title: OAuth for Microservices with API Gateway - architecture
body: <p>In a microservice architecture, an API gateway lays in front of the API. The purpose of this is e.g. changing some request / response parameters, for single entry point or checking authentication etc. Now I would like to protect my API using OAuth2 flows to obtain an access token. The problem is to decide who is the actual OAuth client, I will demonstrate it by using a SPA example:</p>
  
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  <p>a) Is it the SPA that started the oauthorize request (to the api gateway) by using the implicit grant. Then, the Api gateway would simply route the request through to the OAuth authorization server, acting as a single entry point, with the /authorize stuff from the implicit flow</p>
  
  <p>b) Is it the API Gateway itself, meaning the SPA sends the username and password of the enduser to the api gateway (of course, here the SPA needs to be trusted with the end users credentials), which then acts on its own as an oauth client using the resource owner password grant</p>
  
  <p>c) dismiss the api gateway at all and create the oauth authorization server "parallel" to the api gateway, meaning you would loose the single entry point etc.</p>
  
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  <p>The following picture demonstrate a very abstract architecture, and the question is about the numer "[2]", if this request is initiated by the SPA and passed through by the api gateway, or if the api gateway is intercepting the request and acting on its own as an oauth client?</p>
  
  <p><a href="https://i.stack.imgur.com/LhVGt.png" rel="nofollow noreferrer">OAuth with API Gateway</a></p>
  
  <p>My guess is to always use the best fitting grant type for a specific client, regardless of an API gateway being in between or not. This would mean, that when it comes to OAuth, the API Gateway would simply pass the client authorization request through, whatever grant type it used. Therefore, [2] in the picture would come from the client, and not from the API Gateway acting as the OAuth client. Is this correct? As mentioned, this really gets tricky when it comes to first party apps as you probably could use the password credentials grant, which has huge drawbacks,e.g. no refreshing possible for SPAs. </p>