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docs/spec/software/abci.md
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docs/spec/software/abci.md
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# Application Blockchain Interface (ABCI)
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ABCI is the interface between Tendermint (a state-machine replication engine)
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and an application (the actual state machine).
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The ABCI message types are defined in a [protobuf
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file](https://github.com/tendermint/abci/blob/master/types/types.proto).
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For full details on the ABCI message types and protocol, see the [ABCI
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specificaiton](https://github.com/tendermint/abci/blob/master/specification.rst).
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Be sure to read the specification if you're trying to build an ABCI app!
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For additional details on server implementation, see the [ABCI
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readme](https://github.com/tendermint/abci#implementation).
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Here we provide some more details around the use of ABCI by Tendermint and
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clarify common "gotchas".
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## ABCI connections
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Tendermint opens 3 ABCI connections to the app: one for Consensus, one for
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Mempool, one for Queries.
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## Async vs Sync
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The main ABCI server (ie. non-GRPC) provides ordered asynchronous messages.
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This is useful for DeliverTx and CheckTx, since it allows Tendermint to forward
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transactions to the app before it's finished processing previous ones.
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Thus, DeliverTx and CheckTx messages are sent asycnhronously, while all other
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messages are sent synchronously.
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## CheckTx and Commit
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It is typical to hold three distinct states in an ABCI app: CheckTxState, DeliverTxState,
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QueryState. The QueryState contains the latest committed state for a block.
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The CheckTxState and DeliverTxState may be updated concurrently with one another.
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Before Commit is called, Tendermint locks and flushes the mempool so that no new changes will happen
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to CheckTxState. When Commit completes, it unlocks the mempool.
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Thus, during Commit, it is safe to reset the QueryState and the CheckTxState to the latest DeliverTxState
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(ie. the new state from executing all the txs in the block).
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Note, however, that it is not possible to send transactions to Tendermint during Commit - if your app
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tries to send a `/broadcast_tx` to Tendermint during Commit, it will deadlock.
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## EndBlock Validator Updates
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Updates to the Tendermint validator set can be made by returning `Validator`
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objects in the `ResponseBeginBlock`:
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```
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message Validator {
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bytes address = 1;
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PubKey pub_key = 2;
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int64 power = 3;
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}
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message PubKey {
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string type = 1;
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bytes data = 2;
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}
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```
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The `pub_key` currently supports two types:
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- `type = "ed25519" and `data = <raw 32-byte public key>`
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- `type = "secp256k1" and `data = <33-byte OpenSSL compressed public key>`
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If the address is provided, it must match the address of the pubkey, as
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specified [here](/docs/spec/blockchain/encoding.md#Addresses)
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(Note: In the v0.19 series, the `pub_key` is the [Amino encoded public
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key](/docs/spec/blockchain/encoding.md#public-key-cryptography).
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For Ed25519 pubkeys, the Amino prefix is always "1624DE6220". For example, the 32-byte Ed25519 pubkey
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`76852933A4686A721442E931A8415F62F5F1AEDF4910F1F252FB393F74C40C85` would be
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Amino encoded as
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`1624DE622076852933A4686A721442E931A8415F62F5F1AEDF4910F1F252FB393F74C40C85`)
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(Note: In old versions of Tendermint (pre-v0.19.0), the pubkey is just prefixed with a
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single type byte, so for ED25519 we'd have `pub_key = 0x1 | pub`)
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The `power` is the new voting power for the validator, with the
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following rules:
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- power must be non-negative
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- if power is 0, the validator must already exist, and will be removed from the
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validator set
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- if power is non-0:
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- if the validator does not already exist, it will be added to the validator
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set with the given power
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- if the validator does already exist, its power will be adjusted to the given power
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## InitChain Validator Updates
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ResponseInitChain has the option to return a list of validators.
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If the list is not empty, Tendermint will adopt it for the validator set.
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This way the application can determine the initial validator set for the
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blockchain.
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Note that if addressses are included in the returned validators, they must match
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the address of the public key.
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ResponseInitChain also includes ConsensusParams, but these are presently
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ignored.
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## Query
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Query is a generic message type with lots of flexibility to enable diverse sets
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of queries from applications. Tendermint has no requirements from the Query
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message for normal operation - that is, the ABCI app developer need not implement Query functionality if they do not wish too.
