBlock during block execution and committed to disk as the "latest committed state" during Commit.

Updates made to the DeliverTxState by each method call must be readable by each subsequent method - ie. the updates are linearizable.

# Mempool Connection

The Mempool Connection should maintain a CheckTxState to sequentially process pending transactions in the mempool that have not yet been committed. It should be initialized to the latest committed state at the end of every Commit.

The CheckTxState may be updated concurrently with the DeliverTxState, as messages may be sent concurrently on the Consensus and Mempool connections. However, before calling Commit, Tendermint will lock and flush the mempool connection, ensuring that all existing CheckTx are responded to and no new ones can begin.

After Commit, CheckTx is run again on all transactions that remain in the node's local mempool after filtering those included in the block. To prevent the mempool from rechecking all transactions every time a block is committed, set the configuration option mempool.recheck=false. As of Tendermint v0.32.1, an additional Type parameter is made available to the CheckTx function that indicates whether an incoming transaction is new (CheckTxType_New), or a recheck (CheckTxType_Recheck).

Finally, the mempool will unlock and new transactions can be processed through CheckTx again.

Note that CheckTx doesn't have to check everything that affects transaction validity; the expensive things can be skipped. In fact, CheckTx doesn't have to check anything; it might say that any transaction is a valid transaction. Unlike DeliverTx, CheckTx is just there as a sort of weak filter to keep invalid transactions out of the blockchain. It's weak, because a Byzantine node doesn't care about CheckTx; it can propose a block full of invalid transactions if it wants.

# Info Connection

The Info Connection should maintain a QueryState for answering queries from the user, and for initialization when Tendermint first starts up (both described further below). It should always contain the latest committed state associated with the latest committed block.

QueryState should be set to the latest DeliverTxState at the end of every Commit, ie. after the full block has been processed and the state committed to disk. Otherwise it should never be modified.

# Transaction Results

ResponseCheckTx and ResponseDeliverTx contain the same fields.

The Info and Log fields are non-deterministic values for debugging/convenience purposes that are otherwise ignored.

The Data field must be strictly deterministic, but can be arbitrary data.

# Gas

Ethereum introduced the notion of gas as an abstract representation of the cost of resources used by nodes when processing transactions. Every operation in the Ethereum Virtual Machine uses some amount of gas, and gas can be accepted at a market-variable price. Users propose a maximum amount of gas for their transaction; if the tx uses less, they get the difference credited back. Tendermint adopts a similar abstraction, though uses it only optionally and weakly, allowing applications to define their own sense of the cost of execution.

In Tendermint, the ConsensusParams.Block.MaxGas limits the amount of gas that can be used in a block. The default value is -1, meaning no limit, or that the concept of gas is meaningless.

Responses contain a GasWanted and GasUsed field. The former is the maximum amount of gas the sender of a tx is willing to use, and the later is how much it actually used. Applications should enforce that GasUsed <= GasWanted - ie. tx execution should halt before it can use more resources than it requested.

When MaxGas > -1, Tendermint enforces the following rules:

  • GasWanted <= MaxGas for all txs in the mempool
  • (sum of GasWanted in a block) <= MaxGas when proposing a block

If MaxGas == -1, no rules about gas are enforced.

Note that Tendermint does not currently enforce anything about Gas in the consensus, only the mempool. This means it does not guarantee that committed blocks satisfy these rules! It is the application's responsibility to return non-zero response codes when gas limits are exceeded.

The GasUsed field is ignored completely by Tendermint. That said, applications should enforce:

  • GasUsed <= GasWanted for any given transaction
  • (sum of GasUsed in a block) <= MaxGas for every block

In the future, we intend to add a Priority field to the responses that can be used to explicitly prioritize txs in the mempool for inclusion in a block proposal. See #1861.

# CheckTx

If Code != 0, it will be rejected from the mempool and hence not broadcasted to other peers and not included in a proposal block.

Data contains the result of the CheckTx transaction execution, if any. It is semantically meaningless to Tendermint.

Tags include any tags for the execution, though since the transaction has not been committed yet, they are effectively ignored by Tendermint.

# DeliverTx

If DeliverTx returns Code != 0, the transaction will be considered invalid, though it is still included in the block.

Data contains the result of the CheckTx transaction execution, if any. It is semantically meaningless to Tendermint.

Both the Code and Data are included in a structure that is hashed into the LastResultsHash of the next block header.

Tags include any tags for the execution, which Tendermint will use to index the transaction by. This allows transactions to be queried according to what events took place during their execution.

