tendermint/UPGRADING.md

# Upgrading Tendermint Core

This guide provides steps to be followed when you upgrade your applications to
a newer version of Tendermint Core.

## v0.34.0

**This release is not compatible with previous blockchains** due to switching
from amino to proto3 encoding and breaking changes to the header.

### ABCI application changes

New ABCI methods (`ListSnapshots`, `LoadSnapshotChunk`, `OfferSnapshot`, and
`ApplySnapshotChunk`) were added for the highly anticipated state sync feature.
With it, new nodes are able to join the network in a matter of seconds. Read
[the spec](https://docs.tendermint.com/master/spec/abci/apps.html#state-sync)
if you want to learn more & support it (with cosmos-sdk you get it
 automatically). If you don't want to support it, just leave these methods
 empty.

`KV.Pair` has been replaced with `abci.EventAttribute`. `EventAttribute.Index`
field allows ABCI applications to dictate which events should be indexed.

The blockchain can now start from an arbitrary initial height, provided to the
application via `RequestInitChain.InitialHeight`.

ABCI evidence type is now an enum with two recognised types of evidence:
`DUPLICATE_VOTE` and `LIGHT_CLIENT_ATTACK`. Applications should be able to handle these
evidence types.

### P2P Protocol

The default codec is now proto3, not amino. The schema files can be found in the `/proto`
directory. In the future we're considering using gRPC for the remote private
validator and ABCI ( [#4698](https://github.com/tendermint/tendermint/issues/4698) ).

### Blockchain Protocol

`Header#LastResultsHash`, which previously was the root hash of a Merkle tree
built from `ResponseDeliverTx(Code, Data)` responses, became the root hash of a
Merkle tree built from:

- `BeginBlock#Events`;
- root hash of a Merkle tree built from `ResponseDeliverTx(Code, Data,
    GasWanted, GasUsed, Events)` responses;
- `BeginBlock#Events`.

Merkle hashes of empty trees previously returned nothing, but now return the hash of an empty input,
to conform with RFC-6962. This mainly affects `Header#DataHash`, `Header#LastResultsHash`, and
`Header#EvidenceHash`, which are often empty. Non-empty hashes can also be affected, e.g. if their
inputs depend on other (empty) Merkle hashes, giving different results.

### Tx Indexing

- Tendermint will now rely on the application entirely to tell it what txs to index. This means that in the `config.toml`,
generated by Tendermint, there will not be a way to specify which txs to index. `tx.height` & `tx.hash` will always be indexed when using the `kv` indexer.
The application will need to decide if they would like to allow the node operator to decide what to index or if it will enable
indexing for all txs. Application's can notify Tendermint to index a specific tx by setting `Index: bool` to true in the Event Attribute.

```go
[]types.Event{
		{
			Type: "app",
			Attributes: []types.EventAttribute{
				{Key: []byte("creator"), Value: []byte("Cosmoshi Netowoko"), Index: true},
			},
		},
  }
```

### Protobuf

With this release we are happy to announce the full protobuf migration of the Tendermint repo. This consists of changes that you may need to be aware of:

- All proto files have been moved under one directory, `/proto`. This is in line with the recommended file layout by [buf](https://buf.build), you can read more about it [here](https://buf.build/docs/lint-checkers#file_layout)
- We use the generated protobuf types for only on disk and over the wire serialization. This means that these changes should not effect you as user of Tendermint.
- A few notable changes in the abci:
    - In `ValidatorUpdates` the public key type has been migrated to a protobuf `oneof` type. Since Tendermint only supports ed25519 validator keys this is the only available key in the oneof.

### Consensus Params

Various parameters have been added to the consensus parameters.

#### Version Params (New)

- `AppVersion` - this contains the ABCI application version

#### Evidence Params

- `MaxNum` - cap the total amount of evidence by a absolute number (Default: 50)
- `ProofTrialPeriod` - duration (blocks) in which a node has to provide proof of correctly executing a lock change in the event of amnesia evidence (Default: 50000, half MaxAgeNumBlocks)

### Crypto

#### Keys

All keys have removed there type prefix. Ed25519 Pubkey went from `PubKeyEd25519` to `PubKey`. This way when calling the key you are not duplicating information (`ed25519.PubKey`). All keys are now slice of bytes(`[]byte`), previously they were a array of bytes (`[<size>]byte`).

