Query is a generic message type with lots of flexibility to enable diverse sets
of queries from applications. Tendermint has 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.
That said, Tendermint makes a number of queries to support some optional
features. These are:
### 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.
## 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.
```
storeBlockHeight = 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
completely Commit
```
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 simeple 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.
