user2897: I have been researching on how to start a Hydra node in devnet using Docker and I found this link with these two simple commands:
docker pull ghcr.io/input-output-hk/hydra-node:latest
docker run --rm ghcr.io/input-output-hk/hydra-node --help

The problem is that, when I execute this, I get:
hydra-node - A prototype of Hydra Head protocol

Usage: hydra-node [-q|--quiet] [-n|--node-id INTEGER] [-h|--host IP] 
                  [-p|--port PORT] [-P|--peer ARG] [--api-host IP] 
                  [--api-port PORT] [--monitoring-port PORT] 
                  [--hydra-signing-key FILE] [--hydra-verification-key FILE] 
                  [--network-id INTEGER] [--node-socket FILE] 
                  [--cardano-signing-key FILE] [--cardano-verification-key FILE]
                  [--start-chain-from SLOT.HEADER_HASH] [--ledger-genesis FILE] 
                  [--ledger-protocol-parameters FILE] [--version] 
                  [--script-info]

  Starts a Hydra Node

Available options:
  -q,--quiet               Turns off any logging
  -n,--node-id INTEGER     Sets this node's id
  -h,--host IP             The address this node listens on for Hydra network
                           peers connection (default: 127.0.0.1)
  -p,--port PORT           The port this node listens on for Hydra network peers
                           connection (default: 5001)
  -P,--peer ARG            A peer address in the form <host>:<port>, where
                           <host> can be an IP address, or a host name
  --api-host IP            The address this node listens on for client API
                           connections (default: 127.0.0.1)
  --api-port PORT          The port this node listens on for client API
                           connections (default: 4001)
  --monitoring-port PORT   The port this node listens on for monitoring and
                           metrics. If left empty, monitoring server is not
                           started
  --hydra-signing-key FILE Our Hydra multisig signing key.
  --hydra-verification-key FILE
                           Other party multisig verification key.
  --network-id INTEGER     A test network with the given network magic.
  --node-socket FILE       Local (Unix) socket path to connect to cardano node.
  --cardano-signing-key FILE
                           Signing key for the internal wallet use for Chain
                           interactions.
  --cardano-verification-key FILE
                           Cardano verification key of other Hydra participant's
                           wallet.
  --start-chain-from SLOT.HEADER_HASH
                           The point at which to start on-chain component.
                           Defaults to chain tip at startup time.
  --ledger-genesis FILE    Path to a Shelley-compatible genesis JSON file.
  --ledger-protocol-parameters FILE
                           Path to a JSON file describing protocol parameters
                           (same format as returned from 'cardano-cli query
                           protocol-parameters')
  -h,--help                Show this help text
  --version                Show version
  --script-info            Dump script info as JSON

This doesn't seem to start anything.
What is the correct way to start hydra-node?

user6632: You are running with the --help option: 'docker run --rm ghcr.io/input-output-hk/hydra-node --help'
This will only ever show you the help information.

user2897: This is how you run hydra-node in Devnet. You need to have Docker installed.
Keep in mind that, at the moment, this is just a POC:

git clone https://github.com/input-output-hk/hydra-poc
cd hydra-poc/demo/
./prepare-devnet.sh
./run-docker.sh

This will start 1 instance of cardano-node and 3 instances of hydra-node.
If you open a new terminal, you can check they are working correctly with this command (you need to be in the hydra-poc/demo directory):
docker-compose ps
Once this is done, you can check the initial state of the network using this command:
docker-compose exec cardano-node cardano-cli query utxo --testnet-magic 42 --whole-utxo

