Engineering and Data

How Much Testing is Enough Testing?

Ggb by night

One hundred years ago, a proposal took hold to build a bridge across the Golden Gate Strait at the mouth of San Francisco Bay.  For more than a decade, engineer Joseph Strauss drummed up support for the bridge throughout Northern California.  Before the first concrete was poured, his original double-cantilever design was replaced with Leon Moisseiff’s suspension design.  Construction on the latter began in 1933, seventeen years after the bridge was conceived.  Four years later, the first vehicles drove across the bridge.  With the exception of a retrofit in 2012, there have been no structural changes since.  21 years in the making.  Virtually no changes for the next 80.

Now, compare that with a modern Silicon Valley software startup.  Year one: build an MVP.  Year two: funding and product-market fit.  Year three: profitability?…growth? Year four: make it or break it.  Year five: if the company still exists at this point, you’re lucky.

Software in a startup environment is a drastically different engineering problem than building a bridge.  So is the testing component of that problem.  The bridge will endure 100+ years of heavy use and people’s lives depend upon it.  One would be hard-pressed to over-test it.  A software startup endeavor, however, is prone to monthly changes and usually has far milder consequences when it fails (although being in a regulated environment dealing with financial data raises the stakes a bit).  Over-testing could burn through limited developer time and leave the company with an empty bank account and a fantastic product that no one wants.

I want to propose a framework to answer the question of how much testing is enough.  I’ll outline 6 criteria then throw them at few examples.  Skip to the charts at the end and come back if you are a highly visual person like me.  In general, I am proposing that testing efforts be assessed on a spectrum according to the nature of the product under test.  A bridge would be on one end of the spectrum whereas a prototype for a free app that makes funny noises would be on the other.

Assessment Criteria

Cost of Failure

What is the material impact if this thing fails?  If a bridge collapses, it’s life and death and a ton of money.  Similarly, in a stock trading app, there are potentially big dollar and legal impacts when the numbers are wrong.  On the contrary, an occasional failure in a dating app would annoy customers and maybe drive a few of them away, but wouldn’t be catastrophic. Bridges and stock trading have higher costs of failure and thus merit more rigorous testing.

Amount of Use

How often is this thing used and by how many people?  In other words, if a failure happens in this component, how widespread will the impact be?  A custom report that runs once a month gets far less use than the login page.  If the latter fails, a great number of users will feel the impact immediately.  Thus, I really want to make sure my login page (and similar) are well-tested.


How visible is the component?  How easy will it be for customers to see that it’s broken?  If it’s a backend component that only affects engineers, then customers may not know it’s broken until they start to see second-order side effects down the road.  I have some leeway in how I go about fixing such a problem.  In contrast, a payment processing form would have high visibility.  If it breaks, it will give the impression that my app is broken big-time and will cause a fire drill until it is fixed.  I want to increase testing with increased visibility.


This is a matter of return on effort.  If the thing I’ve built is a run-once job, then any bugs will only show up once.  On the other hand, a piece of code that is core to my application will last for years (and produce bugs for years).  Longer lifespans give me greater returns on my testing efforts.  If a little extra testing can avoid a single bug per month, then that adds up to a lot of time savings when the code lasts for years.

Difficulty of Repair

Back to the bridge example, imagine there is a radio transmitter at the top.  If it breaks, a trained technician would have to make the climb (several hours) to the top, diagnose the problem, swap out some components (if he has them on hand), then make the climb down.  Compare that with a small crack in the road.  A worker spends 30 minutes squirting some tar into it at 3am.  The point here is that things which are more difficult to repair will result in a higher cost if they break.  Thus, it’s worth the larger investment of testing up front.  It is also worth mentioning that this can be inversely related to visibility.  That is, low visibility functionality can go unnoticed for long stretches and accumulate a huge pile of bad data.


Complex pieces of code tend to be easier to break than simple code.  There are more edge cases and more paths to consider.  In other words, greater complexity translates to greater probability of bugs.  Hence, complex code merits greater testing.


Golden Gate Bridge

This is a large last-forever sort of project.  If we get it wrong, we have a monumental (literally) problem to deal with.  Test continually as much as possible.

Criterion Score
Cost of failure 5
Amount of use 5
Visibility 5
Lifespan 5
Difficulty of repair 5
Complexity 4

Cat Dating App

Once the word gets out, all of the cats in the neighborhood will be swiping in a cat-like unpredictable manner on this hot new dating app.  No words, just pictures.  Expect it to go viral then die just as quickly.  This thing will not last long and the failure modes are incredibly minor.  Not worth much time spent on testing.

