Number Service

Highly available sequential number generator backed by Cosmos DB with guaranteed uniqueness.

Free numbers here!

PUT https://numberservice-aue.azurewebsites.net/api/numbers/free

Requirements

• Each request for a new number must return a number that is unique and one greater than the last number generated (for all clients).
• Number should be able to be seeded (start at 10,000 for example), but only the first time it is generated
• Number service must be highly available with an uptime of around 99.99%, or less than 5 minutes of total downtime per month.
• RTO = minutes
• RPO = 0

Phase 1

Number generation requires a strong write. It is not possible to have a strong write and multiple write regions, even with a consensus algorithm (not even Cosmos DB offers strong multi-region writes).

The Google Chubby lock service is actually a single write region service. It orchestrates a master¹. The read replicas (which may be thousands of miles away) are mirrors and are eventually consistent. This is OK because chubby is essentially a consistent cache lock service.

Phase 1 fulfils the sequential guarantee requirement with Azure Cosmos DB and Functions at a cost of ~NZ$75 per month for 28 numbers per second.

Analysis

• 1 Cosmos DB Region. Zone Redundant.
• 1 Azure Functions (consumption) region
• 99.94% uptime. No region failover
• RPO and RTO for Zone failure = 0
• RPO for Region failure could be hours and RTO for Region failure could be days, assuming the Region never recovers. However an RPO of 0 and an RTO of hours is more likely IMO.
• ~14 RU/s per number
• Single partition (per key)
• Max RU/s per partition = 10,000, so max throughput is 625 per second
• At 400 RU/s provision, max throughput is 28 per second.
• Highest number currently supported (I assume) is Javascript Number.MAX_SAFE_INTEGER, which is 9,007,199,254,740,991.
• Stored proc write consistency is strong. If proc can't increment number in atomic operation it will fail with an exception that is thrown to client.
• Read consistency is the default (session). While out of sproc/session reads may not get the latest number, ordering will be consistent. Strong consistency of reads is not a requirement for NumberService.

Costs

🧮 Azure Pricing Calculator estimate

• Cosmos DB, single region (Australia East), ZR, 400 RU/s, 1GB data = NZ$51.21 /month
• Azure Functions, Consumption, Australia East, @28RPS = NZ$23.90 /month

References and links

¹ The Chubby lock service for loosely-coupled distributed systems, section 2.12.