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README: Document usage of RPC node #2313

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README: Document usage of RPC node #2313

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33 changes: 33 additions & 0 deletions README.md
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Expand Up @@ -37,6 +37,39 @@ and treat as best.
A Polkadot [collator](https://wiki.polkadot.network/docs/en/learn-collator) for the parachain is
implemented by the `polkadot-parachain` binary (previously called `polkadot-collator`).

### Relaychain Interaction
To operate a parachain node a connection to the corresponding relaychain is necessary. This can be achieved in one of two ways:
1. Run a full relaychain node within the parachain node (default)
2. Connect to an external relaychain node via websocket RPC

#### In-process Relaychain Node
If an external relaychain node is not specified (default behavior) then a full relaychain node will be spawned within the same process.

This node has all of the typical components of a normal Polkadot node, and will have to fully sync with the relaychain to work.
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This node has all of the typical components of a normal Polkadot node, and will have to fully sync with the relaychain to work.
In-process node has all of the typical components of a normal Polkadot node, and will have to fully sync with the relaychain to work.


##### Example command
```shell=
# In-process node with this chainspec is spawned
# |
# |-----------------------|
polkadot-parachain --chain parachain-chainspec.json --tmp -- --chain relaychain-chainspec.json
```

#### External Relaychain Node
An external relaychain node can be connected via websocket RPC by using the `--relay-chain-rpc-urls` command line argument. This option accepts one or more space-separated websocket URLs to a full relay chain node. By default only the first URL will be used, with the rest acting as a backup in case the connection to the first node will be lost.
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@davxy davxy Mar 14, 2023 •
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mmm... Very long lines 😃

Can we stick to max 100 cols per line?
Long lines are best fitted for reading on github or on a proper markdown renderer.
I usually brutally read md files on an editor in the terminal


Parachain nodes using this feature won't have to fully sync with the relay chain to work, so in general they will use significantly less system resources.

**Note:** At this time any parachain nodes using this feature will still spawn a very cut down relaychain node in-process, hence even though they lack the majority of normal Polkadot subsystems they will still need to be able to directly connect to the relay chain network.
##### Example command
```shell=
# Perform runtime calls and fetch Still required since we connect
# data via RPC from here. Backup node to the relaychain network
# | | |
# |---------------------------------| |----------------------------------------| |-----------------------|
polkadot-parachain --chain parachain-chainspec.json --tmp --relay-chain-rpc-urls "ws://relaychain-rpc-endpoint:9944" "ws://relaychain-rpc-endpoint-backup:9944" -- --chain relaychain-chainspec.json
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this is unreadable in raw format.

Maybe we should write long lines containing shell commands like:

> polkadot-parachain \
       --chain parachain-chainspec.json \
       --tmp\
        --relay-chain-rpc-urls "ws://relaychain-rpc-endpoint:9944" "ws://relaychain-rpc-endpoint-backup:9944" \
        -- \
        --chain relaychain-chainspec.json

Then the explanation of the single lines may optionally follow:

 --relay-chain-rpc-urls <primary-endpoint> <backup-endpoint>

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I was a bit fast on this one, I will add your suggestions later 👍

```

## Installation and Setup
Before building Cumulus SDK based nodes / runtimes prepare your environment by following Substrate [installation instructions](https://docs.substrate.io/main-docs/install/).

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