Criterion Score
Cost of failure 1
Amount of use 4
Visibility 4
Lifespan 1
Difficulty of repair 1
Complexity 1

Enterprise App — AMEX Payment Processing Integration

Now, we get into the nuance.  Consider an American Express payment processing integration i.e. the part of a larger app that sends data to AMEX and receives confirmations that the payments were successful.  For this example, let’s assume that only 1% of your customers are AMEX users and they are all monthly auto-pay transactions.  In other words, it’s a small group that will not see payment failures immediately.  Even though this is a money-related feature, it will not merit as much testing as perhaps a VISA integration since it is lightly used with low visibility.

Criterion Score
Cost of failure 2
Amount of use 1
Visibility 1
Lifespan 5
Difficulty of repair 2
Complexity 2

Enterprise App — De-duplication of Persons Based on Demographic Info

This is a real problem for TrueAccord.  Our app imports “people” from various sources.  Sometimes, we get two versions of the same “person”.  It is to our advantage to know this and take action accordingly in other parts of our system.  Person-matching can be quite complex given that two people can easily look very similar from a demographic standpoint (same name, city, zip code, etc.) yet truly be different people.  If we get it wrong, we could inadvertently cross-pollinate private financial information.  To top it all off, we don’t know what shape this will take long term and are in a pre-prototyping phase. In this case, I am dividing the testing assessment into two parts: prototyping phase and production phase.


The functionality will be in dry-run mode.  Other parts of the app will not know it exists and will not take action based on its results.  Complexity alone drives light testing here.

Criterion Score
Cost of failure 1
Amount of use 1
Visibility 1
Lifespan 1
Difficulty of repair 1
Complexity 4


Once adopted, this would become rather core functionality with a wide-sweeping impact.  If it is wrong, then other wrong data will be built upon it, creating a heavy cleanup burden and further customer impact.  That being said, it will still have low visibility since it is an asynchronous backend process.  Moderate to heavy testing is needed here.

Criterion Score
Cost of failure 4
Amount of use 3
Visibility 1
Lifespan 3
Difficulty of repair 4
Complexity 4

Testing at TrueAccord

TrueAccord is three years old.  We’ve found product-market fit and are on the road to success (fingers crossed).  At this juncture, engineering time is a bit scarce, so we have to be wise in how it is allocated.  That means we don’t have the luxury of 100% test coverage.  Though we don’t formally apply the above heuristics, they are evident in the automated tests that exist in our system.  For example, two of our larger test suites are PaymentPlanHelpersSpec and PaymentPlanScannerSpec at 1500 and 1200 lines respectively.  As you might guess, these are related to handling customers’ payment plans.  This is a fairly complex, highly visible, highly used core functionality for us.  Contrast that with TwilioClientSpec at 30 lines.  We use Twilio very lightly with low visibility and low cost of failures.  Since we are only calling a single endpoint on their api, this is a very simple piece of code.  In fact, the testing that exists is just for a helper function, not the api call itself.

I’d love to hear about other real world examples, and I’d love to hear if this way of thinking about testing would work for your software startup.  Please leave us a comment with your point of view!

Skipping Photoshop: How we made ID Badge creation 10x faster by using facial recognition

Recently TrueAccord has grown to the size where our compliance stance requires the addition of photo ID badges. It’s a rite of passage all small-but-growing companies endure and ours is no different.

Since I have previous experience setting up badge systems and dealing with the printers, I volunteered to kickoff this process. I’ve evaluated pre-existing badge creation software in the past and found them all significantly lacking. In a previous environment, I wrote my own badge creation software which fit the needs at the time. The key phrase being “at the time“. For tech startups, it’s not unusual to go from onboarding one person every other week, to 10 people a week in a year or two. That means every manual step for onboarding someone will go from an “oh well, it’s just once every other week” to “we need to dedicate several hours of someone’s time every week to this process.” Typically that same growth period also happens to be when your operations (IT, Facilities, and Office Admin) organizations are the most short staffed and the least likely to have the free time to do that. “Where is this going?” and “How much work does this mean for me?”, you ask? Allow me to share with you how I automated our badge system – Photoshop included.

Continue reading

Repos: How we use MySQL as a key-value store

When we started TrueAccord in 2013, we used MySQL to store our data in pretty traditional way. As business requirements came in, we found ourselves continuously migrating our table schemas to add more columns and more tables. Before MySQL 5.6, these schema changes would lock down the database for the entire duration of a change causing a brief downtime. When the company was smaller and just starting out, this was tolerable, but as we grew the increase in schema complexity was getting harder to manage via SQL migration scripts.

We were looking for an alternative, something like Big Table, the key-value store that I used back at Google. Using a key-value store enables storing an entire document as a value, and thus eliminating the need for migrations. We investigated several publicly available key-value stores, but none of them met our major requirements at the time. As a small engineering team, we wanted a hosted fully managed database solution, so that backups and server migrations are taken care of for us. Additionally we wanted security features like encryption at rest. DynamoDB came the closest to matching our requirements, but was missing encryption at rest.

