CQRS and Event Sourcing with MongoDB and PHP

CQRS and Event Sourcing
with MongoDB and PHP

About me
Davide Bellettini
● Developer at Onebip
● TDD addicted
@SbiellONE — about.bellettini.me

About Onebip
Mobile payment platform.
Start-up born in 2005,
acquired by Neomobile
group in 2011.
Onebip today:
- 70 countries
- 200+ carriers
- 5 billions potential users

LAMP stack
It all started with a Monolith

self-contained services communicating via REST
To a distributed system

First class modern NoSQL distributed dbs
Modern services

But the Monolith is still there

The problem
A reporting horror story

We need three new reports!
― Manager
Sure, no problem!

Deal with the legacy SQL schema

A little bit of queries here,
a little bit of map-reduce there

Your queries are killing production!
― SysAdmin

Still not enough!
Heavy query optimization,
adding indexes

Let’s reuse data from other reports
(don’t do that)

Houston, we have a problem. Reports are
not consistent (with other reports)
― Business guy

It’s hard to compare different data in a distributed
system splitted across multiple domains
#1
Avoid multiple sources of truth

Same words, different concepts across domains
#2
Ubiquitous language

Changing a report shouldn’t have side effects
#3
Fault tolerance to change

#2
Data Warehouse + Consultants

No downtime in production
Consistent across domains
Must have

A system elastic enough to extract any metric
Real time data
Nice to have

Command-query
responsibility segregation
(CQRS)

Commands
Anything that happens in one of your domains
is triggered by a command and generates one
or more events.
Order received -> payment sent -> Items queued
-> Confirmation email sent

Query
Generate read models from events depending
how data need to be actually used (by users
and other application internals)

Event Sourcing
The fundamental idea of Event Sourcing is that of ensuring
every change to the state of an application is captured in an
event object, and that these event objects are themselves
stored in the sequence they were applied.
― Martin Fowler

Starting from the beginning of time, you are
literally unrolling history to reach state in a
given time
Unrolling a stream of events

Idea #1
Every change to the state of your application is
captured in event object.
“UserLoggedIn”, “PaymentSent”, “UserLanded”

Idea #2
Events are stored in
the sequence they
were applied inside
an event store

Idea #3
Everything is an event. No more state.

Idea #4
One way to store data/events but potentially
infinite ways to read them.
A practical example
Tech ops, business control, monitoring,
accounting they all are interested in reading
data from different views.

You start with this
{
"_id": ObjectId("123"),
"username": "Flash",
"city": …,
"phone": …,
"email": …,
}

The more successful your company
is, the more people
…
The more people, the more views

With documental dbs it's magically easy to add new
fields to your collections.

Soon you might end up with
{
"_id": ObjectId("123"),
"username": "Flash",
"city": …,
"phone": …,
"email": …,
"created_at": …,
"updated_at": …,
"ever_tried_to_purchase_something": …,
"canceled_at": …,
"acquisition_channel": …,
"terminated_at": …,
"latest_purchase_date": …,
…
}

A bomb waiting to detonate
It’s impossible to keep adding state changes to your
documents and then expect to be able to extract them with
a single query.

Event Store
● Engineered for event sourcing
● Supports projections
● By the father of CQRS (Greg Young)
● Great performances
http://geteventstore.com/
The bad
Based on Mono, still too unstable.

LevelWHEN
An event store built with Node.js and LevelDB
● Faster than light
● Completely custom, no tools to handle
aggregates
https://github.com/gabrielelana/levelWHEN

The known path
● PHP (any other language
would just do fine)
● MongoDB 2.2.x

Why MongoDB
Events are not relational
Scales well
Awesome aggregation framework

Queues
Recruiter - https://github.com/gabrielelana/recruiter

MongoDB replica set
A MongoDB replica set with two logical dbs:
1. Event store where we would store events
2. Reporting DB where we would store
aggregates and final reports

Anatomy of an event 1/2
{
'_id' : '3318c11e-fe60-4c80-a2b2-7add681492d9',
'type': 'an-event-type',
'data': {
'meta' : {
…
},
'payload' : {
…
}
}
}

Anatomy of an event 2/2
'meta' : {
'creation_date': ISODate("2014-21-11T00:00:01Z"),
'saved_date': ISODate("2014-21-11T00:00:02Z"),
'source': 'some-bounded-context',
'correlation_id': 'a-correlation-id'
},
'payload' : {
'user_id': '1234',
'animal': 'unicorn',
'colour': 'pink',
'purchase_date': ISODate("2014-21-11T00:00:00Z"),
'price': '20/fantaeuros'
}

Don’t trust the network: Idempotence
{
'_id' : '3318c11e-fe60-4c80-a2b2-7add681492d9',
…
}
The _id field is actually defined client side and
ensures idempotence if an event is received
two times

Indexes
● Events collection is huge (~100*N documents)
● Use indexes wisely as they are necessary yet
expensive
● With suggested event structure:
{‘data.meta.created_at’: 1, type:1}

Benchmarking
How many events/second can you store?
Our machines were able to store roughly 150 events/sec.
This number can be greatly increased with dedicated IOPS,
more aggressive inserting policies, etc...

