# Retrieve Messages Using Waku Store

DApps running on a phone or in a browser are often offline:
The browser could be closed or mobile app in the background.

[Waku Relay](https://rfc.vac.dev/spec/18/) is a gossip protocol.
As a user, it means that your peers forward you messages they just received.
If you cannot be reached by your peers, then messages are not relayed;
relay peers do **not** save messages for later.

However, [Waku Store](https://rfc.vac.dev/spec/13/) peers do save messages they relay,
allowing you to retrieve them at a later time.
The Waku Store protocol is best-effort and does not guarantee data availability.
Waku Relay should still be preferred when online;
Waku Store can be used after resuming connectivity:
For example, when the dApp starts.

In this guide, we'll review how you can use Waku Store to retrieve messages.


Before starting, you need to choose a _Content Topic_ for your dApp.
Check out the [how to choose a content topic guide](choose-content-topic.md) to learn more about content topics.

For this guide, we are using a single content topic: `/store-guide/1/news/proto`.

# Installation

You can install [js-waku](https://npmjs.com/package/js-waku) using your favorite package manager:

```shell
npm install js-waku
```

# Create Waku Instance

In order to interact with the Waku network, you first need a Waku instance:

```js
import { Waku } from 'js-waku';

const wakuNode = await Waku.create();
```

# Connect to Other Peers

The Waku instance needs to connect to other peers to communicate with the network.
You are free to choose other methods to bootstrap and DappConnect will ship with new bootstrap mechanisms in the future.

For now, the easiest way is to connect to Waku bootstrap nodes:

```js
import { getBootstrapNodes } from 'js-waku';

try {
  const nodes = await getBootstrapNodes();
  await Promise.all(nodes.map((addr) => waku.dial(addr)));
} catch (e) {
  console.error('Failed to bootstrap to Waku network');
} 
```

# Use Protobuf

Waku v2 protocols use [protobuf](https://developers.google.com/protocol-buffers/) [by default](https://rfc.vac.dev/spec/10/).

Let's review how you can use protobuf to send structured data.

First, define a data structure.
For this guide, we will use a simple news article that contains a date of publication, title and body:

```js
{
  date: Date;
  title: string;
  body: string;
}
```

To encode and decode protobuf payloads, you can use the [protons](https://www.npmjs.com/package/protons) package.

## Install Protobuf Library

First, install protons:

```shell
npm install protons
```

## Protobuf Definition

Then specify the data structure:

```js
import protons from 'protons';

const proto = protons(`
message ArticleMessage {
  uint64 date = 1;
  string title = 2;
  string body = 3;
}
`);
```

You can learn about protobuf message definitions here:
[Protocol Buffers Language Guide](https://developers.google.com/protocol-buffers/docs/proto).

## Decode Messages

To decode the messages retrieved from a Waku Store node,
you need to extract the protobuf payload and decode it using `protons`.

```js
const decodeWakuMessage = (wakuMessage) => {
  // No need to attempt to decode a message if the payload is absent
  if (!wakuMessage.payload) return;

  const { date, title, body } = proto.SimpleChatMessage.decode(
    wakuMessage.payload
  );

  // In protobuf, fields are optional so best to check
  if (!date || !title || !body) return;

  const publishDate = new Date();
  publishDate.setTime(date);

  return { publishDate, title, body };
};
```

## Retrieve messages

You now have all the building blocks to retrieve and decode messages for a store node.

Retrieve messages from a store node:

```js
const ContentTopic = '/store-guide/1/news/proto';

waku.store
  .queryHistory([ContentTopic])
  .catch((e) => {
    // Be sure to catch any potential error
    console.log('Failed to retrieve messages', e);
  })
  .then((retrievedMessages) => {
    const articles = retrievedMessages
      .map(decodeWakuMessage) // Decode messages
      .filter(Boolean); // Filter out undefined values

    console.log(`${articles.length} articles have been retrieved`);
  });
```

Note that `WakuStore.queryHistory` select an available store node for you.
However, it can only select a connected node, which is why the bootstrapping is necessary.
It will throw an error if no store node is available.

## Wait to be connected

Depending on your dApp design, you may want to wait for a store node to be available first.
In this case, you can listen for the [PeerStore's change protocol event](https://github.com/libp2p/js-libp2p/blob/master/doc/API.md#known-protocols-for-a-peer-change)
to know whether any of your connected peers is a store peer:

```js
import { StoreCodec } from 'js-waku';

// Or using a callback
waku.libp2p.peerStore.on('change:protocols', ({ peerId, protocols }) => {
  if (protocols.includes(StoreCodec)) {
    // A Store node is available!
  }
});
```