Fault Tolerance in a High Volume, Distributed System Ben Christensen Software Engineer – API Platform at Netflix @benjchristensen http://www.linkedin.com/in/benjchristensen 1

Dozens of dependencies. One going down takes everything down. 99.99%30 = 99.7% uptime 0.3% of 1 billion = 3,000,000 failures 2+ hours downtime/month even if all dependencies have excellent uptime. Reality is generally worse. 2

3

4

5

No single dependency should take down the entire app. Fail fast. Fail silent. Fallback. Shed load. 6

Options Aggressive Network Timeouts Semaphores (Tryable) Separate Threads Circuit Breaker 7

Options Aggressive Network Timeouts Semaphores (Tryable) Separate Threads Circuit Breaker 8

Options Aggressive Network Timeouts Semaphores (Tryable) Separate Threads Circuit Breaker 9

TryableSemaphore executionSemaphore = getExecutionSemaphore(); // acquire a permit if (executionSemaphore.tryAcquire()) { try { return executeCommand(); } finally { executionSemaphore.release(); } } else { circuitBreaker.markSemaphoreRejection(); // permit not available so return fallback return getFallback(); } Semaphores (Tryable): Limited Concurrency 10

TryableSemaphore executionSemaphore = getExecutionSemaphore(); // acquire a permit if (executionSemaphore.tryAcquire()) { try { return executeCommand(); } finally { executionSemaphore.release(); } } else { circuitBreaker.markSemaphoreRejection(); // permit not available so return fallback return getFallback(); } Semaphores (Tryable): Limited Concurrency if (executionSemaphore.tryAcquire()) { } else { } 11

TryableSemaphore executionSemaphore = getExecutionSemaphore(); // acquire a permit if (executionSemaphore.tryAcquire()) { try { return executeCommand(); } finally { executionSemaphore.release(); } } else { circuitBreaker.markSemaphoreRejection(); // permit not available so return fallback return getFallback(); } Semaphores (Tryable): Limited Concurrency if (executionSemaphore.tryAcquire()) { } else { return getFallback(); } 12

Options Aggressive Network Timeouts Semaphores (Tryable) Separate Threads Circuit Breaker 13

try { if (!threadPool.isQueueSpaceAvailable()) { // we are at the property defined max so want to throw the RejectedExecutionException to simulate // reaching the real max and go through the same codepath and behavior throw new RejectedExecutionException("Rejected command because thread-pool queueSize is at rejection threshold."); } ... define Callable that performs executeCommand() ... // submit the work to the thread-pool return threadPool.submit(command); } catch (RejectedExecutionException e) { circuitBreaker.markThreadPoolRejection(); // rejected so return fallback return getFallback(); } Separate Threads: Limited Concurrency 14

try { if (!threadPool.isQueueSpaceAvailable()) { // we are at the property defined max so want to throw the RejectedExecutionException to simulate // reaching the real max and go through the same codepath and behavior throw new RejectedExecutionException("Rejected command because thread-pool queueSize is at rejection threshold."); } ... define Callable that performs executeCommand() ... // submit the work to the thread-pool return threadPool.submit(command); } catch (RejectedExecutionException e) { circuitBreaker.markThreadPoolRejection(); // rejected so return fallback return getFallback(); } Separate Threads: Limited Concurrency if (!threadPool.isQueueSpaceAvailable()) { throw new RejectedExecutionException } } catch (RejectedExecutionException e) { } 15

try { if (!threadPool.isQueueSpaceAvailable()) { // we are at the property defined max so want to throw the RejectedExecutionException to simulate // reaching the real max and go through the same codepath and behavior throw new RejectedExecutionException("Rejected command because thread-pool queueSize is at rejection threshold."); } ... define Callable that performs executeCommand() ... // submit the work to the thread-pool return threadPool.submit(command); } catch (RejectedExecutionException e) { circuitBreaker.markThreadPoolRejection(); // rejected so return fallback return getFallback(); } Separate Threads: Limited Concurrency if (!threadPool.isQueueSpaceAvailable()) { throw new RejectedExecutionException } } catch (RejectedExecutionException e) { return getFallback(); } 16

