The Reactor Pattern's present in a lot of production infrastructure (Nginx, Eventmachine, 0mq, Redis), yet not very well understood by developers and systems fellas alike. In this talk we'll have a look at what code is doing at a lower level and also how underlying subsystems affect your event driven services.
Below the surface : system calls, file descriptor behavior, event loop internals and buffer management
Evented Patterns : handler types, deferrables and half sync / half async work
Anti patterns : on CPU time, blocking system calls and protocol gotchas
Behind the Scenes From the Manager's Chair: Decoding the Secrets of Successfu...
In the Loop - Lone Star Ruby Conference
1. In the Loop
W
5 R
10
Lone Star
6 Ruby
R
Conference 9
R
2011
7
W 8
W
Lourens Naudé
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2. Agenda
System calls
File descriptor semantics
Blocking, nonblocking and async I/O
The Reactor Pattern
Patterns and gotchas
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3. Caveats
Can’t run before you walk
Patterns are framework agnostic
KISS
read, write and connect only
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4. Head count ?
Eventmachine (or any client libs)
Redis
Nginx
node.js
ZeroMQ
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5. Reactor Pattern sweet spot ?
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6. Soft realtime systems where
throughput is more
important than processing
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7. What does that mean ?
Lots of in flight I/O
Little CPU per request - proxies
Improve ON:OFF cpu time
NEVER about speed for a single client
or request
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sábado, 13 de Agosto de 2011
8. "Upgraded to Thin, my
app's flying!"
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sábado, 13 de Agosto de 2011
9. EDOINGITWRONG
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10. To maintain acceptable
response times for more clients,
with the same or less
infrastructure
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11. Example app
Client 1
Client 2
Broker
Client 3
Client 4
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12. FIX
FIX client
Financial Information eXchange
Compact protocol - encodes a lot of
domain info
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13. FIX message
8=FIX.4.49=4535=049=AB56=CD34=352=20000426-12:05:0610=22
8=FIX.4.4 # FIX version
9=45 # body length
35=0 # Heartbeat msg
49=AB # sender
56=CD # receiver
34=3 # sequence number
52=20000426-12:05:06 # sent at
10=220 # checksum
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14. The kernel
Mediates access to system
resources :
I/O, memory and CPU
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sábado, 13 de Agosto de 2011
15. User mode
Sandboxed execution for
security and stability
requirements
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sábado, 13 de Agosto de 2011
16. How do we access resources ?
System calls ( syscalls )
read(5, &buf, 4000)
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17. Syscalls
Ruby
process
Kernel disk
Ruby
process
read(5, &buf, 4000)
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18. Definition
A protected function call
from user to kernel space
with the intent to interact
with a system resource
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19. Characteristics
Uniform API on POSIX systems
Ditto for behavior
LIES !
Slow emulations
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20. Syscall performance
MUCH slower than function calls
20x and more
Definite context switch
Calls can block - slow upstream peers
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21. Who doesn't know what file
descriptors or file handles
are ?
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22. open syscall
fd = open(“/path/file”, O_READ)
read(fd, &buf, 4000)
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23. Examples
local file
socket
directory
pipe
Consistent API, semantics differ
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24. Definition
Numeric handle
References a kernel allocated resource
Kernel buffer
User space buffer
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25. Blocking I/O
A request to read a chunk of
data from a descriptor may
block depending on
buffer state
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26. Blocked buffer state
read(5, &b, 4000)
Kernel
User buffer
buffer
2000
1000
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27. Nonblocking I/O
fd = socket (PF_INET, SOCK_STREAM, 0);
fcntl (fd, F_SETFL, O_NONBLOCK)
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28. Nonblocking I/O
A read request would only
be initiated if the system
call won’t block
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29. EAGAIN or
EWOULDBLOCK
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30. Not a silver bullet
Guideline only
Not free - invokes a syscall
Not supported by all devices
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31. Async I/O myth
Nonblocking I/O IS NOT
asynchronous I/O
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32. Async I/O
Windows I/O Completion Ports
AIO: POSIX Realtime Extension
Async I/O often skip double buffering
Supports file I/O only
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33. Recap
O_NONBLOCK is a guideline only
Invoked in user space
Data transfer in user space
Blocking and nonblocking I/O terms
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34. Reactor Pattern
Remember, the primary use
case is increased throughput
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35. Challenges
Large number of file descriptors
Respond to descriptor state changes
Execute domain logic
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36. WHILE maintaining acceptable
response times
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37. The Reactor Loop
while true do
# shortened for brevity
add_new_descriptors # attach new client descriptors
modify_descriptors # update descriptor state
break if terminate? # conditional loop break
end
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38. Registered descriptors
W
5 R
10
R
6 Reactor 9
R
7
W 8
W
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39. Multiplexed I/O
Who's familiar with select,
poll, epoll or kqueue ?
