tendermint/docs/tendermint-core/light-client.md

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Light Client

Light clients are an important part of the complete blockchain system for most applications. Tendermint provides unique speed and security properties for light client applications.

See our light package.

Overview

The objective of the light client protocol is to get a commit for a recent block hash where the commit includes a majority of signatures from the last known validator set. From there, all the application state is verifiable with merkle proofs.

Properties

• You get the full collateralized security benefits of Tendermint; no need to wait for confirmations.
• You get the full speed benefits of Tendermint; transactions commit instantly.
• You can get the most recent version of the application state non-interactively (without committing anything to the blockchain). For example, this means that you can get the most recent value of a name from the name-registry without worrying about fork censorship attacks, without posting a commit and waiting for confirmations. It's fast, secure, and free!

Where to obtain trusted height & hash

Trust Options

One way to obtain semi-trusted hash & height is to query multiple full nodes and compare their hashes:

$ curl -s https://233.123.0.140:26657:26657/commit | jq "{height: .result.signed_header.header.height, hash: .result.signed_header.commit.block_id.hash}"
{
  "height": "273",
  "hash": "188F4F36CBCD2C91B57509BBF231C777E79B52EE3E0D90D06B1A25EB16E6E23D"
}

Running a light client as an HTTP proxy server

Tendermint comes with a built-in tendermint light command, which can be used to run a light client proxy server, verifying Tendermint RPC. All calls that can be tracked back to a block header by a proof will be verified before passing them back to the caller. Other than that, it will present the same interface as a full Tendermint node.

You can start the light client proxy server by running tendermint light <chainID>, with a variety of flags to specify the primary node, the witness nodes (which cross-check the information provided by the primary), the hash and height of the trusted header, and more.

For example:

$ tendermint light supernova -p tcp://233.123.0.140:26657 \
  -w tcp://179.63.29.15:26657,tcp://144.165.223.135:26657 \
  --height=10 --hash=37E9A6DD3FA25E83B22C18835401E8E56088D0D7ABC6FD99FCDC920DD76C1C57

For additional options, run tendermint light --help.