A Cloud Spanner instance is a set of allocated resources that supports the Spanner databases it contains. When you create one, you make three choices: the edition, the geographic configuration, and the compute capacity. Those three settings together decide where your data lives, how it is replicated, how much throughput and storage you have, and what availability you can expect. The Professional Cloud Database Engineer exam tends to test whether you can keep these choices straight, and in particular which geographic configurations are even available to you given the edition you pick.
Compute capacity defines the amount of compute and storage resources available to the databases within an instance. It is the fundamental building block for performance. You express it in one of two units, either a number of nodes or a number of processing units, and the two are tied together: one node is equivalent to 1000 processing units. Each unit of capacity represents a set amount of throughput and storage. When you need more performance, you add capacity, which increases the total pool of resources the databases in the instance can draw on. Processing units give you finer-grained control at smaller scales, while nodes describe the same resources in larger increments.
The geographic configuration determines where your data is physically stored and replicated across Google Cloud's infrastructure. There are three options, and they differ in how far the data is spread and how much redundancy that spread provides.
A regional configuration keeps resources within a single Google Cloud region. A dual-region configuration distributes resources across two regions within the same country, which offers a higher level of redundancy than staying in one region. A multi-region configuration distributes resources across multiple regions, and this is the configuration used to achieve global reach and the highest levels of availability.
The practical reading of this for the exam is to match the configuration to the redundancy and reach a scenario asks for. A workload that needs to survive the loss of a region cannot rely on a regional configuration, because all of its resources sit in that one region. A requirement for the broadest geographic distribution and the highest availability points toward multi-region.
The edition determines which features and configurations are available to you, and this is where the geographic choices get gated. There are three editions, and a meaningful part of the difference between them is which configurations they will let you use and what availability they commit to.
The Standard edition supports regional configuration only and provides a 99.99% availability SLA. The Enterprise edition also supports regional configuration only and a 99.99% availability SLA, and it includes everything in Standard plus additional capabilities. The Enterprise Plus edition is the most robust tier. It supports regional, dual-region, and multi-region configurations and offers up to a 99.999% availability SLA, and it includes everything in the Standard and Enterprise editions plus additional capabilities for mission-critical workloads.
The connection between edition and geographic configuration is the part worth holding onto. Dual-region and multi-region are not available under Standard or Enterprise. If a scenario calls for resources spread across two regions in the same country or across multiple regions, the edition has to be Enterprise Plus. Reading it the other direction, the jump from a 99.99% SLA to up to 99.999% also lands on Enterprise Plus.
When you size up a Spanner question on the Professional Cloud Database Engineer exam, take the three choices in order. Decide the compute capacity the workload needs and remember that nodes and processing units are the same resources expressed at different granularities, with one node equal to 1000 processing units. Decide the geographic configuration from the redundancy and reach the scenario describes, ranging from a single region up through two regions in one country to multiple regions. Then confirm the edition actually supports that configuration, because regional fits any edition while dual-region and multi-region require Enterprise Plus. Keeping the edition-to-configuration dependency in mind is usually what separates the correct answer from options that pair a wide geographic spread with an edition that cannot deliver it.
Our Professional Cloud Database Engineer course covers Spanner instance configuration alongside compute capacity sizing and edition selection, with practice questions that drill these distinctions.
