go-ethereum/docs/swarm-guide/contents/usage/feed.rst

Feeds 
========================

.. note::
  Feeds, previously known as *Mutable Resource Updates*, is an experimental feature, available since Swarm POC3. It is under active development, so expect things to change.

Since Swarm hashes are content addressed, changes to data will constantly result in changing hashes. Swarm Feeds provide a way to easily overcome this problem and provide a single, persistent, identifier to follow sequential data.

The usual way of keeping the same pointer to changing data is using the Ethereum Name Service (ENS). However, since ENS is an on-chain feature, it might not be suitable for each use case since:

1. Every update to an ENS resolver will cost gas to execute
2. It is not be possible to change the data faster than the rate that new blocks are mined
3. ENS resolution requires your node to be synced to the blockchain


Swarm Feeds provide a way to have a persistent identifier for changing data without having to use ENS. It is named Feeds for its similarity with a news feed.

If you are using *Feeds* in conjunction with an ENS resolver contract, only one initial transaction to register the "Feed manifest address" will be necessary. This key will resolve to the latest version of the Feed (updating the Feed will not change the key).

You can think of a Feed as a user's Twitter account, where he/she posts updates about a particular Topic. In fact, the Feed object is simply defined as:

.. code-block:: go

  type Feed struct {
    Topic Topic
    User  common.Address
  }

That is, a specific user posting updates about a specific Topic.

Users can post to any topic. If you know the user's address and agree on a particular Topic, you can then effectively "follow" that user's Feed.

.. important::
  How you build the Topic is entirely up to your application. You could calculate a hash of something and use that, the recommendation
  is that it should be easy to derive out of information that is accesible to other users.
  
  For convenience, ``feed.NewTopic()`` provides a way to "merge" a byte array with a string in order to build a Feed Topic out of both.
  This is used at the API level to create the illusion of subtopics. This way of building topics allows using a random byte array (for example the hash of a photo)
  and merge it with a human-readable string such as "comments" in order to create a Topic that could represent the comments about that particular photo.
  This way, when you see a picture in a website you could immediately build a Topic out of it and see if some user posted comments about that photo.

Feeds are not created, only updated. If a particular Feed (user, topic combination) has never posted to, trying to fetch updates will yield nothing.

Feed Manifests
--------------

A Feed Manifest is simply a JSON object that contains the ``Topic`` and ``User`` of a particular Feed (i.e., a serialized ``Feed`` object). Uploading this JSON object to Swarm in the regular way will return the immutable hash of this object. We can then store this immutable hash in an ENS Resolver so that we can have a ENS domain that "follows" the Feed described in the manifest.

Feeds API
---------

There  are 3 different ways of interacting with *Feeds* : HTTP API, CLI and Golang API.

