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https://github.com/decentralized-identity/did-resolver

Universal did-resolver for javascript environments
https://github.com/decentralized-identity/did-resolver

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Universal did-resolver for javascript environments

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# Typescript 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](https://w3c.github.io/did-core/#identifier) spec

from the [W3C Credentials Community Group](https://w3c-ccg.github.io).



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

applications can add.



## 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:



```js

{

  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.



```js

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)

```



### Using legacy DID Method resolvers



DID Method resolvers created before version `3.0.0` of this library can be used as legacy resolvers.



```js

import { Resolver } from 'did-resolver'

import web from 'web-did-resolver'

import sov from 'sov-did-resolver'



//returns an object of { methodName: resolveFunction}

webResolver = web.getResolver()

sovResolver = sov.getResolver()



//If you are using multiple methods you need to flatten them into one object

const resolver = new Resolver({}, {

  legacyResolvers: {

    ...webResolver,

    ...sovResolver,

  }

})



//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.



```js

resolver.resolve('did:ethr:0xF3beAC30C498D9E26865F34fCAa57dBB935b0D74/some/path#fragment=123').then(doc => console.log)



// You can also use ES7 async/await syntax

const doc = await resolver.resolve('did:ethr:0xF3beAC30C498D9E26865F34fCAa57dBB935b0D74/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:



```js

const resolver = new DIDResolver({

  ethr,

  web

}, {

  cache: true

})

```



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



```js

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

}, {

  cache: 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, options: DIDResolutionOptions)`

function. e.g. `{ ethr: resolve }`.



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

type [ParsedDID](https://github.com/decentralized-identity/did-resolver/blob/master/src/resolver.ts#L112)



```js

export function getResolver() {

  async function resolve(

    did: string,

    parsed: ParsedDID,

    didResolver: Resolver,

    options: DIDResolutionOptions

  ): Promise {

    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 the DIDResolutionResult object

    return {

      didResolutionMetadata: { contentType: 'application/did+ld+json' },

      didDocument: didDoc

      didDocumentMetadata: { ... }

    }

  }



  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.



```js

import { DIDResolver } from 'did-resolver'

import MyMethod from 'mymethod-did-resolver'



const myResolver = MyMethod.getResolver()

const resolver = new DIDResolver(myResolver)

```