Export databases from Spanner to Avro

This page describes how to export Spanner databases with the Google Cloud console. To export a Spanner database using the REST API or the gcloud spanner command-line tool, complete the steps in the Before you begin section on this page, and then see the detailed instructions in Spanner to Cloud Storage Avro in the Dataflow documentation. The export process uses Dataflow and writes data to a folder in a Cloud Storage bucket. The resulting folder contains a set of Avro files and JSON manifest files.

Before you begin

To export a Spanner database, first you need to enable the Spanner, Cloud Storage, Compute Engine, and Dataflow APIs:

Enable the APIs

You also need enough quota and the required IAM permissions.

Quota requirements

The quota requirements for export jobs are as follows:

  • Spanner: No additional compute capacity is required to export a database, though you might need to add more compute capacity so that your job finishes in a reasonable amount of time. See Optimize jobs for more details.
  • Cloud Storage: To export, you must create a bucket for your exported files if you do not already have one. You can do this in the Google Cloud console, either through the Cloud Storage page or while creating your export through the Spanner page. You do not need to set a size for your bucket.
  • Dataflow: Export jobs are subject to the same CPU, disk usage, and IP address Compute Engine quotas as other Dataflow jobs.
  • Compute Engine: Before running your export job, you must set up initial quotas for Compute Engine, which Dataflow uses. These quotas represent the maximum number of resources that you allow Dataflow to use for your job. Recommended starting values are:

    • CPUs: 200
    • In-use IP addresses: 200
    • Standard persistent disk: 50 TB

    Generally, you don't have to make any other adjustments. Dataflow provides autoscaling so that you only pay for the actual resources used during the export. If your job can make use of more resources, the Dataflow UI displays a warning icon. The job should finish even if there is a warning icon.

IAM requirements

To export a database, you also need to have IAM roles with sufficient permissions to use all of the services involved in an export job. For information on granting roles and permissions, see Apply IAM roles.

To export a database, you need the following roles:

To use the independent compute resources of Spanner Data Boost during an export, you also need the spanner.databases.useDataBoost IAM permission. For more information, see Data Boost overview.

Export a database

After you satisfy the quota and IAM requirements described previously, you can export an existing Spanner database.

To export your Spanner database to a Cloud Storage bucket, follow these steps.

  1. Go to the Spanner Instances page.

    Go to the instances page

  2. Click the name of the instance that contains your database.

  3. Click the Import/Export menu item in the left pane and then click the Export button.

  4. Under Choose where to store your export, click Browse.

  5. If you do not already have a Cloud Storage bucket for your export:

    1. Click New bucket Screenshot of new bucket UI element.
    2. Enter a name for your bucket. Bucket names must be unique across Cloud Storage.
    3. Select a default storage class and location, then click Create.
    4. Click your bucket to select it.

    If you already have a bucket, either select the bucket from the initial list or click Search Screenshot of search UI element to filter the list, then click your bucket to select it.

  6. Click Select.

  7. Select the database that you want to export in the Choose a database to export drop-down menu.

  8. Optional: To export your database from an earlier point in time, check the box and enter a timestamp.

  9. Select a region in the Choose a region for the export job drop-down menu.

  10. Optional: To encrypt the Dataflow pipeline state with a customer-managed encryption key:

    1. Click Show encryption options.
    2. Select Use a customer-managed encryption key (CMEK).
    3. Select your key from the drop-down list.

    This option does not affect the destination Cloud Storage bucket-level encryption. To enable CMEK for your Cloud Storage bucket, refer to Use CMEK with Cloud Storage.

  11. Optional: To export using Spanner Data Boost, select the Use Spanner Data Boost checkbox. For more information, see Data Boost overview.

  12. Select the checkbox under Confirm charges to acknowledge that there are charges in addition to those incurred by your existing Spanner instance.

  13. Click Export.

    The Google Cloud console displays the Database Import/Export page, which now shows a line item for your export job in the import/export jobs list, including the job's elapsed time:

    Screenshot of in-progress job

When the job finishes or terminates, the status is updated in the import/export list. If the job succeeded, the status Succeeded is displayed:

Export job success message

If the job failed, the status Failed is displayed:

Export job failure message

To view the details of the Dataflow operation for your job, click on the job's name in the Dataflow job name column.

If your job fails, check the job's Dataflow logs for error details.

To avoid Cloud Storage charges for files your failed export job created, delete the folder and its files. See View your export for information on how to find the folder.

A note on exporting generated columns and change streams

The values in a stored generated column are not exported. The column definition is exported to the Avro schema as a null-type record field, with the column definition as custom properties of the field. Until the backfill operation of a newly added generated column completes, the generated column is ignored as if it doesn't exist in the schema.

Change streams exported as Avro files contain only the schema of the change streams, and not any data change records.