HTTP API
~~~~~~~~

Posting to a Feed
.................

Since Feed updates need to be signed, and an update has some correlation with a previous update, it is necessary to retrieve first the Feed's current status. Thus, the first step to post an update will be to retrieve this current status in a ready-to-sign template:

1. Get Feed template

``GET /bzz-feed:/?topic=<TOPIC>&user=<USER>&meta=1``

``GET /bzz-feed:/<MANIFEST OR ENS NAME>/?meta=1``


Where:
 + ``user``: Ethereum address of the user who publishes the Feed
 + ``topic``: Feed topic, encoded as a hex string. Topic is an arbitrary 32-byte string (64 hex chars)

.. note::
  + If ``topic`` is omitted, it is assumed to be zero, 0x000...
  + if ``name=<name>`` (optional) is provided, a subtopic is composed with that name
  + A common use is to omit topic and just use ``name``, allowing for human-readable topics

You will receive a JSON like the below:

.. code-block:: js

  {
    "feed": {
      "topic": "0x6a61766900000000000000000000000000000000000000000000000000000000",
      "user": "0xdfa2db618eacbfe84e94a71dda2492240993c45b"
    },
    "epoch": {
      "level": 16,
      "time": 1534237239
    }
    "protocolVersion" : 0,
  }

2. Post the update

Extract the fields out of the JSON and build a query string as below:

``POST /bzz-feed:/?topic=<TOPIC>&user=<USER>&level=<LEVEL>&time=<TIME>&signature=<SIGNATURE>``

Where:
 + ``topic``: Feed topic, as specified above
 + ``user``: your Ethereum address
 + ``level``: Suggested frequency level retrieved in the JSON above
 + ``time``: Suggested timestamp retrieved in the JSON above
 + ``protocolVersion``: Feeds protocol version. Currently ``0``
 + ``signature``: Signature, hex encoded. See below on how to calclulate the signature
 + Request posted data: binary stream with the update data


Reading a Feed
..............

To retrieve a Feed's last update:

``GET /bzz-feed:/?topic=<TOPIC>&user=<USER>``

``GET /bzz-feed:/<MANIFEST OR ENS NAME>``

.. note::

  + Again, if ``topic`` is omitted, it is assumed to be zero, 0x000...
  + If ``name=<name>`` is provided, a subtopic is composed with that name
  + A common use is to omit ``topic`` and just use ``name``, allowing for human-readable topics, for example:      
    ``GET /bzz-feed:/?name=profile-picture&user=<USER>``


To get a previous update:

Add an addtional ``time`` parameter. The last update before that ``time`` (unix time) will be looked up.

``GET /bzz-feed:/?topic=<TOPIC>&user=<USER>&time=<T>``

``GET /bzz-feed:/<MANIFEST OR ENS NAME>?time=<T>``

Creating a Feed Manifest
........................

To create a ``Feed manifest`` using the HTTP API:

``POST /bzz-feed:/?topic=<TOPIC>&user=<USER>&manifest=1.`` With an empty body.

This will create a manifest referencing the provided Feed.

.. note::
  This API call will be deprecated in the near future.

Go API
~~~~~~~~

Query object
.................

The ``Query`` object allows you to build a query to browse a particular ``Feed``.

The default ``Query``, obtained with ``feed.NewQueryLatest()`` will build a ``Query`` that retrieves the latest update of the given ``Feed``.

You can also use ``feed.NewQuery()`` instead, if you want to build a ``Query`` to look up an update before a certain date.

Advanced usage of ``Query`` includes hinting the lookup algorithm for faster lookups. The default hint ``lookup.NoClue`` will have your node track Feeds you query frequently and handle hints automatically.

Request object
.................

The ``Request`` object makes it easy to construct and sign a request to Swarm to update a particular Feed. It contains methods to sign and add data. We can  manually build the ``Request`` object, or fetch a valid "template" to use for the update.

A ``Request`` can also be serialized to JSON in case you need your application to delegate signatures, such as having a browser sign a Feed update request.

Posting to a Feed
.................

1. Retrieve a ``Request`` object or build one from scratch. To retrieve a ready-to-sign one: 

.. code-block:: go
  
  func (c *Client) GetFeedRequest(query *feed.Query, manifestAddressOrDomain string) (*feed.Request, error)

2. Use ``Request.SetData()`` and ``Request.Sign()`` to load the payload data into the request and sign it

3. Call ``UpdateFeed()`` with the filled ``Request``:

.. code-block:: go
  
  func (c *Client) UpdateFeed(request *feed.Request, createManifest bool) (io.ReadCloser, error) 

Reading a Feed
..............

To retrieve a Feed update, use `client.QueryFeed()`. ``QueryFeed`` returns a byte stream with the raw content of the Feed update.  

.. code-block:: go

  func (c *Client) QueryFeed(query *feed.Query, manifestAddressOrDomain string) (io.ReadCloser, error)

``manifestAddressOrDomain`` is the address you obtained in ``CreateFeedWithManifest`` or an ``ENS`` domain whose Resolver
points to that address.
``query`` is a Query object, as defined above.