Separate Threads: Timeout public K get() throws CancellationException, InterruptedException, ExecutionException { try { long timeout = getCircuitBreaker().getCommandTimeoutInMilliseconds(); return get(timeout, TimeUnit.MILLISECONDS); } catch (TimeoutException e) { // report timeout failure circuitBreaker.markTimeout( System.currentTimeMillis() - startTime); // retrieve the fallback return getFallback(); } } Override of Future.get() 17

Separate Threads: Timeout public K get() throws CancellationException, InterruptedException, ExecutionException { try { long timeout = getCircuitBreaker().getCommandTimeoutInMilliseconds(); return get(timeout, TimeUnit.MILLISECONDS); } catch (TimeoutException e) { // report timeout failure circuitBreaker.markTimeout( System.currentTimeMillis() - startTime); // retrieve the fallback return getFallback(); } } Override of Future.get() try { return get(timeout, TimeUnit.MILLISECONDS); } catch (TimeoutException e) { } } 18

Separate Threads: Timeout public K get() throws CancellationException, InterruptedException, ExecutionException { try { long timeout = getCircuitBreaker().getCommandTimeoutInMilliseconds(); return get(timeout, TimeUnit.MILLISECONDS); } catch (TimeoutException e) { // report timeout failure circuitBreaker.markTimeout( System.currentTimeMillis() - startTime); // retrieve the fallback return getFallback(); } } Override of Future.get() try { return get(timeout, TimeUnit.MILLISECONDS); } catch (TimeoutException e) { return getFallback(); } } 19

Options Aggressive Network Timeouts Semaphores (Tryable) Separate Threads Circuit Breaker 20

if (circuitBreaker.allowRequest()) { return executeCommand(); } else { // short-circuit and go directly to fallback circuitBreaker.markShortCircuited(); return getFallback(); } Circuit Breaker 21

if (circuitBreaker.allowRequest()) { return executeCommand(); } else { // short-circuit and go directly to fallback circuitBreaker.markShortCircuited(); return getFallback(); } Circuit Breaker if (circuitBreaker.allowRequest()) { } else { } 22

if (circuitBreaker.allowRequest()) { return executeCommand(); } else { // short-circuit and go directly to fallback circuitBreaker.markShortCircuited(); return getFallback(); } Circuit Breaker if (circuitBreaker.allowRequest()) { } else { return getFallback(); } 23

Netflix uses all 4 in combination 24

25

Tryable semaphores for “trusted” clients and fallbacks Separate threads for “untrusted” clients Aggressive timeouts on threads and network calls to “give up and move on” Circuit breakers as the “release valve” 26

27

28

29

Benefits of Separate Threads Protection from client libraries Lower risk to accept new/updated clients Quick recovery from failure Client misconfiguration Client service performance characteristic changes Built-in concurrency 30

Drawbacks of Separate Threads Some computational overhead Load on machine can be pushed too far ... Benefits outweigh drawbacks when clients are “untrusted” 31

32

Visualizing Circuits in Realtime (generally sub-second latency) Video available at https://vimeo.com/33576628 33

Rolling 10 second counter – 1 second granularity Median Mean 90th 99th 99.5th Latent Error Timeout Rejected Error Percentage (error+timeout+rejected)/ (success+latent success+error+timeout+rejected). 34

Netflix DependencyCommand Implementation 35

Netflix DependencyCommand Implementation 36

Netflix DependencyCommand Implementation 37

Netflix DependencyCommand Implementation 38

Netflix DependencyCommand Implementation 39

Netflix DependencyCommand Implementation 40

Netflix DependencyCommand Implementation Fallbacks Cache Eventual Consistency Stubbed Data Empty Response 41

Netflix DependencyCommand Implementation 42

Netflix DependencyCommand Implementation 43

Rolling Number Realtime Stats and Decision Making 44

Request Collapsing Take advantage of resiliency to improve efficiency 45

Request Collapsing Take advantage of resiliency to improve efficiency 46

47

Fail fast. Fail silent. Fallback. Shed load. 48

Questions & More Information Fault Tolerance in a High Volume, Distributed System http://techblog.netflix.com/2012/02/fault-tolerance-in-high-volume.html Making the Netflix API More Resilient http://techblog.netflix.com/2011/12/making-netflix-api-more-resilient.html Ben Christensen @benjchristensen http://www.linkedin.com/in/benjchristensen 49