A note on exporting sequences

Sequences (GoogleSQL, PostgreSQL) are schema objects that you use to generate unique integer values. Spanner exports each of the schema object to the Avro schema as a record field, with its sequence kind, skipped range, and counter as properties of the field. Note that to prevent a sequence from being reset and generating duplicate values after import, during schema export, the GET_INTERNAL_SEQUENCE_STATE() (GoogleSQL, PostgreSQL) function captures the sequence counter. Spanner adds a buffer of 1000 to the counter, and writes the new counter value to the record field. This approach avoids duplicate value errors that might happen after import. If there are more writes to the source database during data export, you should adjust the actual sequence counter by using the ALTER SEQUENCE (GoogleSQL, PostgreSQL) statement.

At import, the sequence starts from this new counter instead of the counter found in the schema. Alternatively, you can use the ALTER SEQUENCE (GoogleSQL, PostgreSQL) statement to update the sequence with a new counter.

View your export in Cloud Storage

To view the folder that contains your exported database in the Google Cloud console, navigate to the Cloud Storage browser and choose the bucket you previously selected:

Go to the Cloud Storage browser

The bucket now contains a folder with the exported database inside. The folder name begins with your instance's ID, database name, and the timestamp of your export job. The folder contains:

  • A spanner-export.json file
  • A TableName-manifest.json file for each table in the database you exported.
  • One or more TableName.avro-#####-of-##### files. The first number in the extension .avro-#####-of-##### represents the index of the Avro file, starting at zero, and the second represents the number of Avro files generated for each table.

    For example, Songs.avro-00001-of-00002 is the second of two files that contain the data for the Songs table.

  • A ChangeStreamName-manifest.json file for each change stream in the database you exported.

  • A ChangeStreamName.avro-00000-of-00001 file for each change stream. This file contains empty data with only the Avro schema of the change stream.

Choose a region for your import job

You might want to choose a different region based on the location of your Cloud Storage bucket. To avoid outbound data transfer charges, choose a region that matches your Cloud Storage bucket's location.

  • If your Cloud Storage bucket location is a region, you can take advantage of free network usage by choosing the same region for your import job, assuming that region is available.

  • If your Cloud Storage bucket location is a dual-region, you can take advantage of free network usage by choosing one of the two regions that make up the dual-region for your import job, assuming one of the regions is available.

  • If a co-located region is not available for your import job, or if your Cloud Storage bucket location is a multi-region, outbound data transfer charges apply. Refer to Cloud Storage data transfer pricing to choose a region that incurs the lowest data transfer charges.

Export a subset of tables

If you want to export only the data from certain tables, and not the entire database, then you can specify those tables during export. In this case, Spanner exports the database's entire schema, including the data of tables you specify, and leaving all other tables present but empty in the exported file.

You can specify a subset of tables to export using either the Dataflow page in Google Cloud console or the command line. (The Spanner page doesn't provide this action.)

If you export the data of a table that is the child of another table, then you should export its parent table's data as well. If parents are not exported, then the export job fails.

To export a subset of tables, start the export using Dataflow's Spanner to Cloud Storage Avro template, and specify the tables using either the Dataflow page in Google Cloud console or the Google Cloud CLI, as described below:

Google Cloud console

If you are using the Dataflow page in Google Cloud console, the Cloud Spanner Table name(s) parameter is located in the Optional parameters section of the Create job from template page. Multiple tables can be specified in a comma-separated format.

gcloud CLI

If you are using the Google Cloud CLI, use the tableNames argument to specify the table. For example:

gcloud dataflow jobs run my-export-job \
--gcs-location='gs://dataflow-templates/latest/Cloud_Spanner_to_GCS_Avro' \
--region=us-central1 \
--parameters='instanceId=test-instance,databaseId=example-db,tableNames=table1,outputDir=gs://my-gcs-bucket' \

Specifying multiple tables in gcloud requires dictionary-type argument escaping. The following example uses '|' as the escape character:

 gcloud dataflow jobs run my-export-job \
--gcs-location='gs://dataflow-templates/latest/Cloud_Spanner_to_GCS_Avro' \
--region=us-central1 \
--parameters='^|^instanceId=test-instance|databaseId=example-db|tableNames=table1,table2|outputDir=gs://my-gcs-bucket' \

The shouldExportRelatedTables parameter is a convenient option to automatically export all parent tables of the desired tables. For example, in this schema hierarchy with tables Singers, Albums and Songs, we only need to specify Songs. The shouldExportRelatedTables option will also export Singers and Albums because Songs is a descendant of both.

gcloud dataflow jobs run my-export-job \
--gcs-location='gs://dataflow-templates/latest/Cloud_Spanner_to_GCS_Avro' \
--region=us-central1 \
--parameters='instanceId=test-instance,databaseId=example-db,tableNames=Songs,shouldExportRelatedTables=true,outputDir=gs://my-gcs-bucket' \

View or troubleshoot jobs in the Dataflow UI

After you start an export job, you can view details of the job, including logs, in the Dataflow section of the Google Cloud console.