Final tips
● Use SSD on your storage machines
● Pay attention to write concerns (w=majority)
● Test your replica set fault tolerance

From events
to meaningful metrics

Sequential Projector -> Event Mapper -> Projection -> Aggregation
The event processing pipeline

A real life problem
What is the conversion rate of our registered
users?

#1 The registration event
{
'_id' : '3318c11e-fe60-4c80-a2b2-7add681492d9',
'type': 'user-registered',
'data': {
'meta' : {
'save_date': ISODate("2014-21-11T00:00:09Z"),
'created_at': ISODate("2014-21-11T00:00:01Z"),
'source': 'core-domain',
'correlation_id': 'user-123456'
},
'payload' : {
'user_id': 123,
'username': 'flash',
'email': 'a-dummy-email@gmail.com',
'country': 'IT'
}
}

#2 The purchase event
{
'_id' : '3318c11e-fe60-4c80-a2b2-7add681492d9',
'type': 'user-purchased',
'data': {
'meta' : {
'save_date': ISODate("2014-21-11T00:10:09Z"),
'created_at': ISODate("2014-21-11T00:10:01Z"),
'source': 'payment-gateway',
'correlation_id': 'user-123456'
},
'payload' : {
'user_id': 123,
'amount': 20,
'value': EUR,
'payment': 'credit_card',
'item': 'fluffy cat'
}
}

Sequential projector 1/2
[]->[x]->[]->[x]->[]->[]->[]->[]
|--------------| |------------|
|
|
|
|
---> Projector
Divides the stream of events into batches, filters events by
type and pass those of interest to the mapper

Sequential projector 2/2
● It’s a good idea to select fixed sizes batches to avoid
memory problems when you load your Cursor in memory
● Could be a long-running process selecting events as they
arrive in realtime

Event mapper 1/3
Translates event fields to the Read Model domain
Takes an event as input, applies a bunch of logic and will
return a list of Read Model fields.

Event mapper 2/3
Input event:
user-registered
Output:
$output = [
'user_id' => 123, // simply copied
'user_name' => 'flash', // simply copied
'email' => 'a-dummy-email@gmail.com', // simply copied
'registered_at' => "2014-21-11T00:00:01Z" // From the data.meta.created_at event field
];

Event mapper 3/3
Input event:
user-purchased
Output:
$output = [
'user_id' => 123, // simply copied
'email' => 'a-dummy-email@gmail.com', // simply copied
'purchased_at': "2014-21-11T00:10:01Z" // From the data.meta.created_at event field
];

Projection
Essentially it is your read model.
The data that the business is interested in.

The Projection after event #1
db.users_conversion_rate_projection.findOne()
{
'user_id': 123,
'user_name': 'flash',
'registered_at': "2014-21-11T00:00:01Z"
}

The Projection after event #2
{
'user_id': 123,
'registered_at': "2014-21-11T00:00:01Z"
'purchased_at': "2014-21-11" // Added this field and rewrote others
}

The Projection collection{
'user_id': 123,
'registered_at': "2014-21-11",
}
{
'user_id': 456,
'user_name': 'batman',
}
{
'user_id': 789,
'user_name': 'superman',
}

The Projection - A few thoughts
Note that we didn't copy from events to projection
all the available fields. Just relevant ones.

From these two events we could have
generated infinite read models such as
● List all purchased products and related amounts for the
company buyers
● Map all sales and revenues for our accounting dept
● List transactions for the financial department

One way to write,
infinite ways to read!

The aggregation (1) - Total registered users
var registered = db.users_conversion_rate_projection.aggregate([
{
$match: {
"registered_at": { $gte: ISODate("2015-11-21"), $lte: ISODate("2015-11-22") }
}
},
{
$group: {
_id: { },
count: { $sum:1 }
}
}
]);

The aggregation (2) - User with a purchase
var purchased = db.users_conversion_rate_projection.aggregate([
{
$match: {
"registered_at": { $gte: ISODate("2015-11-21"), $lte: ISODate("2015-11-22") },
"purchased_at": { $exists: true }
}
},
{
$group: {
_id: { },
count: { $sum:1 }
}
}
]);

The aggregation (3) - Automate all the things
● You can easily create the aggregation framework statement
by composition abstracting the concept of Column.
● This way you can dynamically aggregate your projections
on (for example) an API requests.
● If your Projector is a long running process, your projections
will be updated to the second and you automagically get
realtime data.

Another events usage:
Business & Tech Monitoring

Beware of the beast!
No Silver Bullet

Events are expensive
They require a lot of TIME to be parsed

Events are expensive
You will end up with this billion size collection
(and counting)

Fixing wrong events is painful

Moving around events is
horribly painful

Actually it will make your life incredibly
difficult with hidden bugs and leaking
documentation.
Mongo won’t help you

Improvements
● Upgrade from MongoDB 2.2.x to 3.0.x
● Switch to WiredTiger storage engine to save
space

Credits
Based on a talk by Jacopo Nardiello
● Slides: http://bit.ly/es-nardiello-2014
● Video: https://vimeo.com/113370688

@SbiellONE — about.bellettini.me
Thank you!

CQRS and Event Sourcing with MongoDB and PHP

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