Ssb-replication-scheduler NPM

ssb-replication-scheduler

Triggers replication of feeds identified as friendly in the social graph or in private groups.

Depends on ssb-friends and ssb-tribes2, and calls ssb-ebt APIs.

Installation

Prerequisites:

Requires Node.js 12 or higher
Requires ssb-db2 (it may work with ssb-db if you don't use partial replication)
Requires ssb-friends version 5.0 or higher
Requires ssb-ebt version 9.0 or higher

npm install --save ssb-replication-scheduler

Add this secret-stack plugin like this:

 const SecretStack = require('secret-stack')
 const caps = require('ssb-caps')

 const createSsbServer = SecretStack({ caps })
     .use(require('ssb-master'))
     .use(require('ssb-db2'))
     .use(require('ssb-ebt'))
     .use(require('ssb-friends'))
+    .use(require('ssb-replication-scheduler'))
     .use(require('ssb-conn'))
     // ...

Usage

Typically there is nothing you need to do after installing this plugin. As soon as the SSB peer is initialized, ssb-replication-scheduler will automatically query the social graph, and either request replication or stop replication, depending whether the feed is friendly or blocked.

Opinions embedded in the scheduler:

Replication is enabled for:
- The main feed, ssb.id, because this allows you to recover your feed
- Any friendly feed at a distance of at most config.friends.hops
  - Includes your friends (if config.friends.hops >= 1)
  - Includes friends of friends (if config.friends.hops >= 2)
  - Includes friends of friends of friends (if config.friends.hops >= 3)
  - And so forth
Replication is strictly disabled for:
- Any feed you explicitly block

There are two APIs available in case you want to have more control over this module: start() and reconfigure(). Read more about these at the bottom of this file.

Configuration

Some parameters and opinions can be configured by the user or by application code through the conventional ssb-config object. The possible options are listed below:

{
  replicationScheduler: {
    /**
     * Whether the replication scheduler should start automatically as soon as
     * the SSB app is initialized. When `false`, you have to call
     * `ssb.replicationScheduler.start()` manually. Default is `true`.
     */
    autostart: true,

    /**
     * If `partialReplication` is an object, it tells the replication scheduler
     * to perform partial replication, whenever remote feeds support it. If
     * `partialReplication` is `null` (which it is, by default), then all
     * friendly feeds will be requested in full.
     *
     * Read below more about this configuration.
     */
    partialReplication: null,
  }
}

Configuring partial replication

The config.replicationScheduler.partialReplication object describes the tree of meta feeds that we are interested in replicating, for each hops level. For each hops level we have a certain template to describe how replication should work at that level. Notice that this configuration cannot specify who we replicate (that's the job of ssb-friends and your chosen hops, see the Usage section above), this configuration just specifies how should we replicate a friendly peer, in other words, the level of granularity for those peers.

Template per hops

The high-level overview of the partialReplication configuration is:

replicationScheduler: {
  partialReplication: {
    0: TEMPLATE_FOR_HOPS_0,
    1: TEMPLATE_FOR_HOPS_1,
    2: TEMPLATE_FOR_HOPS_2_AND_ABOVE,
    group: TEMPLATE_FOR_GROUP_MEMBERS,
  }
}

Soon we'll show how those TEMPLATE_FOR_HOPS work, but for now notice that the highest number will handle all the hops beyond that number, e.g. notice how 2 is the highest number and it means that TEMPLATE_FOR_HOPS_2_AND_ABOVE configures how to replicate peers at hops 2 or 3 or 4 or higher. There's nothing special about the number 2, it could also have been this:

replicationScheduler: {
  partialReplication: {
    0: TEMPLATE_FOR_HOPS_0,
    1: TEMPLATE_FOR_HOPS_1_AND_ABOVE,
  }
}

Or even this (which means we use the same template for all peers, regardless of their hops distance):

replicationScheduler: {
  partialReplication: {
    0: TEMPLATE_FOR_HOPS_0_AND_ABOVE,
  }
}

Or even fractional numbers:

replicationScheduler: {
  partialReplication: {
    0: TEMPLATE_FOR_HOPS_0,
    0.5: TEMPLATE_FOR_HOPS_HALF,
    1: TEMPLATE_FOR_HOPS_1_AND_ABOVE,
  }
}

Template structure

A Template is JSON which describes how should we do partial replication. If the template is null or a falsy value, then it means that for that hops level we don't do partial replication and we will do full replication (which means pre-2022 SSB replication of the peer's main feed).

