Introduction to Subscriptions

Subscriptions allow two picos to communicate with each other on a secure channel, augmenting the parent-child hierarchy to create peer-to-peer structures.  From each pico's perspective there is a receiving channel, Rx, and a transmitting channel, Tx. The pico receives events and queries from the other pico in the subscriptions over the Rx channel and makes queries and sends events to the other pico using the Tx channel. 

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Subscriptions do not have to have the same parent. For example, in the Forever application, the picos belonged to different people and represented their personal contact information. 
Subscriptions are established by a pico accepting a request from another pico. Subscriptions' details are listed below.

Well Known Rx

When the subscription ruleset is installed, a well known Rx channel is created. As a reminder, a channel contains three pieces of information: the long identifier (eci), a name, and a descriptive type. The well known channel name is wellKnown_Rx with Tx_Rx as its channel type. This channel is used to establish new subscriptions between picos, and is meant to be publicly shared with potential subscribers. In the future this channel will use policies to restrict events to only subscription requests, making your engine more secure.

A wellKnown_Rx can be accessed by querying subscriptions wellKnown_Rx() function.

Simple Subscription

A simple subscription only contains what is needed for secure communication.

To create a simple subscription request from pico A to pico B. Raise a wrangler subscription event to pico A containing an attribute named "wellKnown_Tx" , and the value set to pico B's wellKnown_Rx.

Directional Subscription

A directional subscription is a subscription that stores roles, providing an easier way to identify master slave relationships in a subscription.

To create a directional subscription request from pico A to pico B. Raise a wrangler subscription event to pico A with additional attributes, Rx_role, Tx_role.  Rx_role is pico A's role in the subscription, leaving Tx_role to be pico B's role in the subscription relationship.

Cross Engine Subscription

It is easy to imagine a situation where you want to create a subscritption between picos hosted on different engines. Cross engine subscriptions allow us to do so by storing the host of the other pico in a subscription relationship.

To create a cross engine subscription between pico A and pico B, raise a wrangler subscription event to pico A with event attribute Tx_host.  Tx_host is pico B's engine host url including protocol, for example, "http://localhost:8282".

Accepting Subscription Request

After you raise the initiating event wrangler subscription to pico A, the inner workings of the subscription ruleset will automatically make a subscription request to pico B. This subscription request must be accepted by pico B before the subscription can be established. 

To accept an inbound subscription request from pico A to pico B, raise a wrangler pending_subscription_approval event to pico B with a Rx event attribute. The value of that event attribute will be the "Rx" attribute stored in the incoming request. You can find this value by looking at pico B's subscription ruleset entity variables on the UI, or by querying pico B's subscription ruleset inbound() function.

Auto Accepting Subscription Request

The Subscription ruleset is configurable to auto accept subscription requests if given certain attributes.  A Subscription request with attributes that matches any configured attribute and corresponding value will be auto accepted.

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Subscription Event Flow

To establish a basic subscription between pico A and pico B:

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Upon receiving a wrangler:established_removal event, the pico deletes the specified subscription from the established entity variable.

Initial Subscription Flow

Subscription Approval Flow

Subscription Rejection Flow

Subscription Events

Overview

Events Users Could Raise

• wrangler:subscription
• wrangler:pending_subscription_approval
• wrangler:outbound_cancellation
• wrangler:inbound_rejection
• wrangler:subscription_cancellation

Events Users Could React To

• wrangler:subscription_added
• wrangler:subscription_removed

Event Attributes

wrangler:subscription (pico A)

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Subscription Functions

wellKnown_Rx()

wellKnown_Rx() returns the well known channel, e.g.

{
  "id": "4H2tXa1NkexgyfEbzCFsPE",
  "pico_id": "cjc78fh4s000b9iry4u2j2len",
  "name": "wellKnown_Rx",
  "type": "Tx_Rx",
  "sovrin": {
    "did": "4H2tXa1NkexgyfEbzCFsPE",
    "verifyKey": "2naJczreMuFRiDJcc3SWpP4CkXVSuViKMNq4gRmh3Tzq",
    "encryptionPublicKey": "G3Xq6daaUndzwjiP8H1WFt9fKKb2NCbZtEVwGqXLuGMC"
  }
}

established()

established() returns all established subscription in an array, e.g.

[
  {
    "Rx": "5sk3bNLv1aZC1w4Gz9tg7i",
    "Tx": "BQMnQkRhpwRBspcnz4LdX8",
    "Tx_verify_key": "6frM5S1t6i2SJh1Rp4AXDpsmwQsCqMjLagsyEKrv6HEN",
    "Tx_public_key": "DCVx2iExZb6gnumBoHz5GA2rdrt3BbsU41YUJjcF7Peq"
  }
]

outbound()

outbound() returns all outbound subscriptions in an array, e.g.

[
  {
    "wellKnown_Tx": "B5JMLRzZnz7NQcDJuBAPSr",
    "Rx": "Q4aosn4DiPdEEpqwZZGEbh"
  }
]

inbound()

inbound() returns all inbound subscriptions in an array, e.g.

[
  {
    "Tx": "Q4aosn4DiPdEEpqwZZGEbh",
    "Rx": "MvgUk1pHSFHqtzpUKkJErC",
    "Tx_verify_key": "Da2dgGAEFPLJV4bTP6kYTdphUXcJaQ5Zrnq5rnMXS5UZ",
    "Tx_public_key": "39V93tWrxWBK2Anez4Mz6J6ec8eX3v7ZcTEdsEMSL42a"
  }
  {
    "Rx_role": "teacher",
    "Tx_role": "student",
    "Tx_host": "http://localhost:8040",
    "Tx": "WeEHHsfEVoS2to7LSpyrC8",
    "Rx": "SCmhMnquJrwxoM5SLmofjv",
    "Tx_verify_key": "HA3Dc9K5H4g2njJ6yFGsoi6d3tchs7RN91d9sbpJupi4",
    "Tx_public_key": "BVr8EBML6cbRtUvaxDV44TXjN1dHRiwFS7XpNiAfrGqC"
  }
]

autoAcceptConfig()

autoAcceptConfig() returns all auto accepted attributes keyed to array of possible values. In the future this may be remove for security.

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