We came across this old post from FriendFeed that describes at a high-level design that meets our requirements which inspired our implementation. First, we chose to use MySQL (now Aurora) managed by Amazon RDS as our backing datastore. This solves the requirement for a hosted, managed, encrypted database, and this is a battle-tested database. Then for the key-value interface (to avoid schema migrations), we built a thin library called Repos that provides a key-value interface implemented on top of MySQL. Now we have something that allows us to move quickly on top of a reliable datastore.

Enter Repos

Each repo represents a map from a UUID (key) to an arbitrary array of bytes representing the value. Each repo is stored in MySQL using two tables. The first table is the log table. Every time we wanted to insert or update an entity, we will insert it to this table.

Column name Type Description
pk bigint(20) Auto incremented primary key
uuid binary(16) Unique id for each entry
time_msec bigint(20) Time inserted
format char(1) Describes the format of the entry_bin column.
entry_bin longblog The value.

We always append to this table, never updating an existing row. By doing so, we get the full history of every object. This has proven to be really handy for debugging why a change has occurred, and when.

The format column can take two possible values: ‘1’ means the value in entry_pb is a serialized protocol buffer, and ‘2’ means it is compressed using Snappy (a compression scheme that aims for high speed and reasonable compression)

To optimize look-ups, we have another table, the “latest” table, with the following format:

Column name Type Description
parent_pk bigint(20) PK of this entry in the log table.
uuid binary(16) The unique id of the entry(here it is a primary key)
format Char(1) Describes the format of the entry_bin column.
entry_bin longblog The value.


Whenever we insert an element to the log table, we also upsert it to this table so it always has the latest inserted element. We do this as a transaction to ensure the tables are always in sync.

Secondary Index Implementation

The first hurdle when going in this route is secondary indexes. For example, if your Repo maps a user id to his account information (email, hashed password, full name), how would you look up an account by email? To do so, we implemented index tables. An index table maps the values in the key value store to a primitive value that MySQL can index. A single repo may have multiple indexes, and each one goes to its own table. Index tables have the following layout:

Column name Type Description
parent_pk bigint(20) PK of this entry in the log table.
uuid binary(16) Random id for each entity (here it is a primary key)
value * The indexed value (for example, the email address of the user)


We always insert to the secondary index. Therefore, over time, the index will contain stale values. To solve that, when querying, we join the uuid and parent_pk with the latest value and return the result only if there is a match.

For example, if we have a person with id “idA” and he changed his  email, the log table would look like this:

pk uuid time_msec value (format, entry_bin)
501 idA t1 {“user”: “john”, “email”: “”}
517 idA t2 {“user”: “john”, “email”: “”}


The latest table, would have only the updated row:

parent_pk uuid value (format, entry_bin)
517 idA {“user”: “john”, “email”: “”}


The email index table would have the email value, for each version of the object:

parent_pk uuid value
501 idA
517 idA


Now, to find an account whose latest email value is “”, the Repos library would build a query similar to this:

SELECT l.uuid, l.format, l.entry_bin FROM latest AS l, email_index AS e
  WHERE e.value =" AND
        e.uuid = l.uuid AND e.parent_pk = l.parent_pk

Our Repo library provides a nice Scala api for querying by index. For example,


Would return all the accounts that have this email address.

Using Table Janitor to Manage Our Tables and Indexes

The table janitor is a process implemented as an Akka actor that runs on our JVMs. This actor is responsible for two main tasks:

  1. Ensuring that the underlying MySQL tables are created.It does this by reflecting all of the Repos and indices defined in the code and then creating the corresponding MySQL tables. This makes adding a new repo or adding an index as simple as just defining it in the code.
  2. Ensuring that the indices are up to date. This is necessary since when a new index gets added, there may still be servers that run old version of the code and do not write into the new index. The table janitor regularly monitors the log tables and (re-)indexes every new record. Adding an index to an existing repo is easy – we just declare it in the code.

How we do Analytics

We use AWS data pipeline to incrementally dump our log tables into S3. We then use Spark (with ScalaPB) for Bigdata processing. We also upload a snapshot of it to Google’s Bigquery. As all our repos use Protocol buffers as their value type, we can automatically generate Bigquery schemas for each repo.