You only need to provide either ``manifestAddressOrDomain`` or ``Query`` to ``QueryFeed()``. Set to ``""`` or ``nil`` respectively.

Creating a Feed Manifest
........................

Swarm client (package swarm/api/client) has the following method:

.. code-block:: go 
  
  func (c *Client) CreateFeedWithManifest(request *feed.Request) (string, error) 

``CreateFeedWithManifest`` uses the ``request`` parameter to set and create a  ``Feed manifest``.

Returns the resulting ``Feed manifest address`` that you can set in an ENS Resolver (setContent) or reference future updates using ``Client.UpdateFeed()``

Example Go code
...............

.. code-block:: go

  // Build a `Feed` object to track a particular user's updates
  f := new(feed.Feed)
  f.User = signer.Address()
  f.Topic, _ = feed.NewTopic("weather",nil)

  // Build a `Query` to retrieve a current Request for this feed
  query := feeds.NewQueryLatest(&f, lookup.NoClue)

  // Retrieve a ready-to-sign request using our query
  // (queries can be reused)
  request, err := client.GetFeedRequest(query, "")
  if err != nil {
      utils.Fatalf("Error retrieving feed status: %s", err.Error())
  }

  // set the new data
  request.SetData([]byte("Weather looks bright and sunny today, we should merge this PR and go out enjoy"))

  // sign update
  if err = request.Sign(signer); err != nil {
      utils.Fatalf("Error signing feed update: %s", err.Error())
  }

  // post update
  err = client.UpdateFeed(request)
  if err != nil {
      utils.Fatalf("Error updating feed: %s", err.Error())
  }

Command-Line
~~~~~~~~~~~~~~~~

The CLI API allows us to go through how Feeds work using practical examples. You can look up CL usage by typing ``swarm feed`` into your CLI.

In the CLI examples, we will create and update feeds using the bzzapi on a running local Swarm node that listens by default on port 8500. 

Creating a Feed Manifest
........................

The Swarm CLI allows creating Feed Manifests directly from the console.

``swarm feed create`` is defined as a command to create and publish a ``Feed manifest``.

The feed topic can be built in the following ways:
  * use ``--topic`` to set the topic to an arbitrary binary hex string.
  * use ``--name`` to set the topic to a human-readable name.
      For example, ``--name`` could be set to "profile-picture", meaning this feed allows to get this user's current profile picture.
  * use both ``--topic`` and ``--name`` to create named subtopics. 
      For example, `--topic` could be set to an Ethereum contract address and ``--name`` could be set to "comments", meaning this feed tracks a discussion about that contract.

The ``--user`` flag allows to have this manifest refer to a user other than yourself. If not specified, it will then default to your local account (``--bzzaccount``).

If you don't specify a name or a topic, the topic will be set to ``0 hex`` and name will be set to your username. 

.. code-block:: none

  $ swarm --bzzapi http://localhost:8500 feed create --name test

creates a feed named "test". This is equivalent to the HTTP API way of

.. code-block:: none

  $ swarm --bzzapi http://localhost:8500 feed create --topic 0x74657374    

since ``test string == 0x74657374 hex``. Name and topic are interchangeable, as long as you don't specify both. 

``feed create`` will return the **feed manifest**.

You can also use ``curl`` in the HTTP API, but, here, you have to explicitly define the user (which, in this case, is your account) and the manifest.

.. code-block:: none

  $ curl -XPOST -d 'name=test&user=<your account>&manifest=1' http://localhost:8500/bzz-Feed:/

is equivalent to

.. code-block:: none

  $ curl -XPOST -d 'topic=0x74657374&user=<your account>&manifest=1' http://localhost:8500/bzz-Feed:/


Posting to a Feed
.................

To update a Feed with the CLI, use ``feed update``. The **update** argument has to be in ``hex``. If you want to update your *test* feed with the update *hello*, you can refer to it by name:

.. code-block:: none

  $ swarm --bzzapi http://localhost:8500 feed update --name test 0x68656c6c6f203

You can also refer to it by topic,

.. code-block:: none

  $ swarm --bzzapi http://localhost:8500 feed update --topic 0x74657374 0x68656c6c6f203

or manifest.

.. code-block:: none

  $ swarm --bzzapi http://localhost:8500 feed update --manifest <manifest hash> 0x68656c6c6f203

Reading Feed status
...................

You can read the feed object using ``feed info``. Again, you can use the feed name, the topic, or the manifest hash. Below, we use the name.

.. code-block:: none

  $ swarm --bzzapi http://localhost:8500 feed info --name test

Reading Feed Updates
.....................  

Although the Swarm CLI doesn't have the functionality to retrieve feed updates, we can use ``curl`` and the HTTP api to retrieve them. Again, you can use the feed name, topic, or manifest hash. To return the update ``hello`` for your ``test`` feed, do this:

.. code-block:: none

  $ curl 'http://localhost:8500/bzz-feed:/?user=<your address>&name=test'


Computing Feed Signatures
-------------------------

1. computing the digest:

The digest is computed concatenating the following:
 +  1-byte protocol version (currently 0)
 +  7-bytes padding, set to 0
 +  32-bytes topic
 +  20-bytes user address
 +  7-bytes time, little endian
 +  1-byte level
 +  payload data (variable length)

2. Take the SHA3 hash of the above digest

3. Compute the ECDSA signature of the hash

4. Convert to hex string and put in the ``signature`` field above

JavaScript example
~~~~~~~~~~~~~~~~~~