View Dataflow job details

To see details for any import/export jobs that you ran within the last week, including any jobs currently running:

  1. Navigate to the Database overview page for the database.
  2. Click the Import/Export left pane menu item. The database Import/Export page displays a list of recent jobs.
  3. In the database Import/Export page, click the job name in the Dataflow job name column:

    In-progress job status message

    The Google Cloud console displays details of the Dataflow job.

To view a job that you ran more than one week ago:

  1. Go to the Dataflow jobs page in the Google Cloud console.

    Go to the jobs page

  2. Find your job in the list, then click its name.

    The Google Cloud console displays details of the Dataflow job.

View Dataflow logs for your job

To view a Dataflow job's logs, navigate to the job's details page as described above, then click Logs to the right of the job's name.

If a job fails, look for errors in the logs. If there are errors, the error count displays next to Logs:

Error count example next to Logs button

To view job errors:

  1. Click on the error count next to Logs.

    The Google Cloud console displays the job's logs. You may need to scroll to see the errors.

  2. Locate entries with the error icon Error icon.

  3. Click on an individual log entry to expand its contents.

For more information about troubleshooting Dataflow jobs, see Troubleshoot your pipeline.

Troubleshoot failed export jobs

If you see the following errors in your job logs:

com.google.cloud.spanner.SpannerException: NOT_FOUND: Session not found


com.google.cloud.spanner.SpannerException: DEADLINE_EXCEEDED: Deadline expired before operation could complete.

Check the 99% Read latency in the Monitoring tab of your Spanner database in the Google Cloud console. If it is showing high (multiple second) values, then it indicates that the instance is overloaded, causing reads to timeout and fail.

One cause of high latency is that the Dataflow job is running using too many workers, putting too much load on the Spanner instance.

To specify a limit on the number of Dataflow workers, instead of using the Import/Export tab in the instance details page of your Spanner database in the Google Cloud console, you must start the export using the Dataflow Cloud Spanner to Cloud Storage Avro template and specify the maximum number of workers as described below:
  • If you are using the Dataflow console, the Max workers parameter is located in the Optional parameters section of the Create job from template page.

  • If you are using gcloud, specify the max-workers argument. For example:

    gcloud dataflow jobs run my-export-job \
    --gcs-location='gs://dataflow-templates/latest/Cloud_Spanner_to_GCS_Avro' \
    --region=us-central1 \
    --parameters='instanceId=test-instance,databaseId=example-db,outputDir=gs://my-gcs-bucket' \

Optimize slow running export jobs

If you have followed the suggestions in initial settings, you should generally not have to make any other adjustments. If your job is running slowly, there are a few other optimizations you can try:

  • Optimize the job and data location: Run your Dataflow job in the same region where your Spanner instance and Cloud Storage bucket are located.

  • Ensure sufficient Dataflow resources: If the relevant Compute Engine quotas limit your Dataflow job's resources, the job's Dataflow page in the Google Cloud console displays a warning icon Warning icon and log messages:

    Screenshot of quota limit warning

    In this situation, increasing the quotas for CPUs, in-use IP addresses, and standard persistent disk might shorten the run time of the job, but you might incur more Compute Engine charges.

  • Check the Spanner CPU utilization: If you see that the CPU utilization for the instance is over 65%, you can increase the compute capacity in that instance. The capacity adds more Spanner resources and the job should speed up, but you incur more Spanner charges.

Factors affecting export job performance

Several factors influence the time it takes to complete an export job.

  • Spanner database size: Processing more data takes more time and resources.

  • Spanner database schema, including:

    • The number of tables
    • The size of the rows
    • The number of secondary indexes
    • The number of foreign keys
    • The number of change streams

  • Data location: Data is transferred between Spanner and Cloud Storage using Dataflow. Ideally all three components are located in the same region. If the components are not in the same region, moving the data across regions slows the job down.

  • Number of Dataflow workers: Optimal Dataflow workers are necessary for good performance. By using autoscaling, Dataflow chooses the number of workers for the job depending on the amount of work that needs to be done. The number of workers will, however, be capped by the quotas for CPUs, in-use IP addresses, and standard persistent disk. The Dataflow UI displays a warning icon if it encounters quota caps. In this situation, progress is slower, but the job should still complete.

  • Existing load on Spanner: An export job typically adds a light load on a Spanner instance. If the instance already has a substantial existing load, then the job runs more slowly.

  • Amount of Spanner compute capacity: If the CPU utilization for the instance is over 65%, then the job runs more slowly.