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Zed File #Z-4902: Spanner Spikes

December 7, 2023|4 min read

Zed File # Z-4902: Spanner Spikes

💼 Case facts
Customer: [redacted] // Product: AuthZed Dedicated // Datastore: Spanner

A storm was brewing at AuthZed customer [redacted]- as their traffic grew, so did the number of permission requests, driving check latencies up to a full second at the 95th percentile. This spike in latency degraded the client system's performance - it was time to call in the experts.

AuthZed special agents were on the case.

Read on to discover how our agents used AuthZed’s extensive extant observability toolkit and quickly rolled out additional tooling and optimizations to find the solution among a complex myriad of contributing factors.

Bonus - all improvements are now available in SpiceDB v1.28!

First aid

As an emergency stopgap, AuthZed immediately scaled up resources and tuned Spanner’s configuration, improving overall performance. However, latencies were still spiking every few hours, indicating the system was not yet fully stable. With these initial measures in place, the team started investigating.

Dispatch deduplication

Agent V knew he had to first check the customer’s schema - it might reveal complex permissions that could lead to many subproblems being dispatched. With enough capacity, the system should be capable to handle it, but customer’s cluster was relatively small.

The venerable manual of Agent Operations, the Zanzibar paper, described a clever strategy to reduce fan out of complex permissions: request deduplication. Authzed Agent X introduced improvements in the SpiceDB dispatch with a deduplication middleware that yielded a 30% reduction in dispatched requests, which in turn, led to a proportional reduction in database usage. Despite this notable overall improvement in system performance, persistent latency spikes suggested there still were other underlying issues to address.

Tracking consistency

Initial investigation efforts were hindered due to insufficient observability, leaving key data shrouded in mystery. The situation improved when Agent J improved SpiceDB’s request consistency middleware to produce metrics describing the requested consistency level. This tool uncovered a correlation between the latency spikes and increases in ' fully_consistent' traffic, providing a new lead - one that would crack the case.

Schema spotting

Pursuing the lead, Agent V knew the team was onto something- they just needed more data. He set to improve the readability of OpenTelemetry traces across the dispatch graph and to expose additional metadata, particularly schema traversal details. Direct tracking of Spanner server-side query latency posed a challenge, as using QueryWithStats API method increased Spanner’s CPU usage significantly. Undeterred, Agent V successfully integrated Spanner Client metrics with OpenTelemetry and exposed the metrics through SpiceDB’s Prometheus endpoint. And to add the last bit of information missing, Agent V exposed request and response payloads via new CLI flags.

The solution

Newly equipped with observability data, the team utilized Google Cloud Platform’s log aggregator to locate the timing and source of the latency spikes. They discovered that [redacted] was intermittently issuing up to 800 CheckPermission calls within milliseconds, overwhelming the processing capacity of the provisioned Spanner units.

As it turned out, the database simply had insufficient capacity to handle these spikes.

A crucial log entry showed a single user making over 600 requests within 250 milliseconds for the same permission check, indicating inefficient ACL filtering that fanned out individual checks too quickly for the allocated capacity, all of which is more efficiently computed with BulkCheck or LookupResources.

AuthZed advised the customer to move from fanout CheckPermission calls to BulkCheck, which led to significant latency reduction and improved cache efficiency. As a result, the SpiceDB cluster has achieved stability, eliminating the problematic latency spikes and enhancing the performance of the Permissions System.

These improvements are now available to everyone in SpiceDB v1.28!

Additional Reading

If you’re interested in learning more about Authorization and Google Zanzibar, we recommend reading the following posts:

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