- The multisig that was previously located in Tendermint has now migrated to a new home within the [Cosmos-SDK](https://github.com/cosmos/cosmos-sdk/blob/master/crypto/types/multisig/multisignature.go).
- Secp256k1 has been removed from the Tendermint repo. If you would like to continue using the implementation you can find it in the [Cosmos-SDK](https://github.com/cosmos/cosmos-sdk/tree/443e0c1f89bd3730a731aea30453bd732f7efa35/crypto/keys/secp256k1)

#### Merkle

From the merkle package `SimpleHashFromMap()` and `SimpleProofsFromMap()` were removed along with all the prefixes of `Simple`. If you are looking for `SimpleProof` it has been renamed to `Proof` within the merkle pkg. Previously there were protobuf messages located in the merkle pkg, these have since been moved to the `/proto` directory. The protobuf message `Proof` that contained multiple ProofOp's has been renamed to `ProofOps`. This change effects the ABCI type `ResponseQuery`, the field that was named Proof is now named `ProofOps`.

`HashFromByteSlices` and `ProofsFromByteSlices` now return a hash for empty inputs, to conform with
RFC-6962.

### Libs

The Bech32 pkg has been migrated to a new home, you can find it in the [Cosmos-SDK](https://github.com/cosmos/cosmos-sdk/tree/4173ea5ebad906dd9b45325bed69b9c655504867/types/bech32)

### CLI

`tendermint lite` has been renamed `tendermint light` and has a slightly different API.

Check out [the docs](https://docs.tendermint.com/master/tendermint-core/light-client-protocol.html#http-proxy) for details.

### Light client

The old `lite` package is removed in favor of the new `light` package. Check
out [this
article](https://medium.com/tendermint/everything-you-need-to-know-about-the-tendermint-light-client-f80d03856f98)
if you want to learn why the rewrite was needed and what comprise the new light
  client.

Doc: <https://pkg.go.dev/github.com/tendermint/tendermint/lite2?tab=doc>

`Verifier` was broken up in two pieces: core verification logic (pure `VerifyX`
functions) and `Client` object, which represents the complete light client.

RPC client can be found in `/rpc` directory. HTTP(S) proxy is located in
`/proxy` directory.

### State

A field `State.InitialHeight` has been added to record the initial chain height, which must be `1`
(not `0`) if starting from height `1`. This can be configured via the genesis field
`initial_height`.

### Privval

All requests are now accompanied by the chainID from the network.
This is a optional field and can be ignored by key management systems. It
is recommended to check the chainID if using the same key management system for multiple chains.

### RPC

`/unsafe_start_cpu_profiler`, `/unsafe_stop_cpu_profiler` and
`/unsafe_write_heap_profile` were removed. Please use pprof server, which can
be enabled through `--rpc.pprof_laddr=X` flag or `pprof_laddr=X` config setting
in the rpc section.

## v0.33.4

### Go API

- `rpc/client` HTTP and local clients have been moved into `http` and `local`
  subpackages, and their constructors have been renamed to `New()`.

### Protobuf Changes

When upgrading to version 0.33.4 you will have to fetch the `third_party`
directory along with the updated proto files.

### Block Retention

ResponseCommit added a field for block retention. The application can provide information to Tendermint on how to prune blocks.
If an application would like to not prune any blocks pass a `0` in this field.

```proto
message ResponseCommit {
  // reserve 1
  bytes  data          = 2; // the Merkle root hash
  ++ uint64 retain_height = 3; // the oldest block height to retain ++
}
```

## v0.33.0

This release is not compatible with previous blockchains due to commit becoming
signatures only and fields in the header have been removed.

### Blockchain Protocol

`TotalTxs` and `NumTxs` were removed from the header. `Commit` now consists
mostly of just signatures.

```go
type Commit struct {
	Height     int64
	Round      int
	BlockID    BlockID
	Signatures []CommitSig
}
```

```go
type BlockIDFlag byte

const (
	// BlockIDFlagAbsent - no vote was received from a validator.
	BlockIDFlagAbsent BlockIDFlag = 0x01
	// BlockIDFlagCommit - voted for the Commit.BlockID.
	BlockIDFlagCommit = 0x02
	// BlockIDFlagNil - voted for nil.
	BlockIDFlagNil = 0x03
)

type CommitSig struct {
	BlockIDFlag      BlockIDFlag
	ValidatorAddress Address
	Timestamp        time.Time
	Signature        []byte
}
```

See [\#63](https://github.com/tendermint/spec/pull/63) for the complete spec
change.

