Event streaming: what will go wrong? (Symfony World 2020)

@samuelroze
Event Streaming Some things you want to know about.
@samuelroze

@samuelroze
Introduction
• My name is Samuel Rozé, I am VPoE at Birdie Care.
Core Team member of Symfony, for my work on Messenger.
• This is an architecture talk.
• We will brieﬂy discuss the values of using stream processing event
streaming.
• We will see the consequences of living the dream of managing distributed
systems. TL;DR: plenty of things will go wrong.

@samuelroze@samuelroze
1. Why …is event streaming even interesting?

@samuelroze
Your product works… you split your services.

@samuelroze
Your services are talking to each other.

@samuelroze
Now you need to introduce targeted discounts…

@samuelroze
How will this service get its data?
1. Pull via an API
• A lot of data will be moved each
time the “discount” service computes
discounts for a customer.
• “Discounts” is able to work only
when the 3 other services are
available (cascading failures).
• “Discounts” needs to know about
where are the other services and
how to talk to them.
2. Using “batch”
• Potentially contains loads of duplicated
information (full load each time or the
period is “over X days”)
• Not real-time. “Wait a few days for your
marketing preferences to be propagated”
• A lot can go wrong with all services
properly creating exports every night.

@samuelroze
Event streaming
• Events are ﬂowing in real-time, from and
to multiple services.
• To receive a speciﬁc event, services don’t
have to know who is sending events, just
that they can expect these messages.
• Much higher availability because data
goes to the service that requires it when they
are online.
• (When bus does persistence) New
consumers create their context by going
through all the events that have happened in
the system.
• Writing code that works well with the
nature of the distributed system is hard.
• You need a real governance about how
is the message bus used, they are your
new API contracts.

@samuelroze
Event streaming, as a diagram

@samuelroze
Everything we are going to talk about is true for…

2. What will go wrong? It’s not “if”.

@samuelroze
Let’s start with a simple use-case.
Here we write on the `Basket` entity for example

@samuelroze
Your message is sent to a queue

Problem A Are you sure that the message  
was sent to the queue?

@samuelroze
A. Are you sure that your message has been sent?

@samuelroze
A. Distributed transactions are not really a thing.

@samuelroze
A. What might happen if we don’t care about that?
• Your local “basket” table might have the new product but no worker receives
the “ProductAddedToBasket” event.
• Your local “basket” table might NOT have the new product but workers have
received the “ProductAddedToBasket” event (most likely if you use
database transactions for the entire request)
• Imagine the event being about “payment successful” or even potentially life-
changing like “fall in the home has been detected”… 😬

@samuelroze
A. The outbox pattern

@samuelroze
A. Publishing messages to bus consistently
• In a nutshell, write your message & side effects to your database as part of
one transaction and then get something else to pull the message from the
database and send it to your queue.
• With Symfony, the simplest is actually to use the Doctrine transport for
Symfony Messenger, with the doctrine transaction middleware.
• Alternatively, you can use a dedicated library for this.
• EventSaucePHP/DoctrineOutboxMessageDispatcher
• italolelis/outboxer

@samuelroze
A. Using the Doctrine transport

Problem B You will receive duplicated
messages.

@samuelroze
B. “At least once delivery”

@samuelroze
B. What might happen if we don’t care about that?
• You consume twice “ProductAddedToBasket”: the product is added twice
instead of just once (as per the user request).
• Depending on your business logic, it might be very important. For example,
what if it is about “Money added to bank account” or “Medication dose
taken”.

@samuelroze
B. You need some idempotence.
• You will receive the same message multiple times, it’s just a matter of time.
• There isn’t much a framework could do, you own the business logic; you
need to handle it by yourself.
• Use an idempotency key. A key that represents a single message and
allows you to know whether or not it’s been processed already.
• (By the way, this also applies to HTTP requests. Stripe’s API is a good
example.)

@samuelroze
B. Using the idempotency key in the handler
• One option is to have your idempotency key is part of your message. Your
team needs to know why it is useful and how to use it.

@samuelroze
B. How to use your “idempotency key”

Problem C Processing messages in parallel.

@samuelroze
C. Concurrently processing messages

@samuelroze
C. What might happen if we don’t care about that?
• You will lose some state in whatever you updated based on the events, at
some point.
• The easiest solution: don’t process things concurrently. But, not really
practical when things start to scale.

@samuelroze
C. Locking! Optimistic vs Pessimistic
The optimist…
• Assumes that everything will go
right most of the time.
• It validates that everything has
happened as expected when
writing its state to a consistent
storage.
• a.k.a. HTTP’s If-Match, …
The pessimist…
• Believes that I most cases, this
won’t work.
• Before doing any work, it ensures
nobody else is doing it.
• a.k.a. “mutex”, “advisory locks”,
etc…

@samuelroze
C. Pessimistic locking with Symfony Lock

@samuelroze
C. Optimistic locking with Doctrine’s “versions"

@samuelroze
C. Optimistic locking with Doctrine’s “versions"
What’s happening behind the scene
with optimistic locking:

Problem D Message ordering

@samuelroze
D. Know when there is no ordering guarantee

@samuelroze
D. What might happen if we don’t care about that?
• Hopefully your business logic doesn’t rely too much on the events being
ordered… make sure this is true.
• For example, we rely on “access_granted” and “access_revoked” events to
conﬁgure some permission rules. If they are consumed in the wrong
order… this is a different meaning 💥

@samuelroze
D. There are buses that guarantee order

@samuelroze
D. They scale using partitions

@samuelroze
D. Order guaranteed means blocking messages.

@samuelroze
D. For the infrastructure to guarantee ordering…
• You need a message bus that supports it (Kafka, SQS Fifo, Kinesis, etc…).
• You need to carefully design your partitions (or “shards”) so that you know all
message of a speciﬁc aggregate will always go to the same partition (a.k.a.
routing keys).
• You need to carefully manage all the errors. You can’t afford a wrong
message blocking an entire topic. But you can’t really post-pone only one single message…

To wrap up… A few learnings (hopefully).

@samuelroze
We’ve seen a few ways it can go wrong.
• When publishing a message to a bus.
Outbox pattern FTW.
• When receiving multiple time the same message.
Idempotence FTW.
• When concurrently consuming messages. 
You need to use optimistic or pessimistic locking.
• You can request ordering from your infrastructure.
But needs careful partition design & error management.

@samuelroze
Thank you!
@samuelroze

@samuelroze
Want to read more?
• Martin Kleppman’s book.
https://dataintensive.net
• https://multithreaded.stitchﬁx.com/blog/2017/06/26/patterns-of-soa-
idempotency-key/
• https://microservices.io/patterns/data/transactional-outbox.html

Event streaming: what will go wrong? (Symfony World 2020)

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