See issue #1007 for how the tags will be hashed into the next block header.

# Validator Updates

The application may set the validator set during InitChain, and update it during EndBlock.

Note that the maximum total power of the validator set is bounded by MaxTotalVotingPower = MaxInt64 / 8. Applications are responsible for ensuring they do not make changes to the validator set that cause it to exceed this limit.

Additionally, applications must ensure that a single set of updates does not contain any duplicates - a given public key can only appear in an update once. If an update includes duplicates, the block execution will fail irrecoverably.

# InitChain

ResponseInitChain can return a list of validators. If the list is empty, Tendermint will use the validators loaded in the genesis file. If the list is not empty, Tendermint will use it for the validator set. This way the application can determine the initial validator set for the blockchain.

# EndBlock

Updates to the Tendermint validator set can be made by returning ValidatorUpdate objects in the ResponseEndBlock:

Copymessage ValidatorUpdate { PubKey pub_key int64 power } message PubKey { string type bytes data } Expand

The pub_key currently supports only one type:

  • type = "ed25519" and data = <raw 32-byte public key>

The power is the new voting power for the validator, with the following rules:

  • power must be non-negative
  • if power is 0, the validator must already exist, and will be removed from the validator set
  • if power is non-0:
    • if the validator does not already exist, it will be added to the validator set with the given power
    • if the validator does already exist, its power will be adjusted to the given power
  • the total power of the new validator set must not exceed MaxTotalVotingPower

Note the updates returned in block H will only take effect at block H+2.

# Consensus Parameters

ConsensusParams enforce certain limits in the blockchain, like the maximum size of blocks, amount of gas used in a block, and the maximum acceptable age of evidence. They can be set in InitChain and updated in EndBlock.

# Block.MaxBytes

The maximum size of a complete Amino encoded block. This is enforced by Tendermint consensus.

This implies a maximum tx size that is this MaxBytes, less the expected size of the header, the validator set, and any included evidence in the block.

Must have 0 < MaxBytes < 100 MB.

# Block.MaxGas

The maximum of the sum of GasWanted in a proposed block. This is not enforced by Tendermint consensus. It is left to the app to enforce (ie. if txs are included past the limit, they should return non-zero codes). It is used by Tendermint to limit the txs included in a proposed block.

Must have MaxGas >= -1. If MaxGas == -1, no limit is enforced.

# Block.TimeIotaMs

The minimum time between consecutive blocks (in milliseconds). This is enforced by Tendermint consensus.

Must have TimeIotaMs > 0 to ensure time monotonicity.

# EvidenceParams.MaxAge

This is the maximum age of evidence. This is enforced by Tendermint consensus. If a block includes evidence older than this, the block will be rejected (validators won't vote for it).

Must have MaxAge > 0.

# Updates

The application may set the ConsensusParams during InitChain, and update them during EndBlock. If the ConsensusParams is empty, it will be ignored. Each field that is not empty will be applied in full. For instance, if updating the Block.MaxBytes, applications must also set the other Block fields (like Block.MaxGas), even if they are unchanged, as they will otherwise cause the value to be updated to 0.

# InitChain

ResponseInitChain includes a ConsensusParams. If its nil, Tendermint will use the params loaded in the genesis file. If it's not nil, Tendermint will use it. This way the application can determine the initial consensus params for the blockchain.

# EndBlock

ResponseEndBlock includes a ConsensusParams. If its nil, Tendermint will do nothing. If it's not nil, Tendermint will use it. This way the application can update the consensus params over time.

Note the updates returned in block H will take effect right away for block H+1.

# Query

Query is a generic method with lots of flexibility to enable diverse sets of queries on application state. Tendermint makes use of Query to filter new peers based on ID and IP, and exposes Query to the user over RPC.

Note that calls to Query are not replicated across nodes, but rather query the local node's state - hence they may return stale reads. For reads that require consensus, use a transaction.

The most important use of Query is to return Merkle proofs of the application state at some height that can be used for efficient application-specific lite-clients.

Note Tendermint has technically no requirements from the Query message for normal operation - that is, the ABCI app developer need not implement Query functionality if they do not wish too.

# Query Proofs

The Tendermint block header includes a number of hashes, each providing an anchor for some type of proof about the blockchain. The ValidatorsHash enables quick verification of the validator set, the DataHash gives quick verification of the transactions included in the block, etc.