### P2P Protocol

The secret connection now includes a transcript hashing. If you want to
implement a handshake (or otherwise have an existing implementation), you'll
need to make the same changes that were made
[here](https://github.com/tendermint/tendermint/pull/3668).

### Config Changes

You will need to generate a new config if you have used a prior version of tendermint.

Tags have been entirely renamed throughout the codebase to events and there
keys are called
[compositeKeys](https://github.com/tendermint/tendermint/blob/6d05c531f7efef6f0619155cf10ae8557dd7832f/docs/app-dev/indexing-transactions.md).

Evidence Params has been changed to include duration.

- `consensus_params.evidence.max_age_duration`.
- Renamed `consensus_params.evidence.max_age` to `max_age_num_blocks`.

### Go API

- `libs/common` has been removed in favor of specific pkgs.
    - `async`
    - `service`
    - `rand`
    - `net`
    - `strings`
    - `cmap`
- removal of `errors` pkg

### RPC Changes

- `/validators` is now paginated (default: 30 vals per page)
- `/block_results` response format updated [see RPC docs for details](https://docs.tendermint.com/master/rpc/#/Info/block_results)
- Event suffix has been removed from the ID in event responses
- IDs are now integers not `json-client-XYZ`

## v0.32.0

This release is compatible with previous blockchains,
however the new ABCI Events mechanism may create some complexity
for nodes wishing to continue operation with v0.32 from a previous version.
There are some minor breaking changes to the RPC.

### Config Changes

If you have `db_backend` set to `leveldb` in your config file, please change it
to `goleveldb` or `cleveldb`.

### RPC Changes

The default listen address for the RPC is now `127.0.0.1`. If you want to expose
it publicly, you have to explicitly configure it. Note exposing the RPC to the
public internet may not be safe - endpoints which return a lot of data may
enable resource exhaustion attacks on your node, causing the process to crash.

Any consumers of `/block_results` need to be mindful of the change in all field
names from CamelCase to Snake case, eg. `results.DeliverTx` is now `results.deliver_tx`.
This is a fix, but it's breaking.

### ABCI Changes

ABCI responses which previously had a `Tags` field now have an `Events` field
instead. The original `Tags` field was simply a list of key-value pairs, where
each key effectively represented some attribute of an event occuring in the
blockchain, like `sender`, `receiver`, or `amount`. However, it was difficult to
represent the occurence of multiple events (for instance, multiple transfers) in a single list.
The new `Events` field contains a list of `Event`, where each `Event` is itself a list
of key-value pairs, allowing for more natural expression of multiple events in
eg. a single DeliverTx or EndBlock. Note each `Event` also includes a `Type`, which is meant to categorize the
event.

For transaction indexing, the index key is
prefixed with the event type: `{eventType}.{attributeKey}`.
If the same event type and attribute key appear multiple times, the values are
appended in a list.

To make queries, include the event type as a prefix. For instance if you
previously queried for `recipient = 'XYZ'`, and after the upgrade you name your event `transfer`,
the new query would be for `transfer.recipient = 'XYZ'`.

Note that transactions indexed on a node before upgrading to v0.32 will still be indexed
using the old scheme. For instance, if a node upgraded at height 100,
transactions before 100 would be queried with `recipient = 'XYZ'` and
transactions after 100 would be queried with `transfer.recipient = 'XYZ'`.
While this presents additional complexity to clients, it avoids the need to
reindex. Of course, you can reset the node and sync from scratch to re-index
entirely using the new scheme.

We illustrate further with a more complete example.

Prior to the update, suppose your `ResponseDeliverTx` look like:

```go
abci.ResponseDeliverTx{
  Tags: []kv.Pair{
    {Key: []byte("sender"), Value: []byte("foo")},
    {Key: []byte("recipient"), Value: []byte("bar")},
    {Key: []byte("amount"), Value: []byte("35")},
  }
}
```

The following queries would match this transaction:

```go
query.MustParse("tm.event = 'Tx' AND sender = 'foo'")
query.MustParse("tm.event = 'Tx' AND recipient = 'bar'")
query.MustParse("tm.event = 'Tx' AND sender = 'foo' AND recipient = 'bar'")
```

