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19 April 2017

Build Your Own Serverless Using Firebase & Cloud Functions

As part of a project I'm exploring with a couple friends, I recently decided to try to do "account aggregation" - that is, pull in transactions at several of my "bank accounts" into some coherent portal. I'm trying to do this in this "serverless" way because on the heels of a project that was incredibly complex and involved us building an entire systems framework, I wanted to see if i could do this with a bare minimum of technology "infrastructure development".

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12 January 2017

Protecting Long-running streams


We have long-running entities that we want to operate on with confidence.


  • We need to be able to restart a system in a reasonable timeframe * We really want snapshots in-band (ES)

Canonical question

We have an account with 100k events (Hold, Capture, Release, Deposit) to process. Because of distributed systems, we will receive "duplicate" commands.

Solutions examined

Re-projecting all history to build state

Keeping 'rich' snapshots

  • All 100k transaction ids (UUID, 16 bytes each gives ~ 1.6 mb)
  • The 1.6 mb is too big to put "in-band"
  • This grows monotonically with account usage
  • "Hotspots"

Expiring old (vector) transactions

  • Only keeping, say, the last 1k transactions (16kb is in line with the upper limit we have seen of ~25kb for reasonable performance for ES) in the snapshot
  • This is fine under normal use, but in a "bursty" scenario, your wall time for margin-of-error goes down just when your system comes under load (and presumably more failure-prone)
  • Busy accounts may generate more than 1000 transactions in a single payroll run

Expiring old (wall clock) transactions

  • To do this, you need to also store a timestamp in addition to the UUID in memory, making the per-record size more than 20 bytes
  • On a bursty day, you wind up needing more space, and each record takes more space, so the general snapshotting performance deteriorates

Keeping transaction IDs in small, "external" streams

  • The first implementation we came up with, we could not figure out how to get integrity assured
  • The "command" that gets replayed writes that it was received (transaction-, event 0), this triggers a handler to try to process the request (does an NSF check, then a hold or reply that it's not-held), which either writes a hold in the account stream and then a "held" in the transaction stream, or writes a "NotHeld" in the transaction stream
  • Handler of the transaction stream dispatches a reply to the requesting process

The problem here is subtle.

If the system crashes here:

And handler 1 picks up event A, it will need to check to see whether the account stream has the event already. However, the account stream has a million events. Searching for that event is problematic. If the stream has 1mm events already, scanning for that one event is prohibitively expensive.

ES has a provision for an "Event ID" A unique identifier representing this event. This is used internally for idempotency if you write the same event twice you should use the same identifier both times. So if you can generate an Event ID (GUID) deteriministically, then actually you would be ok. However, that couples more tightly to an ES implementation detail than we would prefer.

But if we write the version number of the account projection in the "HoldRequested" event, then we know that the subsequent "Held" event must be in the events following that, and we can search only that space to ensure that our work is not repeated.

So in the end, we wind up with a messaging pattern that looks like this:

Account Messaging

You'll see that to process a Hold command, we write three events within the Account Service, and then communicate that back out by writing another.

To Capture the held funds, we also write three messages.

One thing that is striking about this pattern is that the handler that is handling Hold events projects entities from the Account stream, does business logic, and writes to the Account stream. These processes are coupled, for all intents and purposes - which is reasonable. They are coupled as a business process, so their implementation is coupled as well. Decoupling is not always necessary (or desirable).

07 August 2016

The Bitfinex Bail-in

What "we" (I have absolutely zero privileged or inside information, though I did have less than a bitcoin on "deposit at Bitfinex") don't know, that frames the discussion a lot about this is:

What does Bitfinex's balance sheet look like?

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11 January 2016

Are there specific health risks to ketosis?

There are not specific health/cardio risks of getting into Ketosis. Ketosis is an adaptation to removing carbohydrates from the diet. That may be due to a fast, starvation, or a very controlled change - like I suggest. If you are otherwise healthy enough to do a three day fast (almost all people are), then you do not face any health risks.

  • Things to look out for are:
  • Pre-existing liver and kidney conditions
  • A compromised immune system
  • Generally any kind of medical dependence you already have

There is some confusion about the fact that you're eating lots of fat and therefore that will carry some kind of risk from high cholesterol or whatever. Studies of low-carb diets (high in fats) have shown that these diets do not raise blood cholesterol, and they have beneficial impacts on cardiovascular disease risk indicators. link

That said, it sounds anecdotally that if you are getting the bulk of your calories from dairy, your blood cholesterol levels may be higher than "normal".