You can also send some ada to Alice, Bob and Carol addresses using:
./seed-devnet.sh
Now, if you check the status of the network again, you'll see something like this:
                           TxHash                                 TxIx        Amount
--------------------------------------------------------------------------------------
0ad134cc87cdf66a6863464b4393501eda7632f7c268b068c48b2aaf84f20e51     1        250000000 lovelace + TxOutDatumNone
2e80d483600acf194661e89bb08aee0cbb2844cf237e43d5e949d11038db9a96     1        100000000 lovelace + TxOutDatumHash ScriptDataInAlonzoEra "a654fb60d21c1fed48db2c320aa6df9737ec0204c0ba53b9b94a09fb40e757f3"
3eeea5c2376b033d5bdeab6fe551950883b04c08a37848c6d648ea03476dce83     1        1000000000 lovelace + TxOutDatumNone
648d71c1cbd835f6e1966c06ba0ad4347c3b399ef00576e59aee18b9e1ec9489     0        897949001186 lovelace + TxOutDatumNone
648d71c1cbd835f6e1966c06ba0ad4347c3b399ef00576e59aee18b9e1ec9489     1        100000000 lovelace + TxOutDatumHash ScriptDataInAlonzoEra "a654fb60d21c1fed48db2c320aa6df9737ec0204c0ba53b9b94a09fb40e757f3"
6db235b8759454654d19baf3ef601a2cb0e4ea3ebdc5e9db466dd07bccf53c7d     1        500000000 lovelace + TxOutDatumNone
94f16fc2a16b7ffa96c7929cbfc1e2f05424b6508612d4513fa23eaf458dc14b     1        100000000 lovelace + TxOutDatumHash ScriptDataInAlonzoEra "a654fb60d21c1fed48db2c320aa6df9737ec0204c0ba53b9b94a09fb40e757f3"

If at any point you'll like to restart the docker containers, I suggest you run the following:
docker-compose stop
docker-compose rm -f

Then you'll be able to do ./run-docker.sh again.
There's also a client which you can use to play a little with the Hydra heads:
docker-compose --profile tui run hydra-tui-1

You can change between hydra-tui-1, hydra-tui-2 and hydra-tui-3, depending on which hydra-node you want to connect with. Only when you close a head you'll see the changes committed to the main chain (cardano-node).

user2901: Here you can find the official documentation on Hydra.
There is also a demo section where it is explained how to use it

2897: <p>I have been researching on how to start a Hydra node in devnet using Docker and I found <a href="https://hydra.family/head-protocol/docs/getting-started/installation" rel="nofollow noreferrer">this link</a> with these two simple commands:</p>
<pre><code>docker pull ghcr.io/input-output-hk/hydra-node:latest
docker run --rm ghcr.io/input-output-hk/hydra-node --help
</code></pre>
<p>The problem is that, when I execute this, I get:</p>
<pre><code>hydra-node - A prototype of Hydra Head protocol

Usage: hydra-node [-q|--quiet] [-n|--node-id INTEGER] [-h|--host IP] 
                  [-p|--port PORT] [-P|--peer ARG] [--api-host IP] 
                  [--api-port PORT] [--monitoring-port PORT] 
                  [--hydra-signing-key FILE] [--hydra-verification-key FILE] 
                  [--network-id INTEGER] [--node-socket FILE] 
                  [--cardano-signing-key FILE] [--cardano-verification-key FILE]
                  [--start-chain-from SLOT.HEADER_HASH] [--ledger-genesis FILE] 
                  [--ledger-protocol-parameters FILE] [--version] 
                  [--script-info]

