Regional and multi-region configurations

This page describes instance configurations and the two types of instance configurations that Cloud Spanner offers: regional configurations and multi-region configurations. It also describes the differences and trade-offs between regional and multi-region configurations.

Instance configurations

An instance configuration defines the geographic placement and replication of the databases in that instance. When you create an instance, you must configure it as either regional (all the resources are contained within a single Google Cloud region) or multi-region (the resources span more than one region). You make this choice by selecting an instance configuration, which determines where your data is stored for that instance.

The instance configurations with fixed regions and replication topologies are referred to as base instance configurations. You can create custom instance configurations and add additional optional read-only replicas. Note that you cannot change the replication topology of base instance configurations. For more information, see Read-only replicas.

You can also move your instance from any instance configuration to any other regional or multi-region instance configuration.

Regional configurations

Google Cloud services are available in locations across North America, South America, Europe, Asia, and Australia. If your users and services are located within a single region, choose a regional instance configuration for the lowest-latency reads and writes.

For any base regional configuration, Spanner maintains three read-write replicas, each within a different Google Cloud zone in that region. Each read-write replica contains a full copy of your operational database that is able to serve read-write and read-only requests. Spanner uses replicas in different zones so that if a single-zone failure occurs, your database remains available.

Available configurations

Spanner offers the following base regional instance configurations:

	Base Configuration Name	Region Description	Optional Region
Americas
	northamerica-northeast1	Montréal Low CO2
	northamerica-northeast2	Toronto Low CO2
	southamerica-east1	São Paulo Low CO2
	southamerica-west1	Santiago
	us-central1	Iowa Low CO2	Read-only: asia-northeast1 1R asia-south1 1R europe-west2 1R europe-west9 1R
	us-east1	South Carolina	Read-only: us-central1 1R us-west1 1R
	us-east4	Northern Virginia
	us-east5	Columbus
	us-south1	Dallas
	us-west1	Oregon Low CO2
	us-west2	Los Angeles
	us-west3	Salt Lake City
	us-west4	Las Vegas
Europe
	europe-central2	Warsaw
	europe-north1	Finland Low CO2
	europe-southwest1	Madrid Low CO2
	europe-west1	Belgium Low CO2
	europe-west2	London
	europe-west3	Frankfurt
	europe-west4	Netherlands
	europe-west6	Zürich Low CO2
	europe-west8	Milan
	europe-west9	Paris Low CO2
	europe-west12	Turin
Asia Pacific
	asia-east1	Taiwan
	asia-east2	Hong Kong
	asia-northeast1	Tokyo
	asia-northeast2	Osaka
	asia-northeast3	Seoul
	asia-south1	Mumbai
	asia-south2	Delhi
	asia-southeast1	Singapore
	asia-southeast2	Jakarta
	australia-southeast1	Sydney
	australia-southeast2	Melbourne
Middle East
	me-central1	Doha
	me-west1	Tel Aviv

Replication

Base regional configurations contain three read-write replicas. Every Spanner mutation requires a write quorum that's composed of a majority of voting replicas. Write quorums are formed from two out of the three replicas in regional configurations. For more information about leader regions and voting replicas, see Replication.

You can create a custom regional instance configuration and add optional read-only replicas. Read-only replicas can help scale reads and support low latency stale reads. These read-only replicas do not take part in the write quorums, and they don't affect the Spanner >= 99.99% SLA for regional instances. For more information, see Read-only replicas.

Performance best practices for regional configurations

For optimal performance, follow these best practices:

• Design a schema that prevents hotspots and other performance issues.
• Place critical compute resources within the same region as your Spanner instance.
• Provision enough compute capacity to keep high priority total CPU utilization under 65%.
• For the amount of throughput per Spanner node, see performance for regional configurations.

Multi-region configurations

Spanner regional configurations replicate data between multiple zones within a single region. However, a regional configuration might not be optimal in the following situations:

• If your application often needs to read data from multiple geographic locations (for example, to serve data to users in both North America and Asia)
• If your writes originate from a different location than your reads (for example, if you have large write workloads in North America and large read workloads in Europe)

Multi-region configurations have other benefits such as:

• Maintain availability in the case of regional failures.
• Provide higher availability and SLAs than regional configurations.
• Serve writes from multiple regions.

Multi-region configurations allow you to replicate the database's data not just in multiple zones, but in multiple zones across multiple regions, as defined by the instance configuration. These additional replicas enable you to read data with low latency from multiple locations close to or within the regions in the configuration. There are trade-offs though, because in a multi-region configuration, the quorum (read-write) replicas are spread across more than one region. Hence, they can incur additional network latency when these replicas communicate with each other to vote on writes. In other words, multi-region configurations enable your application to achieve faster reads in more places at the cost of a small increase in write latency.

