Troubleshooting `test_migrate_external_table` Failure In Databricks UCX

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assert 0 == 1
 +  where 0 = len([])
[gw3] linux -- Python 3.10.18 /home/runner/work/ucx/ucx/.venv/bin/python
05:11 INFO [tests.integration.hive_metastore.test_migrate] dst_catalog=dummy_cua5n, external_table=hive_metastore.dummy_szgzf.dummy_tus2a
05:12 DEBUG [databricks.labs.ucx.framework.crawlers] [hive_metastore.dummy_snb7p.tables] fetching tables inventory
05:12 DEBUG [databricks.labs.ucx.framework.crawlers] Inventory table not found
Traceback (most recent call last):
  File "/home/runner/work/ucx/ucx/src/databricks/labs/ucx/framework/crawlers.py", line 152, in _snapshot
    cached_results = list(fetcher())
  File "/home/runner/work/ucx/ucx/src/databricks/labs/ucx/hive_metastore/tables.py", line 458, in _try_fetch
    for row in self._fetch(f"SELECT * FROM {escape_sql_identifier(self.full_name)}"):
  File "/home/runner/work/ucx/ucx/.venv/lib/python3.10/site-packages/databricks/labs/lsql/core.py", line 344, in fetch_all
    execute_response = self.execute(
  File "/home/runner/work/ucx/ucx/.venv/lib/python3.10/site-packages/databricks/labs/lsql/core.py", line 268, in execute
    self._raise_if_needed(status)
  File "/home/runner/work/ucx/ucx/.venv/lib/python3.10/site-packages/databricks/labs/lsql/core.py", line 478, in _raise_if_needed
    raise NotFound(error_message)
databricks.sdk.errors.platform.NotFound: [TABLE_OR_VIEW_NOT_FOUND] The table or view `hive_metastore`.`dummy_snb7p`.`tables` cannot be found. Verify the spelling and correctness of the schema and catalog.
If you did not qualify the name with a schema, verify the current_schema() output, or qualify the name with the correct schema and catalog.
To tolerate the error on drop use DROP VIEW IF EXISTS or DROP TABLE IF EXISTS. SQLSTATE: 42P01; line 1 pos 14
05:12 DEBUG [databricks.labs.ucx.framework.crawlers] [hive_metastore.dummy_snb7p.tables] crawling new set of snapshot data for tables
05:12 DEBUG [databricks.labs.ucx.hive_metastore.tables] [hive_metastore.dummy_szgzf] listing tables and views
05:12 DEBUG [databricks.labs.ucx.hive_metastore.tables] [hive_metastore.dummy_szgzf.dummy_tus2a] fetching table metadata
05:12 DEBUG [databricks.labs.ucx.framework.crawlers] [hive_metastore.dummy_snb7p.tables] found 1 new records for tables
05:12 DEBUG [databricks.labs.ucx.hive_metastore.table_migrate] Migrating external table hive_metastore.dummy_szgzf.dummy_tus2a to using SQL query: SYNC TABLE `dummy_cua5n`.`dummy_szgzf`.`dummy_tus2a` FROM `hive_metastore`.`dummy_szgzf`.`dummy_tus2a`;
05:12 WARNING [databricks.labs.ucx.hive_metastore.table_migrate] failed-to-migrate: SYNC command failed to migrate table hive_metastore.dummy_szgzf.dummy_tus2a to dummy_cua5n.dummy_szgzf.dummy_tus2a. Status code: EXTERNAL_LOCATION_DOES_NOT_EXIST. Description: parent external location for path `TEST_MOUNT_CONTAINER/a/b/oWni-ra78b42523` does not exist.
05:11 INFO [tests.integration.hive_metastore.test_migrate] dst_catalog=dummy_cua5n, external_table=hive_metastore.dummy_szgzf.dummy_tus2a
05:12 DEBUG [databricks.labs.ucx.framework.crawlers] [hive_metastore.dummy_snb7p.tables] fetching tables inventory
05:12 DEBUG [databricks.labs.ucx.framework.crawlers] Inventory table not found
Traceback (most recent call last):
  File "/home/runner/work/ucx/ucx/src/databricks/labs/ucx/framework/crawlers.py", line 152, in _snapshot
    cached_results = list(fetcher())
  File "/home/runner/work/ucx/ucx/src/databricks/labs/ucx/hive_metastore/tables.py", line 458, in _try_fetch
    for row in self._fetch(f"SELECT * FROM {escape_sql_identifier(self.full_name)}"):
  File "/home/runner/work/ucx/ucx/.venv/lib/python3.10/site-packages/databricks/labs/lsql/core.py", line 344, in fetch_all
    execute_response = self.execute(
  File "/home/runner/work/ucx/ucx/.venv/lib/python3.10/site-packages/databricks/labs/lsql/core.py", line 268, in execute
    self._raise_if_needed(status)
  File "/home/runner/work/ucx/ucx/.venv/lib/python3.10/site-packages/databricks/labs/lsql/core.py", line 478, in _raise_if_needed
    raise NotFound(error_message)
databricks.sdk.errors.platform.NotFound: [TABLE_OR_VIEW_NOT_FOUND] The table or view `hive_metastore`.`dummy_snb7p`.`tables` cannot be found. Verify the spelling and correctness of the schema and catalog.
If you did not qualify the name with a schema, verify the current_schema() output, or qualify the name with the correct schema and catalog.
To tolerate the error on drop use DROP VIEW IF EXISTS or DROP TABLE IF EXISTS. SQLSTATE: 42P01; line 1 pos 14
05:12 DEBUG [databricks.labs.ucx.framework.crawlers] [hive_metastore.dummy_snb7p.tables] crawling new set of snapshot data for tables
05:12 DEBUG [databricks.labs.ucx.hive_metastore.tables] [hive_metastore.dummy_szgzf] listing tables and views
05:12 DEBUG [databricks.labs.ucx.hive_metastore.tables] [hive_metastore.dummy_szgzf.dummy_tus2a] fetching table metadata
05:12 DEBUG [databricks.labs.ucx.framework.crawlers] [hive_metastore.dummy_snb7p.tables] found 1 new records for tables
05:12 DEBUG [databricks.labs.ucx.hive_metastore.table_migrate] Migrating external table hive_metastore.dummy_szgzf.dummy_tus2a to using SQL query: SYNC TABLE `dummy_cua5n`.`dummy_szgzf`.`dummy_tus2a` FROM `hive_metastore`.`dummy_szgzf`.`dummy_tus2a`;
05:12 WARNING [databricks.labs.ucx.hive_metastore.table_migrate] failed-to-migrate: SYNC command failed to migrate table hive_metastore.dummy_szgzf.dummy_tus2a to dummy_cua5n.dummy_szgzf.dummy_tus2a. Status code: EXTERNAL_LOCATION_DOES_NOT_EXIST. Description: parent external location for path `TEST_MOUNT_CONTAINER/a/b/oWni-ra78b42523` does not exist.
[gw3] linux -- Python 3.10.18 /home/runner/work/ucx/ucx/.venv/bin/python

