diff --git a/bin/wrap.js b/bin/wrap.js
index b7d995ffc..e2c0a8deb 100644
--- a/bin/wrap.js
+++ b/bin/wrap.js
@@ -8,6 +8,7 @@ config.isChildProcess = true
 config._processInfo = {
   pid: process.pid,
   ppid: process.ppid,
+  parent: process.env.NYC_PROCESS_ID,
   root: process.env.NYC_ROOT_ID
 }
 if (process.env.NYC_PROCESSINFO_EXTERNAL_ID) {
diff --git a/index.js b/index.js
index 006ff268a..84e370a6c 100755
--- a/index.js
+++ b/index.js
@@ -312,6 +312,7 @@ NYC.prototype._wrapExit = function () {
 }
 
 NYC.prototype.wrap = function (bin) {
+  process.env.NYC_PROCESS_ID = this.processInfo.uuid
   this._addRequireHooks()
   this._wrapExit()
   this._loadAdditionalModules()
@@ -341,7 +342,7 @@ NYC.prototype.writeCoverageFile = function () {
     coverage = this.sourceMaps.remapCoverage(coverage)
   }
 
-  var id = this.generateUniqueID()
+  var id = this.processInfo.uuid
   var coverageFilename = path.resolve(this.tempDirectory(), id + '.json')
 
   fs.writeFileSync(
@@ -355,6 +356,7 @@ NYC.prototype.writeCoverageFile = function () {
   }
 
   this.processInfo.coverageFilename = coverageFilename
+  this.processInfo.files = Object.keys(coverage)
 
   fs.writeFileSync(
     path.resolve(this.processInfoDirectory(), id + '.json'),
@@ -421,8 +423,7 @@ NYC.prototype.writeProcessIndex = function () {
   const infos = fs.readdirSync(dir).filter(f => f !== 'index.json').map(f => {
     try {
       const info = JSON.parse(fs.readFileSync(path.resolve(dir, f), 'utf-8'))
-      // on thiis first read pass, also map the pids to uuids
-      info.uuid = path.basename(f, '.json')
+      info.children = []
       pidToUid.set(info.uuid, info.pid)
       pidToUid.set(info.pid, info.uuid)
       infoByUid.set(info.uuid, info)
@@ -436,34 +437,17 @@ NYC.prototype.writeProcessIndex = function () {
   }).filter(Boolean)
 
   // create all the parent-child links and write back the updated info
-  const needsUpdate = new Set()
   infos.forEach(info => {
-    if (info.ppid && info.ppid !== '0' && !info.parent) {
-      info.parent = pidToUid.get(info.ppid)
-      needsUpdate.add(info)
-    }
     if (info.parent) {
       const parentInfo = infoByUid.get(info.parent)
       if (parentInfo.children.indexOf(info.uuid) === -1) {
         parentInfo.children.push(info.uuid)
-        needsUpdate.add(parentInfo)
       }
     }
   })
 
   // figure out which files were touched by each process.
   const files = infos.reduce((files, info) => {
-    if (!info.files) {
-      try {
-        info.files = Object.keys(JSON.parse(fs.readFileSync(
-          path.resolve(this.tempDirectory(), info.coverageFilename),
-          'utf-8'
-        )))
-      } catch (er) {
-        return files
-      }
-      needsUpdate.add(info)
-    }
     info.files.forEach(f => {
       files[f] = files[f] || []
       files[f].push(info.uuid)
@@ -478,15 +462,16 @@ NYC.prototype.writeProcessIndex = function () {
       index.processes[info.uuid].parent = info.parent
     }
     if (info.externalId) {
+      if (index.externalIds[info.externalId]) {
+        throw new Error(`External ID ${info.externalId} used by multiple processes`)
+      }
       index.processes[info.uuid].externalId = info.externalId
       index.externalIds[info.externalId] = {
         root: info.uuid,
         children: info.children
       }
     }
-    if (info.children && info.children.length) {
-      index.processes[info.uuid].children = Array.from(info.children)
-    }
+    index.processes[info.uuid].children = Array.from(info.children || [])
     return index
   }, { processes: {}, files: files, externalIds: {} })
 
@@ -502,12 +487,6 @@ NYC.prototype.writeProcessIndex = function () {
     }
   })
 
-  needsUpdate.forEach(info => {
-    fs.writeFileSync(
-      path.resolve(dir, info.uuid + '.json'), JSON.stringify(info)
-    )
-  })
-
   fs.writeFileSync(path.resolve(dir, 'index.json'), JSON.stringify(index))
 }
 
diff --git a/lib/process.js b/lib/process.js
index fb62ab6a9..608b5ec29 100644
--- a/lib/process.js
+++ b/lib/process.js
@@ -1,9 +1,12 @@
 const archy = require('archy')
 const libCoverage = require('istanbul-lib-coverage')
+const uuid = require('uuid/v4')
 
 function ProcessInfo (defaults) {
   defaults = defaults || {}
 
+  this.uuid = null
+  this.parent = null
   this.pid = String(process.pid)
   this.argv = process.argv
   this.execArgv = process.execArgv
@@ -12,14 +15,14 @@ function ProcessInfo (defaults) {
   this.ppid = null
   this.root = null
   this.coverageFilename = null
-  this.nodes = [] // list of children, filled by buildProcessTree()
-  this.children = [] // just uuids, not full nodes
-
-  this._coverageMap = null
 
   for (var key in defaults) {
     this[key] = defaults[key]
   }
+
+  if (!this.uuid) {
+    this.uuid = uuid()
+  }
 }
 
 Object.defineProperty(ProcessInfo.prototype, 'label', {
@@ -37,7 +40,7 @@ Object.defineProperty(ProcessInfo.prototype, 'label', {
 })
 
 ProcessInfo.buildProcessTree = function (infos) {
-  var treeRoot = new ProcessInfo({ _label: 'nyc' })
+  var treeRoot = new ProcessInfo({ _label: 'nyc', nodes: [] })
   var nodes = { }
 
   infos = infos.sort(function (a, b) {
@@ -45,6 +48,7 @@ ProcessInfo.buildProcessTree = function (infos) {
   })
 
   infos.forEach(function (p) {
+    p.nodes = []
     nodes[p.root + ':' + p.pid] = p
   })
 
diff --git a/test/processinfo.js b/test/processinfo.js
index 814f050d3..f3a7b151f 100644
--- a/test/processinfo.js
+++ b/test/processinfo.js
@@ -52,8 +52,8 @@ t.test('validate the created processinfo data', t => {
           throw er
         const procInfoData = JSON.parse(procInfoJson)
         t.match(procInfoData, {
-          pid: /^[0-9]+$/,
-          ppid: /^[0-9]+$/,
+          pid: Number,
+          ppid: Number,
           uuid: f.replace(/\.json$/, ''),
           argv: [
             node,
@@ -65,10 +65,7 @@ t.test('validate the created processinfo data', t => {
           time: Number,
           root: /^[0-9a-f]{8}-(?:[0-9a-f]{4}-){3}[0-9a-f]{12}$/,
           coverageFilename: resolve(fixturesCLI, tmp, f),
-          nodes: [],
-          _coverageMap: null,
           files: [ resolvedJS ],
-          children: Array,
         })
       })
     })