Pros and Cons of Our Approach

By writing repos and have all our database access go through it, we get a lot of benefits:

  • Uniformity: having all our key-value maps being repos has the advantage that every optimization and every improvement applies to all our tables. For example, when we build a view that shows an object history, it works for all of our repos.
  • Schema evolution is free when using protocol buffers as values. We can just add optional fields, rename existing fields, or convert an optional to a repeated and it just works.
  • Security: storing data securely on RDS is a breeze. Encryption at rest? Click a checkbox. Require data encryption in transit? SSL is supported by default.
  • Reliability: We never had the RDS MySQL (later Aurora) instances go down (besides rare scheduled maintenance windows which require the instances to be rebooted). We have never lost data. Additionally we can recover the database to any given snapshot in time with RDS by replaying binary logs on top of a snapshot.
  • Ease of use: adding a Repo or an index is trivial. All of our ~60 or so Repos work in exactly the same way, and accessed through the same programmatic interface, our engineers can easily work with any of them using the same programming interface.
  • Optimization/Monitoring/debugging: Since MySQL is a mature and well-understood technology, there is a plethora of documentation on how to tune it, how to debug problems. In addition, AWS provides a lot of metrics for monitoring how an RDS instance is doing.

However, there are also downsides:

  • Storing binary data in MySQL limits what can be done using the command line MySQL client. We had to write a command line tool (and a UI) to look up elements by key so we can debug. For more complex queries, we use Spark and BigQuery for visibility into our data.
  • Being a homegrown solution, we occasionally had to spend time tuning our SQL queries when our repos grew in size. On the positive side, scaling up due to business growth is a good problem to have and fixing it for one repo, made an improvement for all others.
  • JDBC has Multiple Layers: JDBC/HikariCP/Mysql connector: we had quite a few issues where it was tricky to pinpoint the source of the problem.

Alternatives: What the Future Looks Like

As much as we’d like our homegrown solution, we are continuously thinking what our next storage solution will be like.

  • Current versions of both MySQL and Postgres come with built-in support for indexing JSON documents.
  • Google now offers a publicly hosted version of Bigtable.
  • We are moving towards having our data represented as a stream of events which may benefit from a different data store.


The Repos implementation has enabled our engineering team to quickly develop a lot of new functionality, as well as iterating over the data schema. By implementing on top of RDS, we have the peace of mind that our data is safe and our servers are up to date with all the security patches. At the same time, having full control over the implementation details of repos allowed us to quickly implement additional security measure so we can satisfy the stringent requirements of card issuers and other financial institutions, without sacrificing development speed.

Put your alerts in version control with DogPush

At TrueAccord, we take our service availability very seriously. To ensure our service is always up and running, we are tracking hundreds of system metrics (for example, how much heap is used by each web server), as well as many business metrics (how many payment plans have been charged in the past hour).

We set up monitors for each of these metrics on Datadog, that when triggered, will page an on call engineer. The trigger is usually based on some threshold for that metric.

As our team grew and more alerts were added we noticed three problems with Datadog:

  1. Any member of our team can edit or delete alerts in Datadog’s UI. The changes may be intentional or accidental, though our team prefers to review changes before they hit production. In Datadog, the review stage is missing.
  2. Due to the previous problem, sometimes an engineer would add a new alert with uncalibrated thresholds to datadog to get some initial monitoring for a newly written component. As Murphy’s law would have it, the new alert would fire at 3am waking up the on call engineer, and it may not even indicate a real production issue, but a miscalibrated threshold. A review system could better enforce best practices for new alerts.
  3. Datadog also does not expose a way to indicate that an alert should only be sent during business hours. For example, for some of our batch jobs, it is okay if they fail during the night, but we want an engineer to address it first thing in the morning.

To solve these problems, we made DogPush. It lets you manage your alerts as YAML files that you can check in your source control. So you can use your existing code review system to review them, and once they’re approved they get automatically pushed to DataDog — Voila! In addition, it’s straightforward to setup a cron job (or a Jenkins job) to automatically mute the relevant alerts outside business hours.
DogPush is completely free and open source – check it out here.

ScalaPB: TrueAccord’s Protocol Buffer code generator for Scala

When we started working on TrueAccord, we had a limited understanding of various technical aspects of the problem. Naturally, one of those unclear aspects was the data model: what data entities we will need to track, what will be their relationships (one-to-one, one-to-many, and so on), and how easy it is going to be to change the data model as business requirements become known and our domain expertise grows.

Using Protocol Buffers to model the data your service uses for storage or messaging is great for a fast-changing project:

  1. adding and removing fields is trivial, turning an optional field into a repeated field and so on. If we modeled our data using SQL, we will be constantly migrating our database schema.
  2. the data schema (the proto file) serves as an always up-to-date reference documentation for the service’s data structures and messages. People from different teams can easily generate parsers for almost every programming language, and access the same data.

Continue reading

Come Work With Us!

We combine a social mission, huge market, a proven team and early traction + revenue. We're early enough for you to have a huge influence on the company's future (and upside) but late enough so we already reached first funding and product market fit - the perfect timing to join. If you want to learn a lot while your actions have a positive impact on the world - come work with us.

Stay up to date! Subscribe for updates on TrueAccord

SF Scala: Nadav Samet, ScalaPB: Working with Protocol Buffers in Scala