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adr-037-deliver-block.md
# ADR 037: Deliver Block
Author: Daniil Lashin (@danil-lashin)
## Changelog
13-03-2019: Initial draft
## Context
Initial conversation: https://github.com/tendermint/tendermint/issues/2901
Some applications can handle transactions in parallel, or at least some
part of tx processing can be parallelized. Now it is not possible for developer
to execute txs in parallel because Tendermint delivers them consequentially.
## Decision
Now Tendermint have `BeginBlock`, `EndBlock`, `Commit`, `DeliverTx` steps
while executing block. This doc proposes merging this steps into one `DeliverBlock`
step. It will allow developers of applications to decide how they want to
execute transactions (in parallel or consequentially). Also it will simplify and
speed up communications between application and Tendermint.
As @jaekwon [mentioned](https://github.com/tendermint/tendermint/issues/2901#issuecomment-477746128)
in discussion not all application will benefit from this solution. In some cases,
when application handles transaction consequentially, it way slow down the blockchain,
because it need to wait until full block is transmitted to application to start
processing it. Also, in the case of complete change of ABCI, we need to force all the apps
to change their implementation completely. That's why I propose to introduce one more ABCI
type.
# Implementation Changes
In addition to default application interface which now have this structure
```go
type Application interface {
// Info and Mempool methods...
// Consensus Connection
InitChain(RequestInitChain) ResponseInitChain // Initialize blockchain with validators and other info from TendermintCore
BeginBlock(RequestBeginBlock) ResponseBeginBlock // Signals the beginning of a block
DeliverTx(tx []byte) ResponseDeliverTx // Deliver a tx for full processing
EndBlock(RequestEndBlock) ResponseEndBlock // Signals the end of a block, returns changes to the validator set
Commit() ResponseCommit // Commit the state and return the application Merkle root hash
}
```
this doc proposes to add one more:
```go
type Application interface {
// Info and Mempool methods...
// Consensus Connection
InitChain(RequestInitChain) ResponseInitChain // Initialize blockchain with validators and other info from TendermintCore
DeliverBlock(RequestDeliverBlock) ResponseDeliverBlock // Deliver full block
Commit() ResponseCommit // Commit the state and return the application Merkle root hash
}
type RequestDeliverBlock struct {
Hash []byte
Header Header
Txs Txs
LastCommitInfo LastCommitInfo
ByzantineValidators []Evidence
}
type ResponseDeliverBlock struct {
ValidatorUpdates []ValidatorUpdate
ConsensusParamUpdates *ConsensusParams
Tags []kv.Pair
TxResults []ResponseDeliverTx
}
```
Also, we will need to add new config param, which will specify what kind of ABCI application uses.
For example, it can be `abci_type`. Then we will have 2 types:
- `advanced` - current ABCI
- `simple` - proposed implementation
## Status
In review
## Consequences
### Positive
- much simpler introduction and tutorials for new developers (instead of implementing 5 methods whey
will need to implement only 3)
- txs can be handled in parallel
- simpler interface
- faster communications between Tendermint and application
### Negative
- Tendermint should now support 2 kinds of ABCI
![swh spinner](/static/img/swh-spinner.gif)
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