# Block Structure

The tendermint consensus engine records all agreements by a
supermajority of nodes into a blockchain, which is replicated among all
nodes. This blockchain is accessible via various rpc endpoints, mainly
`/block?height=` to get the full block, as well as
`/blockchain?minHeight=_&maxHeight=_` to get a list of headers. But what
exactly is stored in these blocks?

## Block

A
[Block](https://godoc.org/github.com/tendermint/tendermint/types#Block)
contains:

- a [Header](#header) contains merkle hashes for various chain states
- the
  [Data](https://godoc.org/github.com/tendermint/tendermint/types#Data)
  is all transactions which are to be processed
- the [LastCommit](#commit) > 2/3 signatures for the last block

The signatures returned along with block `H` are those validating block
`H-1`. This can be a little confusing, but we must also consider that
the `Header` also contains the `LastCommitHash`. It would be impossible
for a Header to include the commits that sign it, as it would cause an
infinite loop here. But when we get block `H`, we find
`Header.LastCommitHash`, which must match the hash of `LastCommit`.

## Header

The
[Header](https://godoc.org/github.com/tendermint/tendermint/types#Header)
contains lots of information (follow link for up-to-date info). Notably,
it maintains the `Height`, the `LastBlockID` (to make it a chain), and
hashes of the data, the app state, and the validator set. This is
important as the only item that is signed by the validators is the
`Header`, and all other data must be validated against one of the merkle
hashes in the `Header`.

The `DataHash` can provide a nice check on the
[Data](https://godoc.org/github.com/tendermint/tendermint/types#Data)
returned in this same block. If you are subscribed to new blocks, via
tendermint RPC, in order to display or process the new transactions you
should at least validate that the `DataHash` is valid. If it is
important to verify autheniticity, you must wait for the `LastCommit`
from the next block to make sure the block header (including `DataHash`)
was properly signed.

The `ValidatorHash` contains a hash of the current
[Validators](https://godoc.org/github.com/tendermint/tendermint/types#Validator).
Tracking all changes in the validator set is complex, but a client can
quickly compare this hash with the [hash of the currently known
validators](https://godoc.org/github.com/tendermint/tendermint/types#ValidatorSet.Hash)
to see if there have been changes.

The `AppHash` serves as the basis for validating any merkle proofs that
come from the ABCI application. It represents the state of the actual
application, rather that the state of the blockchain itself. This means
it's necessary in order to perform any business logic, such as verifying
an account balance.

**Note** After the transactions are committed to a block, they still
need to be processed in a separate step, which happens between the
blocks. If you find a given transaction in the block at height `H`, the
effects of running that transaction will be first visible in the
`AppHash` from the block header at height `H+1`.

Like the `LastCommit` issue, this is a requirement of the immutability
of the block chain, as the application only applies transactions _after_
they are commited to the chain.

## Commit

The
[Commit](https://godoc.org/github.com/tendermint/tendermint/types#Commit)
contains a set of
[Votes](https://godoc.org/github.com/tendermint/tendermint/types#Vote)
that were made by the validator set to reach consensus on this block.
This is the key to the security in any PoS system, and actually no data
that cannot be traced back to a block header with a valid set of Votes
can be trusted. Thus, getting the Commit data and verifying the votes is
extremely important.

As mentioned above, in order to find the `precommit votes` for block
header `H`, we need to query block `H+1`. Then we need to check the
votes, make sure they really are for that block, and properly formatted.
Much of this code is implemented in Go in the
[light-client](https://github.com/tendermint/light-client) package. If
you look at the code, you will notice that we need to provide the
`chainID` of the blockchain in order to properly calculate the votes.
This is to protect anyone from swapping votes between chains to fake (or
frame) a validator. Also note that this `chainID` is in the
`genesis.json` from _Tendermint_, not the `genesis.json` from the
basecoin app ([that is a different
chainID...](https://github.com/cosmos/cosmos-sdk/issues/32)).

