Resiliency and Availability Design Patterns for the Cloud

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. AWS Region and availability zones Region Availability zone a Availability zone b Availability zone c data center data center data center 1 or more data centers per AZ 2 or more AZs per region (new regions min 3) data center data center data center data center data center data center

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Availability in parallel Component Availability Downtime X 99% (2-nines) 3 days 15 hours Two X in parallel 99.99% (4-nines) 52 minutes Three X in parallel 99.9999% (6-nines) 31 seconds

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Multi-AZ architecture Region Availability zone a Availability zone b Availability zone c Instances Instances Instances DB Instance DB instance standby Elastic Load Balancing (ELB)

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Multi-AZ architecture X Region Availability zone a Availability zone b Availability zone c Instances Instances Instances DB Instance DB instance standby Elastic Load Balancing (ELB)

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Multi-AZ architecture • Enables fault-tolerant applications • AWS regional services designed to withstand AZ failures • Leveraged by AWS regional services such as Amazon S3, Amazon DynamoDB, Amazon Aurora, Amazon ELBs, etc. Region Availability zone a Availability zone b Availability zone c Instances Instances Instances DB Instance DB instance standby Elastic Load Balancing (ELB)

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Availability zone 1 Auto Scaling group AWS Region Availability zone 2 Auto-scaling for self-healing Elastic Load Balancing (ELB) X

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Process A Process B Process A Process B Synchronous Asynchronous Waiting Working Continues get or fetch result Get result Decoupling with async pattern

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. API: {DO foo} PUT JOB: {JobID: 0001, Task: DO foo} API: {JobID: 0001} GET JOB: {JobID: 0001, Task: DO foo} {JobID: 0001, Result: bar} Cache node Worker Instance Worker Instance Queue/Streaming API Instance API Instance API Instance

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Push Notification User Worker Instance Worker Instance API Instance API Instance Cache node Fetch results API Instance Queue/Streaming

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Degrade & prioritize traffic with queues Worker Instance Worker Instance API Instance API Instance API Instance High Priority Queue Low Priority Queue

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Read / Write separation DB Instance DB instance read replica DB instance read replica DB instance read replica Instance Instance Instance Supports degradation through Read-Only mode

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Database Federation Users DB Products DB Instance Instance Instance DB Instance DB instance read replica DB Instance DB instance read replica

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Database Sharding User ShardID 002345 A 002346 B 002347 C 002348 B 002349 A C B A Instance Instance Instance DB Instance DB instance read replica DB Instance DB instance read replica DB Instance DB instance read replica

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Users App DB Conn Pool INSERT INSERT INSERT INSERT What happens if the DB “slows down”? Timeout client side Timeout backend side ? ?

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. User 1 App DB Conn Pool INSERT Timeout client side = 10s Timeout backend side = default = Infinite Retry INSERT Retry INSERT ERROR: Failed to get connection from pool Retry

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https://dev.mysql.com/doc/connector-j/5.1/en/connector-j-reference-configuration-properties.html

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. @timeout_decorator.timeout(5, timeout_exception=StopIteration) def timed_get(url): return requests.get(url) https://pypi.org/project/timeout-decorator/

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. How else could we have prevented the error? User 1 DB Conn Pool INSERT Retry INSERT Retry INSERT Retry ERROR: Failed to get connection from pool

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. User 1 DB Conn Pool INSERT Timeout client side = 10s Timeout backend side = 10s Wait 2s before Retry INSERT INSERT Wait 4s before Retry Wait 8s before Retry Wait 16s before Retry Backing off between retries Releasing connections Backoff

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. No jitter With jitter https://aws.amazon.com/blogs/architecture/exponential-backoff-and-jitter/ Simple Exponential Backoff is not enough: Add Jitter

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Example: add jitter 0-1000ms MAX_TRIES = 12 def get_item(self, url, n=1): try: res = requests.get(url) except: if n > MAX_TRIES: return None n += 1 time.sleep((2 ** n) + (random.randint(0, 1000) / 1000.0)) return self.get_item(url, n) else: return res

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Circuit Breaker • Wrap a protected function call in a circuit breaker object, which monitors for failures. • If failures reach a certain threshold, the circuit breaker trips. Producer Circuit Breaker Consumer Connection Monitoring Timeouts Breaking Circuit

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Auto Scaling group Service A Availability zone 1 Auto Scaling group AWS Region Service A Availability zone 2 Service B Service B database Email Probing for health Cluster

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Don’t be overly optimistic and take on more than you can. Find an operational metric to reject what you cannot take in. Favor cached and static content Prioritize ELB health check (shallow) pings In an overload situation you have precious resources, do not let any of it go to waste. Load Shedding

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Shuffle sharding Nodes = 100 Shard size = 5 Combinations = 75 million! Overlap % customers 0 77% 1 21% 2 1.8% 3 0.06% 4 0.0006% 5 0.0000013%

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. “Chaos Engineering is the discipline of experimenting on a distributed system in order to build confidence in the system’s capability to withstand turbulent conditions in production.” http://principlesofchaos.org

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© 2021, Amazon Web Services, Inc. or its affiliates. All rights reserved. Failure injection Start small & build confidence •Application level •Host failure •Resource attacks (CPU, memory, …) •Network attacks (dependencies, latency, …) •Region attacks