As in all things, variety is good, doing too much of one certain thing is less good.

You should feel great. You will feel healthy, and alive and have great energy.

10 January 2016

What's up with "And fuck cancer"?

A lot of people have touching and compelling stories about cancer that will move you to tears. This is not that.

My attitude here comes more from a "what is the thing you believe that most people don't", which is that modern medicine is still much closer to leeches and bloodletting than anyone in the profession will let on. Scary close. Like, "we still use leeches sometimes" close. And that we will be much much closer to a science as far as "health" goes in 20 years than we are now. We're amazing at fixing trauma. If I get in a car accident, I want to be helicoptered to the closest trauma facility. When I'm kind of overweight and just don't feel great though? I'm not so interested in what medicine has to say.

It also comes very directly from an experience we had in my family. Someone was diagnosed with "Level 0 Breast Cancer". Which is to say, they had a growth that totally wasn't cancer, but maybe it could have become cancer down the line. "Oh god! Stage X Cancer!" the person thought. And of course a full treatment was ordered. This person isn't in super health in the first place (having suffered a stroke some time ago and various other health conditions), but they decided to get an operation to remove the thing-that-might-someday-become-cancerous. So they took their life in their hands, went under the knife, and got an operation. Tough recovery.

Oops. The operation didn't get the entire thing-that-is-not-cancer-but-we-could-totally-tell-was-thinking-about-it in her breast. So back under the knife. Tougher recovery.

Then, of course, radiation treatment. 12 weeks of sickness and incapacitation. A stroke medication interfered with the other drugs. Emergency room visit. More radiation. Finally, finished treatment for the thing-that-wasn't-cancer. No pink ribbons, since we're not "cancer survivors", but instead, "pre-cancer survivors", but oh well. We weren't in it for the pink ribbons.

A couple months later, the person gets sick. And sicker. And sicker. The new doctors (yay American health care system with constant passing-of-the-doctor) are stumped. Eventually get to an oncologist (why are we seeing an oncologist again?) who diagnoses that the surgical wound (from the second surgery) was infected. Back under the knife to clean out the wound and actually close things up sterile-ly this time. Thankfully no more radiation (remember, still no cancer).

So, three surgeries and many hospitalizations later, (I actually have no idea what the cost to the economy for this whole escapade was, but it was not cheap) we're "cured". Fuck that.

So then, I was listening to the Dom D'agostoino episode of the Tim Ferriss podcast and what he said was really intriguing to me for so many reasons. And I decided that even though the word "paleo" to describe a diet that people consume these days is asinine in so many ways, maybe there was something worth looking into with this low carb high fat diet. We threw out all the carbs in the house, bought an ass-load of bacon, and have been going strong for about 6 weeks now. Everyone in the family is losing weight (well, all the adults anyway), feeling the best they have in ages, and, as an interesting side effect, any pre-cancerous "things" are getting starved of the glucose they need to get big enough to endanger our lives in the cancer/scalpel lottery.

I'm super happy that I've found this, and want to make it easier for others to get here, too.

08 January 2016

React Native & Jest Testing


Have a way of testing my RN components with Jest that Doesn't Suck.


There exists a reasonable corpus of knowledge around testing React components with Jest. React is relatively new, Jest is newer, and though each have documentation, their interop has some nuance. Then, React Native gets added to the mix with a whole 'nother set of nuances and things kind of get crazy. So I spent a few days trying to figure out something reasonable.

This example is of some components whose render functionality will be tested. The project uses Redux, so there is not much store interaction, and a lot of passing of big prop trees through components. It shouldn't matter too much to the testing, but knowing that may give more context.

High Level Strategy:

We're going to use ReactTestUtils' shallowRender function to generate a "one" (really two) layer virtual DOM tree to inspect, then do some assertions on that. The object we are inspecting never actually gets written to HTML, and we are not doing "real" HTML DOM test/inspect. Rather we are just inspecting the objects that React Native is going to paint to a screen (which, being RN, is not a web browser), and can make sure our logic is working based on that.

In order to do this we need a convenient way to "render" a component to something we can inspect, and we need some tools for making assertions about that result.




I'll start with the "header" of my test file.

