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1. IL-0279 One failed Redis Cluster replica left only about 1 in 21 calls to Grab's Apollo service succeeding for a minute

   Grab2019

   Symptoms

   On 15 February 2019 a slave node of the AWS ElastiCache Redis Cluster behind Apollo, Grab's driver-side state machine for transport bookings, failed and had to be replaced. For about one minute only roughly 1 in 21 calls to Apollo succeeded, a call failure rate above 95% against the Redis Cluster, and Grab rides were significantly down.

   High error rateStale endpoint routing

   Trigger

   A slave node of Apollo's Redis Cluster failed and needed to be replaced.

   Root cause

   Software defect

   Component

   In-memory cache

   Contributing factor

   Unverified assumption

   Lessons

   Having several replicas did not protect Apollo, because the client side, not the Redis Cluster server, caused the failure. Grab suggests three ways to avoid it: a separate Redis Cluster client used only for pipelining with RouteByLatency set to true, so pipelined reads go through the master nodes when latency is under 1ms, watching master CPU because they take more load; checking the error of each command in a pipeline instead of failing the whole call, so successful queries are not affected; and changing the Go-Redis library so that the pipeline path reloads the nodes' state when an error happens, as defaultProcessor already does.

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