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40. Multiplexed I/O
Nonblocking I/O is inefficient
Retry on EAGAIN is polling
Multiplexed I/O: concurrent blocking
Notified of state changes on any
registered fd
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41. Multiplexed I/O
fd 5
fd 6
fd 7
fd 8
I/O bound
App Multiplexer
fd 9
fd 10
fd 11
fd 12
User space Kernel space
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42. Multiplexed I/O
1. Tell me when fds 1..200
are ready for a read or a
write
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43. Multiplexed I/O
2. Do other work until one
of them's ready
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44. Multiplexed I/O
3. Get woken up by the
multiplexer on state
changes
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45. Multiplexed I/O
4. Which of fd set 1..200
are in a ready state ?
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46. Multiplexed I/O
5. Handle all fds ready for
I/O
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47. Event driven I/O
These notifications, or
rather state changes in
readiness for reading or
writing, that's the Events in
Event Driven I/O
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48. Event handlers / callbacks
module MyFixApp
def connection_completed
end
def receive_data(buffer)
end
end
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49. Callbacks
Reactor provides I/O concurrency
NO concurrent handling of user
code
Fire on the reactor thread
Runs within the reactor loop
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sábado, 13 de Agosto de 2011
50. Ticks
A turn through the event loop
The reactor can do several thousand
ticks per second
Time slice for doing work
Aim to use minimal CPU per tick
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sábado, 13 de Agosto de 2011
51. Best practices, patterns and
gotchas
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52. #1: Coupling Layers
App handler
Dispatch handler
I/O Multiplexer
Kernel
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53. Dispatch Handler
Handles low level events from the
multiplexed I/O framework
eg. readiness for read or write
Hide complexities from apps
Buffering
Transfer
Conversion to and from Ruby strings
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54. Dispatch Handler
class EventableDescriptor
def read; end # Tell don't ask interface
def write; end
def heartbeat; end
def select_for_read; end # I/O multiplexer
def select_for_write; end # agnostic
end
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55. Application Handlers
Domain layer
Higher level events
Connection accepted, read,
disconnected
Encode and decode protocols
Perform business logic
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56. Application Handlers
class FixConnection
def receive_data(data); end # callbacks
def connection_completed; end
def unbind; end
end
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57. Glue
class FixConnection
def receive_data(buffer)
# Encapsulate excess business logic in callbacks
@queue.push *FIX::Parser.parse(buffer)
end
end
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58. #2: Testing
You don’t need a transport
for testing
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59. Testing
class FixConnection
def receive_data(data)
@queue << data
end
end
def test_enqueue_on_receive
conn.receive_data "stub data"
assert_equal 1, conn.queue.size
end
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60. #3: Confusing sync and async
Events occur
asynchronously, but they're
handled synchronously
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61. App handler
Dispatch handler
I/O Multiplexer
Kernel
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62. Defer work to a thread pool
class FixConnection
def receive_data(buffer)
# enqueue to a thread pool
EM.defer{ FIX::Parser.parse(buffer) }
end
end
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63. Schedule on the reactor thread
class FixConnection
def receive_data(buffer)
# allow the reactor to service other clients also
EM.schedule{ FIX::Parser.parse(buffer) }
end
end
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64. #4: Too much ON cpu time
Reactor is single threaded
Single core on SMP systems
Pegging the core blocks the event loop
Time sharing
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65. Tight loops
[1..n].each do |i|
process(i) # expensive work
end
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66. Prefer a tick loop
work = [1..n]
EM.tick_loop do
if work.empty?