Following the upgrade, your `ResponseDeliverTx` would look something like:
the following `Events`:

```go
abci.ResponseDeliverTx{
  Events: []abci.Event{
    {
      Type: "transfer",
      Attributes: kv.Pairs{
        {Key: []byte("sender"), Value: []byte("foo")},
        {Key: []byte("recipient"), Value: []byte("bar")},
        {Key: []byte("amount"), Value: []byte("35")},
      },
    }
}
```

Now the following queries would match this transaction:

```go
query.MustParse("tm.event = 'Tx' AND transfer.sender = 'foo'")
query.MustParse("tm.event = 'Tx' AND transfer.recipient = 'bar'")
query.MustParse("tm.event = 'Tx' AND transfer.sender = 'foo' AND transfer.recipient = 'bar'")
```

For further documentation on `Events`, see the [docs](https://github.com/tendermint/tendermint/blob/60827f75623b92eff132dc0eff5b49d2025c591e/docs/spec/abci/abci.md#events).

### Go Applications

The ABCI Application interface changed slightly so the CheckTx and DeliverTx
methods now take Request structs. The contents of these structs are just the raw
tx bytes, which were previously passed in as the argument.

## v0.31.6

There are no breaking changes in this release except Go API of p2p and
mempool packages. Hovewer, if you're using cleveldb, you'll need to change
the compilation tag:

Use `cleveldb` tag instead of `gcc` to compile Tendermint with CLevelDB or
use `make build_c` / `make install_c` (full instructions can be found at
<https://tendermint.com/docs/introduction/install.html#compile-with-cleveldb-support>)

## v0.31.0

This release contains a breaking change to the behaviour of the pubsub system.
It also contains some minor breaking changes in the Go API and ABCI.
There are no changes to the block or p2p protocols, so v0.31.0 should work fine
with blockchains created from the v0.30 series.

### RPC

The pubsub no longer blocks on publishing. This may cause some WebSocket (WS) clients to stop working as expected.
If your WS client is not consuming events fast enough, Tendermint can terminate the subscription.
In this case, the WS client will receive an error with description:

```json
{
  "jsonrpc": "2.0",
  "id": "{ID}#event",
  "error": {
    "code": -32000,
    "msg": "Server error",
    "data": "subscription was cancelled (reason: client is not pulling messages fast enough)" // or "subscription was cancelled (reason: Tendermint exited)"
  }
}

Additionally, there are now limits on the number of subscribers and
subscriptions that can be active at once. See the new
`rpc.max_subscription_clients` and `rpc.max_subscriptions_per_client` values to
configure this.
```

### Applications

Simple rename of `ConsensusParams.BlockSize` to `ConsensusParams.Block`.

The `ConsensusParams.Block.TimeIotaMS` field was also removed. It's configured
in the ConsensusParsm in genesis.

### Go API

See the [CHANGELOG](CHANGELOG.md). These are relatively straight forward.

## v0.30.0

This release contains a breaking change to both the block and p2p protocols,
however it may be compatible with blockchains created with v0.29.0 depending on
the chain history. If your blockchain has not included any pieces of evidence,
or no piece of evidence has been included in more than one block,
and if your application has never returned multiple updates
for the same validator in a single block, then v0.30.0 will work fine with
blockchains created with v0.29.0.

The p2p protocol change is to fix the proposer selection algorithm again.
Note that proposer selection is purely a p2p concern right
now since the algorithm is only relevant during real time consensus.
This change is thus compatible with v0.29.0, but
all nodes must be upgraded to avoid disagreements on the proposer.

### Applications

Applications must ensure they do not return duplicates in
`ResponseEndBlock.ValidatorUpdates`. A pubkey must only appear once per set of
updates. Duplicates will cause irrecoverable failure. If you have a very good
reason why we shouldn't do this, please open an issue.

## v0.29.0

This release contains some breaking changes to the block and p2p protocols,
and will not be compatible with any previous versions of the software, primarily
due to changes in how various data structures are hashed.

Any implementations of Tendermint blockchain verification, including lite clients,
will need to be updated. For specific details:

- [Merkle tree](https://github.com/tendermint/spec/blob/master/spec/blockchain/encoding.md#merkle-trees)
- [ConsensusParams](https://github.com/tendermint/spec/blob/master/spec/blockchain/state.md#consensusparams)

There was also a small change to field ordering in the vote struct. Any
implementations of an out-of-process validator (like a Key-Management Server)
will need to be updated. For specific details:

- [Vote](https://github.com/tendermint/spec/blob/master/spec/consensus/signing.md#votes)

Finally, the proposer selection algorithm continues to evolve. See the
[work-in-progress
specification](https://github.com/tendermint/tendermint/pull/3140).

