did-resolver

Javascript DID Resolver

This library is intended as a simple common interface for javascript applications to resolve DID documents from Decentralized Identifiers (DIDs).

This is intended to support the proposed Decentralized Identifiers spec from the W3C Credentials Community Group.

The library does not implement any specific DID method, but allows DID method implementors to release npm packages that applications can add.

Breaking changes (0.1.0)

This removes the global resolver. We've learnt that this ended up causing many problems. Instead you should now instantiate a Resolver object and use that to resolve did's within your consuming app/library.

Changes required for developers of DID resolvers

You now no longer register your DID resolvers using the global registerMethod() function. Instead export the resolver function and show how to configure it in a local resolver object.

Configure Resolver object

You are now required to preconfigure a resolver during instantiation. The Resolver constructor expects a registry of methods mapped to a resolver function. For example:

{ 
  ethr: resolve,
  web: resolve
}

Each method resolver should expose a function called getResolver which will return an object containing one of these key/value pairs. Then you can flatten them into one object to pass into the Resolver constructor.

import { Resolver } from 'did-resolver'
import ethr from 'ethr-did-resolver'
import web from 'web-did-resolver'
import sov from 'sov-did-resolver'

//returns an object of { methodName: resolveFunction}
ethrResolver = ethr.getResolver()
webResolver = web.getResolver()

//If you are using multiple methods you need to flatten them into one object
const resolver = new Resolver({
  ...ethrResolver,
  ...webResolver,
})

//If you are using one method you can simply pass the result of getResolver( into the constructor
const resolver = new Resolver(ethrResolver)

Resolving a DID document

The resolver presents a simple resolve() function that returns a ES6 Promise returning the DID document.

resolver.resolve('did:uport:2nQtiQG6Cgm1GYTBaaKAgr76uY7iSexUkqX/some/path#fragment=123').then(doc => console.log)

// You can also use ES7 async/await syntax
const doc = await resolver.resolve('did:uport:2nQtiQG6Cgm1GYTBaaKAgr76uY7iSexUkqX/some/path#fragment=123')

Caching

Resolving DID Documents can be expensive. It is in most cases best to cache DID documents. Caching has to be specifically enabled using the cache parameter

The built in cache uses a Map, but does not have an automatic TTL, so entries don't expire. This is fine in most web, mobile and serverless contexts. If you run a long running process you may want to use an existing configurable caching system.

The built in Cache can be enabled by passing in a true value to the constructor:

const resolver = new DIDResolver({
  ethr,
  web
}, true)

Here is an example using js-cache which has not been tested.

var cache = require('js-cache')
const customCache : DIDCache = (parsed, resolve) => {
  // DID spec requires to not cache if no-cache param is set
  if (parsed.params && parsed.params['no-cache'] === 'true') return await resolve()
  const cached = cache.get(parsed.didUrl)
  if (cached !== undefined) return cached
  const doc = await resolve()
  cache.set(parsed, doc, 60000)
  return doc
}

const resolver = new DIDResolver({
  ethr,
  web
}, customCache)

Implementing a DID method

Each DID method will have its own methods for looking up an identifier on its respective blockchain or other decentralized storage mechanism.

To avoid misconfiguration, method implementers should export a getResolver() function which returns an object mapping the method name to a resolve(did: string, parsed: ParsedDID, didResolver: DIDResolver) function. e.g. { ethr: resolve }.

The resolve function should accept a did string, and an object of type ParsedDID

export function getResolver() {
  async function resolve(
    did: string,
    parsed: ParsedDID,
    didResolver: Resolver
  ): Promise<DIDDocument> {
    console.log(parsed)
    // {method: 'mymethod', id: 'abcdefg', did: 'did:mymethod:abcdefg/some/path#fragment=123', path: '/some/path', fragment: 'fragment=123'}
    const didDoc = ...// lookup doc
    // If you need to lookup another did as part of resolving this did document, the primary DIDResolver object is passed in as well
    const parentDID = await didResolver.resolve(...)
    //
    return didDoc
  }

  return { myMethod: resolve }
}

The MyMethod getResolver() result could then be passed into the DIDResolver constructor. Note that it should be flattened if used with other methods as well.

import { DIDResolver } from 'did-resolver'
import MyMethod from 'mymethod-did-resolver'

const myResolver = MyMethod.getResolver()
const resolver = new DIDResolver(myResolver)