The document discusses the principles of reactive systems, focusing on responsiveness, resilience, elasticity, and message-driven architecture. It provides a case study of scaling the metric delivery system Graphite, detailing challenges faced and solutions implemented with Gruffalo as a proxy to manage load and improve availability. It highlights the importance of handling system failures gracefully and the role of back-pressure in maintaining performance under high loads.

1. Reactive By Example
Eran Harel - @eran_ha

2. source: http://www.reactivemanifesto.org/
The Reactive Manifesto

3. Responsive
The system responds in a timely manner if at
all possible.
source: http://www.reactivemanifesto.org/

4. Resilient
The system stays responsive in the face of
failure.
source: http://www.reactivemanifesto.org/

5. Elastic
The system stays responsive under varying
workload.
source: http://www.reactivemanifesto.org/

6. Message Driven
Reactive Systems rely on asynchronous message-passing
to establish a boundary between components that ensures
loose coupling, isolation, location transparency, and
provides the means to delegate errors as messages.
source: http://www.reactivemanifesto.org/

7. Case Study
Scaling our metric delivery system

8. Graphite
● Graphite is a highly scalable real-time
graphing system.
● Graphite performs two pretty simple tasks:
storing numbers that change over time and
graphing them.
● Sources:
○ http://graphite.wikidot.com/faq
○ http://aosabook.org/en/graphite.html

9. Graphite
http://aosabook.org/en/graphite.html

10. Graphite plain-text Protocol
<dotted.metric.name> <value> <unix epoch>n
For example:
servers.foo1.load.shortterm 4.5 1286269260n

11. Brief History - take I
App -> Graphite
This kept us going for a while…
The I/O interrupts were too much for Graphite.

12. Brief History - take II
App -> LogStash -> RabbitMQ -> LogStash ->
Graphite
The LogStash on localhost couldn’t handle the load, crashed and
hung on regular basis.
The horror...

13. Brief History - take III
App -> Gruffalo -> RabbitMQ -> LogStash -> Graphite
The queue consuming LogStash was way too slow.
Queues build up hung RabbitMQ, and stopped the producers on
Gruffalo.
Total failure.

14. Brief History - take IV
App -> Gruffalo -> Graphite (single carbon relay)
A single relay couldn’t take all the load, and losing it means
graphite is 100% unavailable.

15. Brief History - take V
App -> Gruffalo -> Graphite (multi carbon relays)
Great success, but not for long.
As we grew our metric count we had to take additional measures
to make it stable.

16. Introducing Gruffalo
● Gruffalo acts as a proxy to Graphite; it
○ Uses non-blocking IO (Netty)
○ Protects Graphite from the herd of clients,
minimizing context switches and interrupts
○ Replicates metrics between Data Centers
○ Batches metrics
○ Increases the Graphite availability
https://github.com/outbrain/gruffalo

17. Metrics Delivery HL Design
Carbon
RelayCarbon
RelayCarbon
Relay
Carbon
RelayCarbon
RelayCarbon
Relay
DC1
DC2

18. Graphite (Gruffalo) Clients
● GraphiteReporter
● Collectd
● StatsD
● JmxTrans
● Bucky
● netcat
● Slingshot

19. Metrics Clients Behavior
● Most clients open up a fresh connection,
once per minute, and publish ~1000K -
5000K metrics
● Each metric is flushed immediately

20. Scale (Metrics / Min)
More than 4M metrics per minute sent to graphite

21. Scale (Concurrent Connections)

22. Scale (bps)

23. Hardware
● We handle the load using 2 Gruffalo
instances in each Data Center (4 cores
each)
● A single instance can handle the load, but
we need redundancy

24. The Gruffalo Pipeline
(Inbound)
IdleState
Handler
Line
Framer
String
Decoder
Batch
Handler
Publish
Handler
Graphite Client
Helps detect
dropped /
leaked
connections
Handling ends
here unless the
batch is full
(4KB)

25. The Graphite Client Pipeline
(Outbound)
IdleState
Handler
String
Decoder
String
Encoder
Graphite
Handler
Handles
reconnects,
back-pressure,
and dropped
connections
Helps detect
dropped
connections

26. Graphite Client Load Balancing
Carbon Relay 1
Carbon Relay 2
Carbon Relay n
Metric batches
...

27. Graphite Client Retries
● A connection to a carbon relay may be
down. But we have more than one relay.
● We make a noble attempt to find a target to
publish metrics to, even if some relay
connections are down.

28. Graphite Client Reconnects
Processes crash, the network is *not* reliable,
and timeouts do occur...

29. Graphite Client Metric Replication
● For DR purposes we replicate each metric to
2 Data Centers.
● ...Yes it can be done elsewhere…
● Sending millions of metrics across the WAN,
to a remote data center is what brings most
of the challenges

30. Handling Graceless Disconnections
● We came across an issue where an
unreachable data center was not detected
by the TCP stack.
● This renders the outbound channel
unwritable
● Solution: Trigger reconnection when no
writes are performed on a connection for 10
sec.

31. Queues Everywhere
● SO_Backlog - queue of incoming
connections
● EventLoop queues (inbound and outbound)
● NIC driver queues - and on each device on
the way 0_o

32. Why are queues bad?
● If queues grow unbounded, at some point,
the process will exhaust all available RAM
and crash, or become unresponsive.
● At this point you need to apply either
○ Back-Pressure
○ Drop requests: SLA--
○ Crash: is this an option?

33. Why are queues bad?
● Queues can increase latency by a
magnitude of the size of the queue (in the
worst case).

34. ● When one component is struggling to keep-
up, the system as a whole needs to respond
in a sensible way.
● Back-pressure is an important feedback
mechanism that allows systems to gracefully
respond to load rather than collapse under it.
Back-Pressure

35. Back-Pressure (take I)
● Netty sends an event when the channel
writability changes
● We use this to stop / resume reads from all
inbound connections, and stop / resume
accepting new connections
● This isn’t enough under high loads

36. Back-Pressure (take II)
● We implemented throttling based on
outstanding messages count
● Setup metrics and observe before applying
this

37. Idle / Leaked Inbound Connections
Detection
● Broken connections can’t be detected by the
receiving side.
● Half-Open connections can be caused by
crashes (process, host, routers), unplugging
network cables, etc
● Solution: We close all idle inbound
connections

38. The Load Balancing Problem
● TCP Keep-alive?
● HAProxy?
● DNS?
● Something else?

39. Consul Client Side Load Balancing
● We register Gruffalo instances in Consul
● Clients use Consul DNS and resolve a
random host on each metrics batch
● This makes scaling, maintenance, and
deployments easy with zero client code
changes :)

40. Auto Scaling?
[What can be done to achieve auto-scaling?]

41. Questions?
“Systems built as Reactive Systems are more flexible,
loosely-coupled and scalable. This makes them easier to
develop and amenable to change. They are significantly
more tolerant of failure and when failure does occur they
meet it with elegance rather than disaster. Reactive
Systems are highly responsive, giving users effective
interactive feedback.”
source: http://www.reactivemanifesto.org/

42. Wouldn’t you want to do this daily?
We’re recruiting ;)

43. Links
● http://www.reactivemanifesto.org/
● https://www.youtube.com/watch?v=IGW5VcnJLuU
● https://github.com/outbrain/gruffalo
● http://aosabook.org/en/graphite.html
● http://ferd.ca/queues-don-t-fix-overload.html