How Google Photos is configured.
Spanner is a distributed SQL database management and storage service developed by Google.
It provides features such as global transactions, strongly consistent reads, and automatic multi-site replication and failover.
Cloud Spanner is the only enterprise-grade, globally-distributed, and strongly-consistent database service built for the cloud, specifically to combine the benefits of relational database structure with non-relational horizontal scale.
It is a unique database that combines transactions, SQL queries, and relational structure with the scalability that you typically associate with non-relational or NoSQL databases.
Cloud Spanner instances provide compute and storage in one or more regions. A distributed clock called TrueTime guarantees transactions are strongly consistent even across regions.
Data is automatically "split" for scalability and replicated using a synchronous, Paxos-based scheme for availability.
How does Spanner work?
In the image you see a four-node regional Cloud Spanner instance hosting two databases.
A node is a measure of compute in Spanner. Node servers serve the read and write/commit transaction requests, but they don’t store the data.
node is replicated across three zones in the region. The database storage is also replicated across the three zones.
Nodes in a zone are responsible for reading and writing to the storage in their zone. The data is stored in Google’s underlying Colossus distributed replicated file system. This provides huge advantages when it comes to redistributing load, as the data is not linked to individual nodes.
If a node or a zone fails, the database remains available, being served by the remaining nodes. No manual intervention is needed to maintain availability.
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