When the template is a JSON array, it means we want to replicate only some leaf feeds in the "metafeed tree", where the root of the tree is always the root meta feed. The structure of the tree is assumed to follow the "tree structure v1", which means we're only concerned about the leaf feeds.

Each item in the template should be an object describing which keys in a leaf feed must match exactly the values given for that leaf to be replicated. So that if we write {purpose: 'git-ssb'}, it means we are interested in matching the leaf feed that has the field purpose exactly matching the value "git-ssb". All specified fields must match, but omitted fields are allowed to be any value. If you omit all the fields, i.e. if you pass the empty object {}, then this means "replicate ALL leaf feeds".

Special variables

Some values are special, in the sense that they are not taken literally, but are going to be substituted by other context-relative values. These special variables are always prefixed with $.

Special values
- $main
- $root
- $groupSecret (only in purpose field)

If the value of a field, e.g. in ssb-ql-0 queries, are the special strings "$main" or "$root", then they respectively refer to the IDs of the main feed and of the root meta feed. The shape {purpose: '$groupSecret'} corresponds to any leaf feed where the purpose matches one of the group secrets known by the local peer.

Example

In the example below, we set up partial replication with the meaning:

For hops 0 (that is, "yourself"), replicate some app feeds and all index feeds
For hops 1 (direct friends), replicate only 5 specific index feeds
For hops 2 and beyond, replicate only 2 specific index feeds

partialReplication: {
  0: [
    { purpose: 'main' },
    { purpose: 'coolgame' },
    { purpose: 'git-ssb' },
    { purpose: 'index' }
  ],

  1: [
    {
      purpose: 'index',
      metadata: {
        querylang: 'ssb-ql-0',
        query: { author: '$main', type: null, private: true },
      },
    },
    {
      purpose: 'index',
      metadata: {
        querylang: 'ssb-ql-0',
        query: { author: '$main', type: 'post', private: false },
      },
    },
    {
      purpose: 'index',
      metadata: {
        querylang: 'ssb-ql-0',
        query: { author: '$main', type: 'vote', private: false },
      },
    },
    {
      purpose: 'index',
      metadata: {
        querylang: 'ssb-ql-0',
        query: { author: '$main', type: 'about', private: false },
      },
    },
    {
      purpose: 'index',
      metadata: {
        querylang: 'ssb-ql-0',
        query: { author: '$main', type: 'contact', private: false },
      },
    },
  ],

  2: [
    {
      purpose: 'index',
      metadata: {
        querylang: 'ssb-ql-0',
        query: { author: '$main', type: 'about', private: false },
      },
    },
    {
      purpose: 'index',
      metadata: {
        querylang: 'ssb-ql-0',
        query: { author: '$main', type: 'contact', private: false },
      },
    },
  ],

  3: [
    {
      purpose: 'index',
      metadata: {
        querylang: 'ssb-ql-0',
        query: { author: '$main', type: 'about', private: false },
      },
    },
  ],
}

APIs

`ssb.replicationScheduler.start() => void` (sync)

`ssb.replicationScheduler.reconfigure(config) => void` (sync)

At any point during the execution of your program, you can reconfigure the replication rules using this API. The configuration object passed to this API has the same shape as config.replicationScheduler (see above) has.

Security considerations

The exclusion spec says that we should stop replicating new messages from an excluded member. That is not implemented so far (see also relevant proposed updates to the spec) because of a lack of time. So an excluded member could in theory keep posting to the group, even if they wouldn't be able to see things remaining members posted.