For everything else, please see the [CHANGELOG](./CHANGELOG.md#v0.29.0).

## v0.28.0

This release breaks the format for the `priv_validator.json` file
and the protocol used for the external validator process.
It is compatible with v0.27.0 blockchains (neither the BlockProtocol nor the
P2PProtocol have changed).

Please read carefully for details about upgrading.

**Note:** Backup your `config/priv_validator.json`
before proceeding.

### `priv_validator.json`

The `config/priv_validator.json` is now two files:
`config/priv_validator_key.json` and `data/priv_validator_state.json`.
The former contains the key material, the later contains the details on the last
message signed.

When running v0.28.0 for the first time, it will back up any pre-existing
`priv_validator.json` file and proceed to split it into the two new files.
Upgrading should happen automatically without problem.

To upgrade manually, use the provided `privValUpgrade.go` script, with exact paths for the old
`priv_validator.json` and the locations for the two new files. It's recomended
to use the default paths, of `config/priv_validator_key.json` and
`data/priv_validator_state.json`, respectively:

```sh
go run scripts/privValUpgrade.go <old-path> <new-key-path> <new-state-path>
```

### External validator signers

The Unix and TCP implementations of the remote signing validator
have been consolidated into a single implementation.
Thus in both cases, the external process is expected to dial
Tendermint. This is different from how Unix sockets used to work, where
Tendermint dialed the external process.

The `PubKeyMsg` was also split into separate `Request` and `Response` types
for consistency with other messages.

Note that the TCP sockets don't yet use a persistent key,
so while they're encrypted, they can't yet be properly authenticated.
See [#3105](https://github.com/tendermint/tendermint/issues/3105).
Note the Unix socket has neither encryption nor authentication, but will
add a shared-secret in [#3099](https://github.com/tendermint/tendermint/issues/3099).

## v0.27.0

This release contains some breaking changes to the block and p2p protocols,
but does not change any core data structures, so it should be compatible with
existing blockchains from the v0.26 series that only used Ed25519 validator keys.
Blockchains using Secp256k1 for validators will not be compatible. This is due
to the fact that we now enforce which key types validators can use as a
consensus param. The default is Ed25519, and Secp256k1 must be activated
explicitly.

It is recommended to upgrade all nodes at once to avoid incompatibilities at the
peer layer - namely, the heartbeat consensus message has been removed (only
relevant if `create_empty_blocks=false` or `create_empty_blocks_interval > 0`),
and the proposer selection algorithm has changed. Since proposer information is
never included in the blockchain, this change only affects the peer layer.

### Go API Changes

#### libs/db

The ReverseIterator API has changed the meaning of `start` and `end`.
Before, iteration was from `start` to `end`, where
`start > end`. Now, iteration is from `end` to `start`, where `start < end`.
The iterator also excludes `end`. This change allows a simplified and more
intuitive logic, aligning the semantic meaning of `start` and `end` in the
`Iterator` and `ReverseIterator`.

### Applications

This release enforces a new consensus parameter, the
ValidatorParams.PubKeyTypes. Applications must ensure that they only return
validator updates with the allowed PubKeyTypes. If a validator update includes a
pubkey type that is not included in the ConsensusParams.Validator.PubKeyTypes,
block execution will fail and the consensus will halt.

By default, only Ed25519 pubkeys may be used for validators. Enabling
Secp256k1 requires explicit modification of the ConsensusParams.
Please update your application accordingly (ie. restrict validators to only be
able to use Ed25519 keys, or explicitly add additional key types to the genesis
file).

## v0.26.0

This release contains a lot of changes to core data types and protocols. It is not
compatible to the old versions and there is no straight forward way to update
old data to be compatible with the new version.

To reset the state do:

```sh
tendermint unsafe_reset_all
```

Here we summarize some other notable changes to be mindful of.

### Config Changes

All timeouts must be changed from integers to strings with their duration, for
instance `flush_throttle_timeout = 100` would be changed to
`flush_throttle_timeout = "100ms"` and `timeout_propose = 3000` would be changed
to `timeout_propose = "3s"`.

### RPC Changes

The default behaviour of `/abci_query` has been changed to not return a proof,
and the name of the parameter that controls this has been changed from `trusted`
to `prove`. To get proofs with your queries, ensure you set `prove=true`.