'use strict';


const shallowHelpers = require('react-shallow-renderer-helpers');
const findMatchingType = require('./findMatching').findMatchingType;
const objectAssign = require('object-assign');

import React, { View, Text } from 'react-native';

const NewLoan = require('../NewLoan');
const NewLoanForm = require('../NewLoanForm');
const PrepareSchedule = require('../PrepareSchedule');
  • jest.autoMockOff() - is important because if I don't have that, one of the components in my NewLoanForm object I include later will blow up. It's a bit of a liability, but not something that I've gone after fixing yet.
  • const shallowHelpers = require('react-shallow-renderer-helpers'); - this pulls in a bunch of React functionality for testing component trees (which is totally useless to us since RN components wind up being structured pretty differently from React components), but does pull in some nice functionality wrapping the shallowRender function.
  • var findMatchingType = require('./findMatching').findMatchingType; - this is a tool I built for doing assertions in the next section.
  • const objectAssign = require('object-assign'); this is useful for having our "default" props and being able to add new ones per-test. You could get similar functionality from lodash or similar, but I would prefer to keep that dependency out of my projects until necessary.

  • import React, { View, Text } from 'react-native'; - notice a couple of things here. First, this is the ES6 import syntax, where everything else uses require. This is because 1) I much prefer this syntax and 2) if I try using this syntax for the other lines, the autoMockOff(); won't work and these would be mocked - even if I ask for them not to be. This is due to JS hoisting, and may be fixed in the near future. The module that's being imported is a mock that will be detailed elsewhere.

  • const NewLoan, const NewLoanForm, const PrepareSchedule - these are not mocked and required in for doing assertions. I'm wanting to assert that when I render NewLoan with certain props, then it will have a NewLoanForm as a child, whereas if I give it different props, it will render a PrepareSchedule as a child. It may be nice to just do the comparison with string names, but this is good for now.


It's nice extract a reusable render method for your components that you will be testing in different state, with different props, etc.

describe('NewLoan', () => {
  let newLoan;

  function renderNewLoan(props) {
    const defaultProps = {
      isFetching: false,
      loans: {
        preparedLoan: null,
        preparedSchedule: null,
        form: {
          fields: {},
          isFetching: false
    const testProps = objectAssign(defaultProps, props)

    const shallowRenderer = shallowHelpers.createRenderer();
    shallowRenderer.render(() => <NewLoan {...testProps}/>);
    const output = shallowRenderer.getRenderOutput();

    return {
  // Your assertions will go here

So I have a set of defaultProps that the NewLoan view will generally depend on to make its rendering decisions.

This defaultProps object then gets objectAssignd (merged, basically), with the provided props.

A renderer is constructed, the component rendered into it, and the output returned in an object.

Note that

return {

Will really return an object like:

return {
  props: props,
  output: output,
  shallowRenderer: shallowRenderer

(thanks ES6 - and Obama).

Now let's render a component:

it('should display the NewLoanForm if there is no loan prepared', () => {
  const testProps = {};

  newLoan = renderNewLoan(testProps);
  const { output } = newLoan;

Sweet! We have something to look at. What is this output thing? console.log tells us:

Object {
  '$$typeof': Symbol(react.element),
  type: [Function: View],
  key: null,
  ref: null,
   Object {
     style: Object { flexDirection: 'column', flex: 1, width: 300, marginTop: 30 },
     children: [ [Object], [Object] ] },
  _owner: null,
  _store: Object {}

What I'm going to be asserting on generally is the type, props, and children - more likely the props of children when changing component state (though since this is Redux, MOST MOST MOST state should wind up being in the main state object and components only care about props & actions).

I was so excited about having this to assert on that my first tests when I got to this point were things like:


I pasted that into our development Slack channel and immediately lost the respect of most of our engineering team. Well, at least that's my deepest fear. I'm sure they still love me. There were some (very correct) criticisms, and it was clearly time to move on to step 2. Making decent assertions.



I'm not sure where this fits in the flow of this post, but I should mention something horrifying that I found. If you look at the assertion: expect(output.type.displayName).toEqual('NewLoanForm'); you may think, "Cool, it seems my React components have a property called DisplayName within their type that I can test against. Thanks React!" And you may even test against that for a little while and it will work. But then you will do that for a new component and get this result:

- Expected: undefined toEqual: 'NewLoan'


It turns out that if you declare a component using the syntax: var NewLoan = React.createClass({}), JSX will helpfully add a property displayName to the component. If you do: export default class NewLoan extends Component {}, then no such luck. You have no displayName. I would like that day of my life to figure all that out back please.

Ok, Assertions For Real

There is a nice module for doing assertions against React components generated by shallowRender - Sadly, almost none of the matchers (isComponentOfType, findAllWithClass, etc.) work in RN because a RN component is pretty different from a React component. Its findAll function does work pretty well, though (and it seemed kind of magical until I looked through it and realized it was just like an interview-question-type tree traversal).

