Limit per exchange polling via LRU cache.

Author: dlongley

lib/config.js

@@ -51,7 +51,16 @@ config.ensureConfigOverride.fields.push(meterServiceName);
 // optional interaction config
 config.interactions = {
   enabled: false,
-  // optional named workflows for interactions
+  caches: {
+    exchangePolling: {
+      // each cache value is only a boolean (the key is ~64 bytes); one entry
+      // per exchange being actively polled, 1M = ~60 MiB
+      max: 1000000,
+      // polling allowed no more than once per second by default
+      ttl: 1000
+    }
+  },
+  // named workflows for interactions
   /* Spec:
   {
     <workflow name>: {

lib/interactions.js

@@ -5,48 +5,57 @@ import * as bedrock from '@bedrock/core';
 import * as schemas from '../schemas/bedrock-profile-http.js';
 import {asyncHandler} from '@bedrock/express';
 import {ensureAuthenticated} from '@bedrock/passport';
+import {LRUCache} from 'lru-cache';
 import {createValidateMiddleware as validate} from '@bedrock/validation';
 import {ZCAP_CLIENT} from './zcapClient.js';
 
 const {config, util: {BedrockError}} = bedrock;
 
 let WORKFLOWS_BY_NAME_MAP;
 let WORKFLOWS_BY_ID_MAP;
+let EXCHANGE_POLLING_CACHE;
 
 bedrock.events.on('bedrock.init', () => {
-  const cfg = bedrock.config['profile-http'];
+  const cfg = config['profile-http'];
 
-  // parse workflow configs when interactions are enabled
+  // interactions feature is optional, return early if not enabled
   const {interactions} = cfg;
-  if(interactions?.enabled) {
-    const {workflows = {}} = interactions;
-    WORKFLOWS_BY_NAME_MAP = new Map();
-    WORKFLOWS_BY_ID_MAP = new Map();
-    for(const workflowName in workflows) {
-      const {localInteractionId, zcaps} = workflows[workflowName];
-      if(!localInteractionId) {
-        throw new TypeError(
-          '"bedrock.config.profile-http.interactions.workflows" must each ' +
-          'have "localInteractionId".');
-      }
-      const workflow = {
-        localInteractionId,
-        name: workflowName,
-        zcaps: new Map()
-      };
-      for(const zcapName in zcaps) {
-        const zcap = zcaps[zcapName];
-        workflow.zcaps.set(zcapName, JSON.parse(zcap));
-      }
-      if(!workflow.zcaps.has('readWriteExchanges')) {
-        throw new TypeError(
-          '"bedrock.config.profile-http.interactions.workflows" must each ' +
-          'have "zcaps.readWriteExchanges".');
-      }
-      WORKFLOWS_BY_NAME_MAP.set(workflowName, workflow);
-      WORKFLOWS_BY_ID_MAP.set(localInteractionId, workflow);
+  if(!interactions?.enabled) {
+    return;
+  }
+
+  // parse workflow configs when interactions are enabled
+  const {workflows = {}} = interactions;
+  WORKFLOWS_BY_NAME_MAP = new Map();
+  WORKFLOWS_BY_ID_MAP = new Map();
+  for(const workflowName in workflows) {
+    const {localInteractionId, zcaps} = workflows[workflowName];
+    if(!localInteractionId) {
+      throw new TypeError(
+        '"bedrock.config.profile-http.interactions.workflows" must each ' +
+        'have "localInteractionId".');
+    }
+    const workflow = {
+      localInteractionId,
+      name: workflowName,
+      zcaps: new Map()
+    };
+    for(const zcapName in zcaps) {
+      const zcap = zcaps[zcapName];
+      workflow.zcaps.set(zcapName, JSON.parse(zcap));
+    }
+    if(!workflow.zcaps.has('readWriteExchanges')) {
+      throw new TypeError(
+        '"bedrock.config.profile-http.interactions.workflows" must each ' +
+        'have "zcaps.readWriteExchanges".');
     }
+    WORKFLOWS_BY_NAME_MAP.set(workflowName, workflow);
+    WORKFLOWS_BY_ID_MAP.set(localInteractionId, workflow);
   }
+
+  // setup caches
+  const {caches} = interactions;
+  EXCHANGE_POLLING_CACHE = new LRUCache(caches.exchangePolling);
 });
 
 bedrock.events.on('bedrock-express.configure.routes', app => {
@@ -64,6 +73,8 @@ bedrock.events.on('bedrock-express.configure.routes', app => {
     interaction: `${interactionsPath}/:localInteractionId/:localExchangeId`
   };
 
+  const retryAfter = Math.ceil(EXCHANGE_POLLING_CACHE.ttl / 1000);
+
   // create an interaction to exchange VCs
   app.post(
     routes.interactions,
@@ -125,8 +136,14 @@ bedrock.events.on('bedrock-express.configure.routes', app => {
           });
       }
 
-      // FIXME: use in-memory cache to return exchange state if it was
-      // polled recently
+      // check if entry is in exchange polling cache, if so, return
+      // "too many requests" error response
+      const key = `${localExchangeId}/${localExchangeId}`;
+      if(EXCHANGE_POLLING_CACHE.get(key)) {
+        res.set('retry-after', retryAfter);
+        res.status(429).json({retryAfter});
+        return;
+      }
 
       // fetch exchange
       const capability = workflow.zcaps.get('readWriteExchanges');
@@ -135,6 +152,9 @@ bedrock.events.on('bedrock-express.configure.routes', app => {
         capability
       });
 
+      // prevent more polling on the same exchange for another second
+      EXCHANGE_POLLING_CACHE.set(key, true);
+
       // ensure `accountId` matches exchange variables
       const {exchange: {state, variables}} = response;
       if(variables.accountId !== accountId) {

package.json

@@ -30,7 +30,8 @@
   "homepage": "https://github.com/digitalbazaar/bedrock-profile-http",
   "dependencies": {
     "@digitalbazaar/ed25519-signature-2020": "^5.2.0",
-    "@digitalbazaar/ezcap": "^4.0.0"
+    "@digitalbazaar/ezcap": "^4.0.0",
+    "lru-cache": "^10.2.2"
   },
   "peerDependencies": {
     "@bedrock/app-identity": "^4.0.0",