Various version fields like `amino_version`, `p2p_version`, `consensus_version`,
and `rpc_version` have been removed from the `node_info.other` and are
consolidated under the tendermint semantic version (ie. `node_info.version`) and
the new `block` and `p2p` protocol versions under `node_info.protocol_version`.

### ABCI Changes

Field numbers were bumped in the `Header` and `ResponseInfo` messages to make
room for new `version` fields. It should be straight forward to recompile the
protobuf file for these changes.

#### Proofs

The `ResponseQuery.Proof` field is now structured as a `[]ProofOp` to support
generalized Merkle tree constructions where the leaves of one Merkle tree are
the root of another. If you don't need this functionality, and you used to
return `<proof bytes>` here, you should instead return a single `ProofOp` with
just the `Data` field set:

```go
[]ProofOp{
    ProofOp{
        Data: <proof bytes>,
    }
}
```

For more information, see:

- [ADR-026](https://github.com/tendermint/tendermint/blob/30519e8361c19f4bf320ef4d26288ebc621ad725/docs/architecture/adr-026-general-merkle-proof.md)
- [Relevant ABCI
  documentation](https://github.com/tendermint/tendermint/blob/30519e8361c19f4bf320ef4d26288ebc621ad725/docs/spec/abci/apps.md#query-proofs)
- [Description of
  keys](https://github.com/tendermint/tendermint/blob/30519e8361c19f4bf320ef4d26288ebc621ad725/crypto/merkle/proof_key_path.go#L14)

### Go API Changes

#### crypto/merkle

The `merkle.Hasher` interface was removed. Functions which used to take `Hasher`
now simply take `[]byte`. This means that any objects being Merklized should be
serialized before they are passed in.

#### node

The `node.RunForever` function was removed. Signal handling and running forever
should instead be explicitly configured by the caller. See how we do it
[here](https://github.com/tendermint/tendermint/blob/30519e8361c19f4bf320ef4d26288ebc621ad725/cmd/tendermint/commands/run_node.go#L60).

### Other

All hashes, except for public key addresses, are now 32-bytes.

## v0.25.0

This release has minimal impact.

If you use GasWanted in ABCI and want to enforce it, set the MaxGas in the genesis file (default is no max).

## v0.24.0

New 0.24.0 release contains a lot of changes to the state and types. It's not
compatible to the old versions and there is no straight forward way to update
old data to be compatible with the new version.

To reset the state do:

```sh
tendermint unsafe_reset_all
```

Here we summarize some other notable changes to be mindful of.

### Config changes

`p2p.max_num_peers` was removed in favor of `p2p.max_num_inbound_peers` and
`p2p.max_num_outbound_peers`.

```toml
# Maximum number of inbound peers
max_num_inbound_peers = 40

# Maximum number of outbound peers to connect to, excluding persistent peers
max_num_outbound_peers = 10
```

As you can see, the default ratio of inbound/outbound peers is 4/1. The reason
is we want it to be easier for new nodes to connect to the network. You can
tweak these parameters to alter the network topology.

### RPC Changes

The result of `/commit` used to contain `header` and `commit` fields at the top level. These are now contained under the `signed_header` field.

### ABCI Changes

The header has been upgraded and contains new fields, but none of the existing
fields were changed, except their order.

The `Validator` type was split into two, one containing an `Address` and one
containing a `PubKey`. When processing `RequestBeginBlock`, use the `Validator`
type, which contains just the `Address`. When returning `ResponseEndBlock`, use
the `ValidatorUpdate` type, which contains just the `PubKey`.

### Validator Set Updates

Validator set updates returned in ResponseEndBlock for height `H` used to take
effect immediately at height `H+1`. Now they will be delayed one block, to take
effect at height `H+2`. Note this means that the change will be seen by the ABCI
app in the `RequestBeginBlock.LastCommitInfo` at block `H+3`. Apps were already
required to maintain a map from validator addresses to pubkeys since v0.23 (when
pubkeys were removed from RequestBeginBlock), but now they may need to track
multiple validator sets at once to accomodate this delay.

### Block Size

The `ConsensusParams.BlockSize.MaxTxs` was removed in favour of
`ConsensusParams.BlockSize.MaxBytes`, which is now enforced. This means blocks
are limitted only by byte-size, not by number of transactions.