Fortunately, this is all just JavaScript, and you can make your own matchers. These are what I came up with:


This will look for an element that matches what you're looking for. Simple. The All variant returns the array, the non-all variant just blows up if you don't have 1 exactly.

export function findAllMatchingType(tree, match) {
  return findAll(tree, (el) => {
    const typeMatch = match.type ? el.type === match.type : true;

    return typeMatch;

export function findMatchingType(tree, match) {
  const found = findAllMatchingType(tree, match);
  if (found.length !== 1) throw new Error('Did not find exactly one match');
  return found[0];

It should be noted that the type attribute that you are looking at here is sometimes friendly, sometimes a big nasty function. If you do the export default class... syntax, it has one form:

function PrepareSchedule() {
  _classCallCheck(this, PrepareSchedule);

  _get(Object.getPrototypeOf(PrepareSchedule.prototype), 'constructor', this).apply(this, arguments);

Otherwise, it can be:

function (props, context, updater) {
  // This constructor is overridden by mocks. The argument is used
  // by mocks to assert on what gets mounted.

  if (process.env.NODE_ENV !== 'production') {
    process.env.NODE_ENV !== 'production' ? warning(this instanceof Constructor, 'Something is calling a React component directly. Use a factory or ' + 'JSX instead. See:') : undefined;

  // Wire up auto-binding
  if (this.__reactAutoBindMap) {

  this.props = props;
  this.context = context;
  this.refs = emptyObject;
  this.updater = updater || ReactNoopUpdateQueue;

  this.state = null;

  // ReactClasses doesn't have constructors. Instead, they use the
  // getInitialState and componentWillMount methods for initialization.

  var initialState = this.getInitialState ? this.getInitialState() : null;
  if (process.env.NODE_ENV !== 'production') {
    // We allow auto-mocks to proceed as if they're returning null.
    if (typeof initialState === 'undefined' && this.getInitialState._isMockFunction) {
      // This is probably bad practice. Consider warning here and
      // deprecating this convenience.
      initialState = null;
  !(typeof initialState === 'object' && !Array.isArray(initialState)) ? process.env.NODE_ENV !== 'production' ? invariant(false, '%s.getInitialState(): must return an object or null', Constructor.displayName || 'ReactCompositeComponent') : invariant(false) : undefined;

  this.state = initialState;

There doesn't seem to be anything in there that makes it seem like it shuold be the right component, but if I compare it with a "dummy" component:

var FooClass = React.createClass({
  render() {
    return <View />

It does not match. Who'da thunk.


This matcher will try to match both the type of the element and the props. It's a trivial extension of the previous matcher:

export function findAllMatching(tree, match) {
  return findAll(tree, (el) => {
    const typeMatch = match.type ? el.type === match.type : true;
    const propsMatch = objectMatches(match.props, el.props);

    return typeMatch && propsMatch;

export function findMatching(tree, match) {
  const found = findAllMatching(tree, match);
  if (found.length !== 1) throw new Error('Did not find exactly one match');
  return found[0];

I have extracted those modules into a file on my filesystem, and will probably extract it out to a react-native-shallow-testutils or similar as it gets more robust.

Putting it all together

This is a full test case:

it('should display the NewLoanForm if there is no loan prepared', () => {
  const testProps = {};

  newLoan = renderNewLoan(testProps);
  const { output } = newLoan;

  const match = findMatchingType(output, <NewLoanForm />);


There we go. The contents of match in this case are the NewLoanForm component instance that got rendered, but mostly all I'm checking at THIS level of testing is whether by "default" it will render a form. In other tests I vary the props to have it render other things (more loan info collection, confirmation view, submission successful view, etc)


This has been hard. I'm pretty good at the internet, and still, it's been very hard. I'm very grateful to Facebook for getting this great tech out there, and really look forward to watching these projects evolve.


  • • I did not like that this uses the shallowRender technique, and tried to avoid it. However, @cpojer suggested that that is a good way to test components
  • Reactiflux Discord - #flux

14 December 2014

Internalizing Dependencies

So I saw an interesting conversation today on Twitter between @DHH and @thijs this morning. They were discussing the NewRelic IPO and the subject got around to running ancient Rails versions, and the fact that a lot of the migration projects from Rails 2.3 have been driven in large part because of the movement of the surrounding ecosystem (read, gem updates).

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