The AppHash is unique in that it is application specific, and allows for application-specific Merkle proofs about the state of the application. While some applications keep all relevant state in the transactions themselves (like Bitcoin and its UTXOs), others maintain a separated state that is computed deterministically from transactions, but is not contained directly in the transactions themselves (like Ethereum contracts and accounts). For such applications, the AppHash provides a much more efficient way to verify lite-client proofs.

ABCI applications can take advantage of more efficient lite-client proofs for their state as follows:

  • return the Merkle root of the deterministic application state in ResponseCommit.Data.
  • it will be included as the AppHash in the next block.
  • return efficient Merkle proofs about that application state in ResponseQuery.Proof that can be verified using the AppHash of the corresponding block.

For instance, this allows an application's lite-client to verify proofs of absence in the application state, something which is much less efficient to do using the block hash.

Some applications (eg. Ethereum, Cosmos-SDK) have multiple "levels" of Merkle trees, where the leaves of one tree are the root hashes of others. To support this, and the general variability in Merkle proofs, the ResponseQuery.Proof has some minimal structure:

Copymessage Proof { repeated ProofOp ops } message ProofOp { string type = 1; bytes key = 2; bytes data = 3; } Expand

Each ProofOp contains a proof for a single key in a single Merkle tree, of the specified type. This allows ABCI to support many different kinds of Merkle trees, encoding formats, and proofs (eg. of presence and absence) just by varying the type. The data contains the actual encoded proof, encoded according to the type. When verifying the full proof, the root hash for one ProofOp is the value being verified for the next ProofOp in the list. The root hash of the final ProofOp in the list should match the AppHash being verified against.

# Peer Filtering

When Tendermint connects to a peer, it sends two queries to the ABCI application using the following paths, with no additional data:

  • /p2p/filter/addr/<IP:PORT>, where <IP:PORT> denote the IP address and the port of the connection
  • p2p/filter/id/<ID>, where <ID> is the peer node ID (ie. the pubkey.Address() for the peer's PubKey)

If either of these queries return a non-zero ABCI code, Tendermint will refuse to connect to the peer.

# Paths

Queries are directed at paths, and may optionally include additional data.

The expectation is for there to be some number of high level paths differentiating concerns, like /p2p, /store, and /app. Currently, Tendermint only uses /p2p, for filtering peers. For more advanced use, see the implementation of Query in the Cosmos-SDK.

# Crash Recovery

On startup, Tendermint calls the Info method on the Info Connection to get the latest committed state of the app. The app MUST return information consistent with the last block it succesfully completed Commit for.

If the app succesfully committed block H but not H+1, then last_block_height = H and last_block_app_hash = <hash returned by Commit for block H>. If the app failed during the Commit of block H, then last_block_height = H-1 and last_block_app_hash = <hash returned by Commit for block H-1, which is the hash in the header of block H>.

We now distinguish three heights, and describe how Tendermint syncs itself with the app.

CopystoreBlockHeight = height of the last block Tendermint saw a commit for stateBlockHeight = height of the last block for which Tendermint completed all block processing and saved all ABCI results to disk appBlockHeight = height of the last block for which ABCI app succesfully completed Commit Expand

Note we always have storeBlockHeight >= stateBlockHeight and storeBlockHeight >= appBlockHeight Note also we never call Commit on an ABCI app twice for the same height.

The procedure is as follows.

First, some simple start conditions:

If appBlockHeight == 0, then call InitChain.

If storeBlockHeight == 0, we're done.

Now, some sanity checks:

If storeBlockHeight < appBlockHeight, error If storeBlockHeight < stateBlockHeight, panic If storeBlockHeight > stateBlockHeight+1, panic

Now, the meat:

If storeBlockHeight == stateBlockHeight && appBlockHeight < storeBlockHeight, replay all blocks in full from appBlockHeight to storeBlockHeight. This happens if we completed processing the block, but the app forgot its height.

If storeBlockHeight == stateBlockHeight && appBlockHeight == storeBlockHeight, we're done. This happens if we crashed at an opportune spot.

If storeBlockHeight == stateBlockHeight+1 This happens if we started processing the block but didn't finish.

If appBlockHeight < stateBlockHeight replay all blocks in full from appBlockHeight to storeBlockHeight-1, and replay the block at storeBlockHeight using the WAL. This happens if the app forgot the last block it committed.

If appBlockHeight == stateBlockHeight, replay the last block (storeBlockHeight) in full. This happens if we crashed before the app finished Commit

If appBlockHeight == storeBlockHeight update the state using the saved ABCI responses but dont run the block against the real app. This happens if we crashed after the app finished Commit but before Tendermint saved the state.