:stop
else
process(work.shift) # expensive work
end
end
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67. Slow Protocol parsers
Protocol parsers incur a cost
for both encoding and
decoding from buffers
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68. Negate encoding / decoding costs
Prefer a fast C extension /
parser to native Ruby code
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69. #5: Name resolution and connects
EM.connect("slow-broker.net", 2000, FixConnection)
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70. Async resolver
d = EM::DNS::Resolver.resolve "slow-broker.net"
d.callback do |addrs|
# connect to broker API when resolved
EM.connect(addrs.first, 2000, FixConnection)
end
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71. Async HTTP APIs
conn = EM::HttpRequest.new('http://api.a-broker.net')
req1 = conn.get :path => “x”, :keepalive => true
req1.callback {
req2 = conn.get :path => “y”
req2.callback {
# same connection as req1
}
}
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sábado, 13 de Agosto de 2011
72. #6: Blocking I/O
Not all file descriptors
support O_NONBLOCK
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sábado, 13 de Agosto de 2011
73. Ruby wrapper for libeio
$ gem install eio
EIO.open(__FILE__) do |fd|
EIO.read(fd, 1000) do |buf|
p buf
EIO.close(fd)
end
end
http://github.com/methodmissing/eio
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74. #7: All libs have to be evented
redis = EM::Protocols::Redis.connect
redis.errback do |code|
puts "Error code: #{code}"
end
redis.set "a", "foo" do |response|
redis.get "a" do |response|
puts response
end
end
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sábado, 13 de Agosto de 2011
75. Evented MySQL
client = Mysql2::EM::Client.new
defer = client.query "SELECT sleep(3) as query"
defer.callback do |result|
puts "Result: #{result.to_a.inspect}"
end
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sábado, 13 de Agosto de 2011
76. #8: Memory
The Garbage Collector
could be invoked at any
time during request
processing
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sábado, 13 de Agosto de 2011
77. Minimize GC pressure
Be careful with sloppy
allocation patterns
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78. Connection level proxy
module ProxyConnection
def initialize(client, request)
@client, @request = client, request
end
def post_init
EM::enable_proxy(self, @client)
end
def connection_completed
send_data @request
end
def proxy_target_unbound
close_connection
# end
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sábado, 13 de Agosto de 2011
79. #9: Buffer sizes
class FixConnection
def receive_data(inbound)
# inbound.size can be a single protocol
# message or a batch - stream of data
end
def send_data(outbound)
# enqueued on a write queue
end
end
Wildfire Interactive, Inc. | 1600 Seaport Boulevard, Suite 500, Redwood City, CA 94063 | (888) 274-0929
sábado, 13 de Agosto de 2011
80. Scheduled parsing
class FixConnection
def receive_data(buffer)
# buffer packs multiple FIX messages
Parser.parse(buffer).each do |msg|
EM.next_tick { process(msg) }
end
end
end
Wildfire Interactive, Inc. | 1600 Seaport Boulevard, Suite 500, Redwood City, CA 94063 | (888) 274-0929
sábado, 13 de Agosto de 2011
81. Takeaways
Time sharing for I/O bound services
System call overheads and behavior
Blocking, nonblocking and async I/O
Schedule through ticks
OS, dispatch and app layers
Transport last, interfaces first
Wildfire Interactive, Inc. | 1600 Seaport Boulevard, Suite 500, Redwood City, CA 94063 | (888) 274-0929
sábado, 13 de Agosto de 2011
82. Questions ?
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83. Wildfire Interactive, Inc. is hiring
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84. Thanks !
http://wildfireapp.com/buzz/jobs
follow @methodmissing
fork github.com/methodmissing
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sábado, 13 de Agosto de 2011