.. code-block:: javascript

  var web3 = require("web3");

  if (module !== undefined) {
    module.exports = {
      digest: feedUpdateDigest
    }
  }

  var topicLength = 32;
  var userLength = 20;
  var timeLength = 7;
  var levelLength = 1;
  var headerLength = 8;
  var updateMinLength = topicLength + userLength + timeLength + levelLength + headerLength;




  function feedUpdateDigest(request /*request*/, data /*UInt8Array*/) {
    var topicBytes = undefined;
      var userBytes = undefined;
      var protocolVersion = 0;
    
      protocolVersion = request.protocolVersion

    try {
      topicBytes = web3.utils.hexToBytes(request.feed.topic);
    } catch(err) {
      console.error("topicBytes: " + err);
      return undefined;
    }

    try {
      userBytes = web3.utils.hexToBytes(request.feed.user);
    } catch(err) {
      console.error("topicBytes: " + err);
      return undefined;
    }

    var buf = new ArrayBuffer(updateMinLength + data.length);
    var view = new DataView(buf);
      var cursor = 0;
      
      view.setUint8(cursor, protocolVersion) // first byte is protocol version.
      cursor+=headerLength; // leave the next 7 bytes (padding) set to zero

    topicBytes.forEach(function(v) {
      view.setUint8(cursor, v);
      cursor++;
    });

    userBytes.forEach(function(v) {
      view.setUint8(cursor, v);
      cursor++;
    });
    
    // time is little-endian
    view.setUint32(cursor, request.epoch.time, true);
    cursor += 7;

    view.setUint8(cursor, request.epoch.level);
    cursor++;

    data.forEach(function(v) {
      view.setUint8(cursor, v);
      cursor++;
      });
      console.log(web3.utils.bytesToHex(new Uint8Array(buf)))

    return web3.utils.sha3(web3.utils.bytesToHex(new Uint8Array(buf)));
  }

  // data payload
  data = new Uint8Array([5,154,15,165,62])

  // request template, obtained calling http://localhost:8500/bzz-feed:/?user=<0xUSER>&topic=<0xTOPIC>&meta=1
  request = {"feed":{"topic":"0x1234123412341234123412341234123412341234123412341234123412341234","user":"0xabcdefabcdefabcdefabcdefabcdefabcdefabcd"},"epoch":{"time":1538650124,"level":25},"protocolVersion":0}

  // obtain digest
  digest = feedUpdateDigest(request, data)

  console.log(digest)