This article delves into the intricacies of a specific test failure encountered within the Databricks Labs UCX project: test_migrate_external_table. This failure, identified during nightly testing, presents a valuable opportunity to explore the underlying issues related to external table migration in Databricks. The primary focus will be on dissecting the error logs, understanding the root cause of the failure, and proposing potential solutions or areas for further investigation. By examining this particular case, we aim to provide insights into the challenges of managing and migrating external tables, especially within the context of complex data ecosystems like Databricks.

Understanding the test_migrate_external_table Failure

The core of this issue lies within the test_migrate_external_table function, which is designed to verify the successful migration of external tables between catalogs in Databricks. The failure is clearly indicated by the initial assertion error: assert 0 == 1. This assertion implies that the test expected a certain number of migrations to succeed, but in reality, none did. The len([]) part of the assertion suggests that the list of successfully migrated tables is empty, leading to the failure. The subsequent logs provide crucial context, revealing a series of events that culminated in this failed assertion.

The debugging information highlights several key areas. First, the test attempts to migrate an external table named hive_metastore.dummy_szgzf.dummy_tus2a to dummy_cua5n.dummy_szgzf.dummy_tus2a. This migration is performed using a SYNC TABLE SQL command. The logs also indicate a problem during the crawling process, specifically when fetching the tables inventory. The system encounters a NotFound error while trying to access hive_metastore.dummy_snb7p.tables. This suggests a potential issue with the availability or accessibility of the metadata related to the tables being migrated.

Furthermore, the logs reveal a warning message during the table migration process: failed-to-migrate: SYNC command failed... Status code: EXTERNAL_LOCATION_DOES_NOT_EXIST. Description: parent external location for path TEST_MOUNT_CONTAINER/a/b/oWni-ra78b42523 does not exist. This is a critical piece of information, indicating that the migration failed because the underlying external storage location for the table does not exist or is not accessible. This could be due to various reasons, such as incorrect configuration of the external location, missing permissions, or the location genuinely not existing in the storage system. The combination of these error messages points towards a multi-faceted issue, involving both metadata access problems and external storage location unavailability.

Deep Dive into the Error Logs and Root Cause Analysis

To effectively address the test_migrate_external_table failure, a meticulous examination of the error logs is essential. Let's dissect the key error messages and their implications to pinpoint the root cause of the problem.

1. `databricks.sdk.errors.platform.NotFound: [TABLE_OR_VIEW_NOT_FOUND] The table or view

hive_metastore. dummy_snb7p. tables cannot be found.`

This error message is a critical indicator of a metadata access issue. The UCX framework uses crawlers to inventory and snapshot table metadata. The crawler attempts to fetch information about the tables view within the hive_metastore.dummy_snb7p schema. The NotFound error suggests that this view either does not exist or the system lacks the necessary permissions to access it. This could stem from several potential issues:

  • Incorrect Schema or Table Name: A typographical error or a misconfiguration in the schema or table name could lead to this error. It's crucial to verify the spelling and case sensitivity of the identifiers.
  • Missing Permissions: The user or service account running the test might not have the required permissions to access the tables view. Databricks employs a robust access control system, and insufficient privileges can prevent metadata retrieval.
  • Table or View Does Not Exist: The tables view might not have been created or might have been dropped accidentally. This scenario requires investigation into the table creation process and potential deletion events.
  • Catalog Inconsistency: In environments with multiple catalogs, the crawler might be looking in the wrong catalog. Ensuring that the crawler is configured to access the correct catalog is crucial.