  Starts a Hydra Node

Available options:
  -q,--quiet               Turns off any logging
  -n,--node-id INTEGER     Sets this node's id
  -h,--host IP             The address this node listens on for Hydra network
                           peers connection (default: 127.0.0.1)
  -p,--port PORT           The port this node listens on for Hydra network peers
                           connection (default: 5001)
  -P,--peer ARG            A peer address in the form &lt;host&gt;:&lt;port&gt;, where
                           &lt;host&gt; can be an IP address, or a host name
  --api-host IP            The address this node listens on for client API
                           connections (default: 127.0.0.1)
  --api-port PORT          The port this node listens on for client API
                           connections (default: 4001)
  --monitoring-port PORT   The port this node listens on for monitoring and
                           metrics. If left empty, monitoring server is not
                           started
  --hydra-signing-key FILE Our Hydra multisig signing key.
  --hydra-verification-key FILE
                           Other party multisig verification key.
  --network-id INTEGER     A test network with the given network magic.
  --node-socket FILE       Local (Unix) socket path to connect to cardano node.
  --cardano-signing-key FILE
                           Signing key for the internal wallet use for Chain
                           interactions.
  --cardano-verification-key FILE
                           Cardano verification key of other Hydra participant's
                           wallet.
  --start-chain-from SLOT.HEADER_HASH
                           The point at which to start on-chain component.
                           Defaults to chain tip at startup time.
  --ledger-genesis FILE    Path to a Shelley-compatible genesis JSON file.
  --ledger-protocol-parameters FILE
                           Path to a JSON file describing protocol parameters
                           (same format as returned from 'cardano-cli query
                           protocol-parameters')
  -h,--help                Show this help text
  --version                Show version
  --script-info            Dump script info as JSON
</code></pre>
<p>This doesn't seem to start anything.</p>
<p>What is the correct way to start <code>hydra-node</code>?</p>
 6632: <p>You are running with the <code>--help</code> option: 'docker run --rm ghcr.io/input-output-hk/hydra-node <strong>--help</strong>'</p>
<p>This will only ever show you the help information.</p>
 2897: <p>This is how you run <code>hydra-node</code> in Devnet. You need to have <a href="https://www.docker.com/get-started/" rel="nofollow noreferrer">Docker</a> installed.</p>
<p>Keep in mind that, at the moment, this is just a POC:</p>
<ol>
<li><code>git clone https://github.com/input-output-hk/hydra-poc</code></li>
<li><code>cd hydra-poc/demo/</code></li>
<li><code>./prepare-devnet.sh</code></li>
<li><code>./run-docker.sh</code></li>
</ol>
<p>This will start 1 instance of <code>cardano-node</code> and 3 instances of <code>hydra-node</code>.</p>
<p>If you open a new terminal, you can check they are working correctly with this command (you need to be in the <code>hydra-poc/demo</code> directory):</p>
<p><code>docker-compose ps</code></p>
<p>Once this is done, you can check the initial state of the network using this command:</p>
<pre><code>docker-compose exec cardano-node cardano-cli query utxo --testnet-magic 42 --whole-utxo
</code></pre>
<p>You can also send some ada to Alice, Bob and Carol addresses using:</p>
<p><code>./seed-devnet.sh</code></p>
<p>Now, if you check the status of the network again, you'll see something like this:</p>
<pre><code>                           TxHash                                 TxIx        Amount
--------------------------------------------------------------------------------------
0ad134cc87cdf66a6863464b4393501eda7632f7c268b068c48b2aaf84f20e51     1        250000000 lovelace + TxOutDatumNone
2e80d483600acf194661e89bb08aee0cbb2844cf237e43d5e949d11038db9a96     1        100000000 lovelace + TxOutDatumHash ScriptDataInAlonzoEra &quot;a654fb60d21c1fed48db2c320aa6df9737ec0204c0ba53b9b94a09fb40e757f3&quot;
3eeea5c2376b033d5bdeab6fe551950883b04c08a37848c6d648ea03476dce83     1        1000000000 lovelace + TxOutDatumNone
648d71c1cbd835f6e1966c06ba0ad4347c3b399ef00576e59aee18b9e1ec9489     0        897949001186 lovelace + TxOutDatumNone
648d71c1cbd835f6e1966c06ba0ad4347c3b399ef00576e59aee18b9e1ec9489     1        100000000 lovelace + TxOutDatumHash ScriptDataInAlonzoEra &quot;a654fb60d21c1fed48db2c320aa6df9737ec0204c0ba53b9b94a09fb40e757f3&quot;
6db235b8759454654d19baf3ef601a2cb0e4ea3ebdc5e9db466dd07bccf53c7d     1        500000000 lovelace + TxOutDatumNone
94f16fc2a16b7ffa96c7929cbfc1e2f05424b6508612d4513fa23eaf458dc14b     1        100000000 lovelace + TxOutDatumHash ScriptDataInAlonzoEra &quot;a654fb60d21c1fed48db2c320aa6df9737ec0204c0ba53b9b94a09fb40e757f3&quot;
</code></pre>
<p>If at any point you'll like to restart the docker containers, I suggest you run the following:</p>
<pre><code>docker-compose stop
docker-compose rm -f
</code></pre>
<p>Then you'll be able to do <code>./run-docker.sh</code> again.</p>
<p>There's also a client which you can use to play a little with the Hydra heads:</p>
<pre><code>docker-compose --profile tui run hydra-tui-1
</code></pre>
<p>You can change between <code>hydra-tui-1</code>, <code>hydra-tui-2</code> and <code>hydra-tui-3</code>, depending on which <code>hydra-node</code> you want to connect with. Only when you close a head you'll see the changes committed to the main chain (<code>cardano-node</code>).</p>
 2901: <p><a href="https://hydra.family/head-protocol/" rel="nofollow noreferrer">Here</a> you can find the official documentation on Hydra.</p>
<p>There is also a demo section where it is explained how to use it</p>