Available configurations

Spanner offers the following base multi-region instance configurations:

One continent

Base Configuration Name	Location	Read-Write Regions	Read-Only Regions	Witness Region	Optional Region
asia1	Asia	Tokyo: asia-northeast1 L,2R Osaka: asia-northeast2 2R	None	Seoul: asia-northeast3
eur3	Europe	Belgium: europe-west1 L,2R Netherlands: europe-west4 2R	None	Finland: europe-north1
eur5	Europe	London: europe-west2 L,2R Belgium: europe-west1 2R	None	Netherlands: europe-west4
eur6	Europe	Netherlands: europe-west4 L,2R Frankfurt: europe-west3 2R	None	Zurich: europe-west6	Read-only: us-east1 2R
nam3	North America	Northern Virginia: us-east4 L,2R South Carolina: us-east1 2R	None	Iowa: us-central1	Read-only: us-west2 1R
nam6	North America	Iowa: us-central1 L,2R South Carolina: us-east1 2R	Oregon: us-west1 1R Los Angeles: us-west2 1R	Oklahoma: us-central2
nam7	North America	Iowa: us-central1 L,2R Northern Virginia: us-east4 2R	None	Oklahoma: us-central2	Read-only: us-east1 2R us-south1 1R europe-west1 2R
nam8	North America	Los Angeles: us-west2 L,2R Oregon: us-west1 2R	None	Salt Lake City: us-west3
nam9	North America	Northern Virginia: us-east4 L,2R Iowa: us-central1 2R	Oregon: us-west1 2R	South Carolina: us-east1
nam10	North America	Iowa: us-central1 L,2R Salt Lake City: us-west3 2R	None	Oklahoma: us-central2
nam11	North America	Iowa: us-central1 L,2R South Carolina: us-east1 2R	None	Oklahoma: us-central2	Read-only: us-west1 1R
nam12	North America	Iowa: us-central1 L,2R Northern Virginia: us-east4 2R	Oregon: us-west1 2R	Oklahoma: us-central2
nam13	North America	Oklahoma: us-central2 L,2R Iowa: us-central1 2R	None	Salt Lake City: us-west3
nam14	North America	Northern Virginia: us-east4 L,2R Montréal: northamerica-northeast1 2R	None	South Carolina: us-east1
nam15	North America	Dallas: us-south1 L,2R Northern Virginia: us-east4 2R	None	Iowa: us-central1

Three continents

Base Configuration Name	Locations	Read-Write Regions	Read-Only Regions	Witness Region
nam-eur-asia1	North America Europe Asia	Iowa: us-central1 L,2R Oklahoma: us-central2 2R	Belgium: europe-west1 2R Taiwan: asia-east1 2R	South Carolina: us-east1
nam-eur-asia3	North America Europe Asia	Iowa: us-central1 L,2R South Carolina: us-east1 2R	Belgium: europe-west1 1R Netherlands: europe-west4 1R Taiwan: asia-east1 2R	Oklahoma: us-central2

• L: default leader region. See also Modify the leader region of a database.

• 1R: 1 replica in the region

• 2R: 2 replicas in the region

Benefits

Multi-region instances offer these primary benefits:

• 99.999% availability, which is greater than the 99.99% availability that Spanner regional configurations provide.

• Data distribution: Spanner automatically replicates your data between regions with strong consistency guarantees. This allows your data to be stored where it's used, which can reduce latency and improve the user experience.

• External consistency: Even though Spanner replicates across geographically distant locations, you can still use Spanner as if it were a database running on a single machine. Transactions are guaranteed to be serializable, and the order of transactions within the database is the same as the order in which clients observe the transactions to have been committed. External consistency is a stronger guarantee than "strong consistency," which is offered by some other products. Read more about this property in TrueTime and external consistency.

Replication

Each base multi-region configuration contains two regions that are designated as read-write regions, each of which contains two read-write replicas. One of these read-write regions is designated as the default leader region, which means that it contains your database's leader replicas. Spanner also places a witness replica in a third region called a witness region.

Each time a client issues a mutation to your database, a write quorum forms, consisting of one of the replicas from the default leader region and any two of the additional four voting replicas. (The quorum could be formed by replicas from two or three of the regions that make up your configuration, depending on which other replicas participate in the vote.) In addition to these five voting replicas, some base multi-region configurations contain read-only replicas for serving low-latency reads. The regions that contain read-only replicas are called read-only regions.