Once we have those votes, and we calculated the proper [sign
bytes](https://godoc.org/github.com/tendermint/tendermint/types#Vote.WriteSignBytes)
using the chainID and a [nice helper
function](https://godoc.org/github.com/tendermint/tendermint/types#SignBytes),
we can verify them. The light client is responsible for maintaining a
set of validators that we trust. Each vote only stores the validators
`Address`, as well as the `Signature`. Assuming we have a local copy of
the trusted validator set, we can look up the `Public Key` of the
validator given its `Address`, then verify that the `Signature` matches
the `SignBytes` and `Public Key`. Then we sum up the total voting power
of all validators, whose votes fulfilled all these stringent
requirements. If the total number of voting power for a single block is
greater than 2/3 of all voting power, then we can finally trust the
block header, the AppHash, and the proof we got from the ABCI
application.

### Vote Sign Bytes

The `sign-bytes` of a vote is produced by taking a
[stable-json](https://github.com/substack/json-stable-stringify)-like
deterministic JSON [wire](./wire-protocol.html) encoding of the vote
(excluding the `Signature` field), and wrapping it with
`{"chain_id":"my_chain","vote":...}`.

For example, a precommit vote might have the following `sign-bytes`:

```
{"chain_id":"my_chain","vote":{"block_hash":"611801F57B4CE378DF1A3FFF1216656E89209A99","block_parts_header":{"hash":"B46697379DBE0774CC2C3B656083F07CA7E0F9CE","total":123},"height":1234,"round":1,"type":2}}
```

## Block Hash

The [block
hash](https://godoc.org/github.com/tendermint/tendermint/types#Block.Hash)
is the [Simple Tree hash](./merkle.html#simple-tree-with-dictionaries)
of the fields of the block `Header` encoded as a list of `KVPair`s.

## Transaction

A transaction is any sequence of bytes. It is up to your ABCI
application to accept or reject transactions.

## BlockID

Many of these data structures refer to the
[BlockID](https://godoc.org/github.com/tendermint/tendermint/types#BlockID),
which is the `BlockHash` (hash of the block header, also referred to by
the next block) along with the `PartSetHeader`. The `PartSetHeader` is
explained below and is used internally to orchestrate the p2p
propogation. For clients, it is basically opaque bytes, but they must
match for all votes.

## PartSetHeader

The
[PartSetHeader](https://godoc.org/github.com/tendermint/tendermint/types#PartSetHeader)
contains the total number of pieces in a
[PartSet](https://godoc.org/github.com/tendermint/tendermint/types#PartSet),
and the Merkle root hash of those pieces.

## PartSet

PartSet is used to split a byteslice of data into parts (pieces) for
transmission. By splitting data into smaller parts and computing a
Merkle root hash on the list, you can verify that a part is legitimately
part of the complete data, and the part can be forwarded to other peers
before all the parts are known. In short, it's a fast way to securely
propagate a large chunk of data (like a block) over a gossip network.

PartSet was inspired by the LibSwift project.

Usage:

```
data := RandBytes(2 << 20) // Something large

partSet := NewPartSetFromData(data)
partSet.Total()     // Total number of 4KB parts
partSet.Count()     // Equal to the Total, since we already have all the parts
partSet.Hash()      // The Merkle root hash
partSet.BitArray()  // A BitArray of partSet.Total() 1's

header := partSet.Header() // Send this to the peer
header.Total        // Total number of parts
header.Hash         // The merkle root hash

// Now we'll reconstruct the data from the parts
partSet2 := NewPartSetFromHeader(header)
partSet2.Total()    // Same total as partSet.Total()
partSet2.Count()    // Zero, since this PartSet doesn't have any parts yet.
partSet2.Hash()     // Same hash as in partSet.Hash()
partSet2.BitArray() // A BitArray of partSet.Total() 0's

// In a gossip network the parts would arrive in arbitrary order, perhaps
// in response to explicit requests for parts, or optimistically in response
// to the receiving peer's partSet.BitArray().
for !partSet2.IsComplete() {
    part := receivePartFromGossipNetwork()
    added, err := partSet2.AddPart(part)
    if err != nil {
    // A wrong part,
        // the merkle trail does not hash to partSet2.Hash()
    } else if !added {
        // A duplicate part already received
    }
}

data2, _ := ioutil.ReadAll(partSet2.GetReader())
bytes.Equal(data, data2) // true
```