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That said, Tendermint makes a number of queries to support some optional
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features. These are:
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### Peer Filtering
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When Tendermint connects to a peer, it sends two queries to the ABCI application
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using the following paths, with no additional data:
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- `/p2p/filter/addr/<IP:PORT>`, where `<IP:PORT>` denote the IP address and
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the port of the connection
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- `p2p/filter/id/<ID>`, where `<ID>` is the peer node ID (ie. the
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pubkey.Address() for the peer's PubKey)
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If either of these queries return a non-zero ABCI code, Tendermint will refuse
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to connect to the peer.
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## Info and the Handshake/Replay
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On startup, Tendermint calls Info on the Query connection to get the latest
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committed state of the app. The app MUST return information consistent with the
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last block it succesfully completed Commit for.
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If the app succesfully committed block H but not H+1, then `last_block_height =
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H` and `last_block_app_hash = <hash returned by Commit for block H>`. If the app
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failed during the Commit of block H, then `last_block_height = H-1` and
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`last_block_app_hash = <hash returned by Commit for block H-1, which is the hash
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in the header of block H>`.
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We now distinguish three heights, and describe how Tendermint syncs itself with
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the app.
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```
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storeBlockHeight = height of the last block Tendermint saw a commit for
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stateBlockHeight = height of the last block for which Tendermint completed all
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block processing and saved all ABCI results to disk
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appBlockHeight = height of the last block for which ABCI app succesfully
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completely Commit
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```
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Note we always have `storeBlockHeight >= stateBlockHeight` and `storeBlockHeight >= appBlockHeight`
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Note also we never call Commit on an ABCI app twice for the same height.
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The procedure is as follows.
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First, some simeple start conditions:
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If `appBlockHeight == 0`, then call InitChain.
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If `storeBlockHeight == 0`, we're done.
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Now, some sanity checks:
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If `storeBlockHeight < appBlockHeight`, error
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If `storeBlockHeight < stateBlockHeight`, panic
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If `storeBlockHeight > stateBlockHeight+1`, panic
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Now, the meat:
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If `storeBlockHeight == stateBlockHeight && appBlockHeight < storeBlockHeight`,
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replay all blocks in full from `appBlockHeight` to `storeBlockHeight`.
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This happens if we completed processing the block, but the app forgot its height.
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If `storeBlockHeight == stateBlockHeight && appBlockHeight == storeBlockHeight`, we're done
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This happens if we crashed at an opportune spot.
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If `storeBlockHeight == stateBlockHeight+1`
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This happens if we started processing the block but didn't finish.
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If `appBlockHeight < stateBlockHeight`
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replay all blocks in full from `appBlockHeight` to `storeBlockHeight-1`,
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and replay the block at `storeBlockHeight` using the WAL.
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This happens if the app forgot the last block it committed.
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If `appBlockHeight == stateBlockHeight`,
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replay the last block (storeBlockHeight) in full.
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This happens if we crashed before the app finished Commit
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If appBlockHeight == storeBlockHeight {
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update the state using the saved ABCI responses but dont run the block against the real app.
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This happens if we crashed after the app finished Commit but before Tendermint saved the state.
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33
docs/spec/software/wal.md
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docs/spec/software/wal.md
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# WAL
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Consensus module writes every message to the WAL (write-ahead log).
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It also issues fsync syscall through
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[File#Sync](https://golang.org/pkg/os/#File.Sync) for messages signed by this
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node (to prevent double signing).
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Under the hood, it uses
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[autofile.Group](https://godoc.org/github.com/tendermint/tmlibs/autofile#Group),
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which rotates files when those get too big (> 10MB).
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The total maximum size is 1GB. We only need the latest block and the block before it,
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but if the former is dragging on across many rounds, we want all those rounds.
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## Replay
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Consensus module will replay all the messages of the last height written to WAL
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before a crash (if such occurs).
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The private validator may try to sign messages during replay because it runs
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somewhat autonomously and does not know about replay process.
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For example, if we got all the way to precommit in the WAL and then crash,
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after we replay the proposal message, the private validator will try to sign a
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prevote. But it will fail. That's ok because we’ll see the prevote later in the
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WAL. Then it will go to precommit, and that time it will work because the
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private validator contains the `LastSignBytes` and then we’ll replay the
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precommit from the WAL.
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Make sure to read about [WAL
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corruption](https://tendermint.readthedocs.io/projects/tools/en/master/specification/corruption.html#wal-corruption)
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and recovery strategies.
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