External Backups/Snapshots for Google Cloud Spanner recommends to use queries with timestamp bounds to create snapshots for export. On the bottom of the Timestamp Bounds documentation it states:
Cloud Spanner continuously garbage collects deleted and overwritten data in the background to reclaim storage space. This process is known as version GC. By default, version GC reclaims versions after they are one hour old. Because of this, Cloud Spanner cannot perform reads at a read timestamp more than one hour in the past.
So any export would need to complete within an hour. A single reader (i.e. select * from table; using timestamp X) would not be able to export the entire table within an hour.
How can multiple parallel readers be implemented in spanner?
Note: It is mentioned in one of the comments that support for Apache Beam is coming, but it looks like that uses a single reader:
/** A simplest read function implementation. Parallelism support is coming. */
Is there a way to do the parallel reader that beam requires today using exising APIs? Or will Beam need to use something that isn't released yet on google spanner?