# Light mode¶

The tezos-client described here forwards all RPCs to a node. This page describes the light mode, a mode where the client performs protocol RPCs locally; like the proxy mode. However, contrary to the proxy mode, the light mode provides a high-level of security. For that it obtains data from multiple (hopefully unrelated) endpoints and make sure all endpoints send the same data, by using Merkle proofs. Such proofs make very hard for unrelated endpoints to craft fake data.

This mode is akin to a light client or thin client in Bitcoin terms.

While the existing implementation of the light mode is entirely functional, it still has room for improvement, in particular communications over the network can be reduced. Users are encouraged to manifest themselves, so that enhancements to the light mode are made high priority: please submit issues here on GitLab.

## Executing commands in light mode¶

The CLI interface of the client in light mode (the light client in short) is the same as the default client. To turn light mode ON, you must pass two arguments to tezos-client:

• --mode light, and

• --sources sources.json.

The sources.json file contains:

• The list of endpoints to contact for retrieving data, as a list of URIs in the uris field. Because --sources must be the sole source of truth for incoming data, it supersedes --endpoint. This is enforced by checking that the URI passed to --endpoint also appears in the uris field of the file passed to --sources. Therefore, it is recommended to use the first member of the uris field as the value of --endpoint.

• An optional min_agreement field, which must be a float from 0.0 (excluded) to 1.0 (included). This field specifies the ratio of endpoints that must agree for data to be accepted. The default value is 1.0, which means that all endpoints must agree for data to be considered valid (no rogue endpoint is tolerated).

Here is an example valid --sources file:

{ "min_agreement": 1.0,
"uris": [
"http://localhost:19733",
"http://localhost:19735"
]
}


Because computations done locally are protocol dependent, the light mode does not support all protocols. However, at any given time, it should support the protocol being developed (alpha) and the three most recent protocols.

If --protocol is omitted when calling the light client, it tries to match the node’s protocol. On the one hand, this is handy when testing. On the other hand, in a production environment, it is recommended to specify --protocol if the protocol is known, to avoid an extra RPC at every call tezos-client --mode light ....

## Examples with the sandbox¶

In this section, we show examples of usage of the light mode when using the sandboxed node. For convenience we repeat instructions for the sandboxed mode here, but refer the reader to the sandboxed mode page for further details. In a terminal, start a sandboxed node:

$./src/bin_node/tezos-sandboxed-node.sh 1 --connections 1 # This node listens to p2p events on localhost:19731  Leave that terminal running. In a second terminal, start another node: $ ./src/bin_node/tezos-sandboxed-node.sh 2 --connections 1
# This node listens to p2p events on localhost:19732


Leave that terminal running. In a third terminal, prepare the appropriate environment for using the light client (from now on, all commands happen in this terminal):

$eval ./src/bin_client/tezos-init-sandboxed-client.sh 1  Then upgrade the node to protocol alpha: $ tezos-activate-alpha  # Triggers output in terminal of first node
$tezos-client bake for bootstrap1 # Triggers output in terminal of first node  Now, connect the two nodes together: $ tezos-admin-client --endpoint localhost:19731 connect address 127.0.0.1:19732
# This triggers many lines of output in the second node's terminal,
# as it catches up with the first node


To avoid warnings being printed in upcoming commands (optional):

$export TEZOS_CLIENT_UNSAFE_DISABLE_DISCLAIMER=y  The last step before being able to use the light client is to prepare the JSON file to pass to --sources. In our scenario, this file specifies the two endpoints to use: $ echo '{ "uris": [ "http://localhost:18731", "http://localhost:18732" ] }' > sources.json


You’re now ready to use the light client. For example, bake a block:

$tezos-client --mode light --sources sources.json bake for bootstrap1 protocol of light mode unspecified, using the node's protocol: ProtoALphaALphaALphaALphaALphaALphaALphaALphaDdp3zK Apr 8 16:42:24.202 - alpha.baking.forge: found 0 valid operations (0 refused) for timestamp 2021-04-08T14:42:24.000-00:00 (fitness 01::0000000000000004) Injected block BMAHozsNCos2  Well that doesn’t seem very different from what the default client would return. Indeed, it’s the same; that was the point! To see what the light client is doing differently, you may use the environment variable TEZOS_LOG. Set it as follows: $ export TEZOS_LOG="light_mode->debug"


Variable light_mode shows how the light mode is obtaining data from the different endpoints.

For convenience, let’s define an alias before continuing, to save keystrokes and the protocol of light mode unspecified warning:

$alias light-client="tezos-client --mode light --protocol ProtoALphaALphaALphaALphaALphaALphaALphaALphaDdp3zK --sources sources.json"  And then bake a new block: $ light-client bake for bootstrap1
Apr  8 16:49:28.172 - light_mode: light mode's core created for chain main and block head
Apr  8 16:49:28.173 - light_mode: API call: do_rpc v1
Apr  8 16:49:28.175 - light_mode: integrated data for key v1 from one endpoint, about to validate from 1 other
Apr  8 16:49:28.175 - light_mode:   endpoints
Apr  8 16:49:28.177 - light_mode: API call: get v1;constants
Apr  8 16:49:28.177 - light_mode: API call: get v1;first_level
Apr  8 16:49:28.177 - light_mode: API call: do_rpc pending_migration_balance_updates
Apr  8 16:49:28.179 - light_mode: integrated data for key pending_migration_balance_updates from one endpoint,
Apr  8 16:49:28.179 - light_mode:   about to validate from 1 other endpoints
Apr  8 16:49:28.180 - light_mode: API call: get pending_migration_balance_updates
...
... A lot of output prefixed with light_mode: ...
...
Injected block BMdbKufTymQJ


Here’s the meaning of these lines:

• Line light mode's core created indicates that the light mode was initialized. It should be printed once per block being inspected.

• Line API call: do_rpc v1 indicates that the light mode needs the data associated to the low-level storage’s key v1

• Line integrated data for key v1 ... indicates that the light mode obtained data for v1 from a single endpoint and that it is about to fetch Merkle proofs for this key from other endpoints.

• Lines API call: get ... indicate that tezos-client is requesting data from the light mode’s cache. In this snippet, after the light mode gathered data for key v1; the client is requesting data for the children keys v1;constants and v1;first_level (the ; indicates nesting). This example shows how the light mode sometimes batches requests, to avoid querying many keys in a row. Here it did a single request for v1 instead of doing one request for v1;constants and a second one for v1;first_level.

To see that computations are done locally by the light client, we refer to the proxy mode’s documentation. Debug variables of the proxy mode apply to the light mode, as internally, the light mode is a more complex instance of the proxy mode.

## How to deploy to relieve nodes from some RPCs¶

Again, we refer to the corresponding section in the page of the proxy mode proxy mode. The exact same recommendations apply for the light mode.