In general, the voting regions in a multi-region configuration are placed geographically close—less than a thousand miles apart—to form a low-latency quorum that enables fast writes (learn more). However, the regions are still far enough apart—typically, at least a few hundred miles—to avoid coordinated failures.

You can create a custom multi-region instance configuration with optional read-only replicas. Any custom read-only replicas you create cannot be included in write quorums. For more information, see Read-only replicas.

Note: if you are interested in more detailed information about how data is replicated in Spanner, and about the different types of replicas and their roles in reads and writes, see Replication.

Performance best practices for multi-region configurations

For optimal performance, follow these best practices:

• Design a schema that prevents hotspots and other performance issues.
• For optimal write latency, place compute resources for write-heavy workloads within or close to the default leader region.
• For optimal read performance outside of the default leader region, use staleness of at least 15 seconds.
• To avoid single-region dependency for your workloads, place critical compute resources in at least two regions. A good option is to place them next to the two different read-write regions so that any single region outage will not impact all of your application.
• Provision enough compute capacity to keep high priority total CPU utilization under 45% in each region.
• For the amount of throughput per Spanner node, see performance for multi-region configurations.

Region types

Read-write regions

Each multi-region configuration contains two read-write regions, each of which contains two read-write replicas.

One of these read-write regions is designated the default leader region. A leader is selected from the replicas in the default leader region for each split. In the event of a leader replica failure, the other replica in the default leader region automatically assumes leadership. In fact, leaders run health checks on themselves and can preemptively give up leadership if they detect they are unhealthy. In most cases, when the default leader region returns to a healthy state, it automatically re-assumes the leadership.

Writes are first processed in the default leader region. You can monitor the percentage of replicas within a given region by using the instance/leader_percentage_by_region monitoring metric. For more information, see Spanner metrics.

The second read-write region contains additional replicas that serve reads and participate in voting to commit writes. These additional replicas in the second read-write region are eligible to be leaders. In the unlikely event of the loss of all replicas in the default leader region, new leader replicas are chosen from the second read-write region.

Key Point: Place the bulk of your read and write workloads in the default leader region. In the event of the loss of a default leader region, read and write workloads are fastest from the second read-write region.

You can configure the leader region of a database by following the instructions at Change the leader region of a database. For more information, see Configure the default leader region.

Read-only regions

Read-only regions contain read-only replicas, which can serve low-latency reads to clients that are outside of the read-write regions. Read-only replicas maintain a full copy of your data, which is replicated from read-write replicas. They do not participate in voting to commit writes so they never contribute to any write latency.

Some base multi-region configurations contain read-only replicas. You can also create a custom instance configuration and add read-only replicas to your custom instance configurations to scale reads and support low latency stale reads. All read-only replicas are subject to compute capacity and database storage costs. Furthermore, adding read-only replicas to an instance configuration doesn't change the Spanner SLAs of the instance configuration. For more information, see Read-only replicas.

Key Point: Place additional read workloads in read-only regions.

Witness regions

A witness region contains a witness replica, which is used to vote on writes. Witnesses become important in the rare event that the read-write regions become unavailable.

Key Point: You can think of the witness region as a system-configured region for achieving write quorums.

Move an instance

You can move your Spanner instance from any instance configuration to any other instance configuration, including between regional and multi-region configurations. Moving your instance does not cause downtime, and Spanner continues to provide the usual transaction guarantees, including strong consistency, during the move.

To learn more about Spanner instance move, see Move an instance.

Configure the default leader region

To change the location of your database's default leader region to be closer to connecting clients to reduce application latency, you can change the leader region for any Spanner instance that uses a multi-region configuration. For instructions on changing the location of the leader region, see Change the leader region of a database. The only regions eligible to become the default leader region for your database are the read-write regions in your multi-region configuration.

The leader region is responsible for handling all database writes, therefore if most of your traffic comes from one geographic region, you can move it to that region to reduce latency. Updating the default leader region is cheap and does not involve any data moves. The new value takes a few minutes to take effect.

Note: In the event that the default leader region fails or is not available, Spanner automatically places your database's leader replicas in the second read-write region of your instance configuration, ensuring high availability and no impact to your application.

Changing the default leader region is a schema change, which uses a long-running operation. If needed, you can Get the status of the long-running operation.

Trade-offs: regional versus multi-region configurations

Configuration	Availability	Latency	Cost	Data Locality
Regional	99.99%	Lower write latencies within region.	Lower cost; see pricing.	Enables geographic data governance.
Multi-region	99.999%	Lower read latencies from multiple geographic regions.	Higher cost; see pricing.	Distributes data across multiple regions within the configuration.

What's next

• Learn how to create a Spanner instance.
• Learn more about Google Cloud regions and zones.