Instance architecture
Quick answer
What sits behind your URL: nodes, database, load balancing, and what that means day to day.
Key takeaways
- Nodes share a database but hold their own caches
- Sub production instances are smaller, so performance results differ
- Semaphores and caches are per node
- Failover is managed by ServiceNow, not by customers
The shape of an instance
An instance is a set of application nodes sharing one database, behind a load balancer. Each node runs the same code and holds its own caches and semaphore pools, which is why a problem can affect one node and not another.
Instance types
Production plus sub production instances for development, test and training. Sub production instances are refreshed from production by cloning, and they are deliberately smaller, so performance tests there are indicative rather than definitive.
What it means for admins
Architecture explains behaviour you will meet often.
- Cache flushes affect one node at a time, so changes can appear inconsistently for a moment
- A long transaction consumes a semaphore on one node, not across the estate
- Scheduled jobs run on whichever node picks them up
- High availability failover happens at the datacenter level and is handled by ServiceNow
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Check your understanding
What do application nodes share?
- A. Caches
- B. The database
- C. Semaphores
- D. Sessions
Show answer
B. The database
Nodes share the database, everything else is local to the node.
Where is high availability handled?
- A. By the customer
- B. By ServiceNow at datacenter level
- C. By the MID server
- D. By the browser
Show answer
B. By ServiceNow at datacenter level
Datacenter failover is part of the service.