2. `WARNING [databricks.labs.ucx.hive_metastore.table_migrate] failed-to-migrate: SYNC command failed to migrate table

hive_metastore. dummy_szgzf. dummy_tus2a to dummy_cua5n. dummy_szgzf. dummy_tus2a. Status code: EXTERNAL_LOCATION_DOES_NOT_EXIST. Description: parent external location for path TEST_MOUNT_CONTAINER/a/b/oWni-ra78b42523 does not exist.`

This warning message directly points to a problem with the external storage location associated with the table being migrated. The EXTERNAL_LOCATION_DOES_NOT_EXIST status code clearly indicates that the specified path TEST_MOUNT_CONTAINER/a/b/oWni-ra78b42523 is either not a valid location or is not accessible. This error can arise due to several reasons:

  • Missing External Location Configuration: The external location might not be properly configured in Databricks. External locations need to be explicitly defined and granted appropriate permissions.
  • Incorrect Path: The path specified in the table metadata might be incorrect. This could be due to manual errors or inconsistencies in the metadata management process.
  • Storage Connectivity Issues: There might be connectivity problems between Databricks and the external storage system (e.g., AWS S3, Azure Blob Storage, Google Cloud Storage). Network issues, firewall rules, or incorrect credentials can prevent access to the storage location.
  • Missing Storage Bucket or Container: The storage bucket or container specified in the path might not exist. This could be due to accidental deletion or misconfiguration of the storage infrastructure.

Root Cause Synthesis

Based on the error messages, the root cause of the test_migrate_external_table failure appears to be a combination of factors:

  1. Metadata Access Issues: The crawler's inability to access the hive_metastore.dummy_snb7p.tables view suggests a problem with metadata availability or permissions.
  2. External Storage Location Unavailability: The EXTERNAL_LOCATION_DOES_NOT_EXIST error indicates that the underlying storage location for the table being migrated is either missing or inaccessible.

It's plausible that these issues are related. For instance, if the metadata about the external table is incomplete or inaccurate due to the metadata access problem, it could lead to the SYNC TABLE command referencing a non-existent external location. Therefore, addressing the metadata access issue might be a prerequisite for resolving the external storage location problem.

Potential Solutions and Mitigation Strategies

To resolve the test_migrate_external_table failure, a systematic approach is required, addressing both the metadata access issues and the external storage location problems. Here are some potential solutions and mitigation strategies:

Addressing Metadata Access Issues

  1. Verify Table and Schema Names:
    • Double-check the spelling and case sensitivity of the table and schema names (hive_metastore.dummy_snb7p.tables).
    • Ensure that the names used in the test configuration match the actual names in the Hive Metastore.
  2. Check Permissions:
    • Verify that the user or service principal running the test has the necessary permissions to access the tables view in the hive_metastore.dummy_snb7p schema.
    • Grant SELECT privileges on the view or, if necessary, broader permissions on the schema or catalog.
  3. Confirm Table Existence:
    • Use SQL queries or Databricks CLI commands to confirm that the tables view exists in the hive_metastore.dummy_snb7p schema.
    • If the view is missing, investigate the table creation process and ensure that it is being created correctly.
  4. Review Catalog Configuration:
    • If using multiple catalogs, ensure that the crawler is configured to access the correct catalog where the tables view resides.
    • Check the catalog settings in the UCX configuration and update them if necessary.

Resolving External Storage Location Issues

  1. Validate External Location Configuration:
    • Use Databricks SQL or the Databricks CLI to verify that the external location TEST_MOUNT_CONTAINER/a/b/oWni-ra78b42523 is properly configured.
    • Check the external location's URI, storage credentials, and any associated access control policies.
  2. Verify Table Metadata:
    • Examine the table metadata for hive_metastore.dummy_szgzf.dummy_tus2a to ensure that the external location path is correct.
    • If the path is incorrect, update the table metadata using ALTER TABLE commands.
  3. Check Storage Connectivity:
    • Test the connectivity between Databricks and the external storage system.
    • Verify network settings, firewall rules, and storage account credentials.
    • Ensure that the Databricks cluster has the necessary access to the storage bucket or container.
  4. Confirm Storage Bucket/Container Existence:
    • Manually check if the storage bucket or container specified in the path exists in the external storage system.
    • If the bucket or container is missing, recreate it or update the table metadata to point to a valid location.

Step-by-Step Troubleshooting

To effectively troubleshoot the test_migrate_external_table failure, follow these steps:

  1. Isolate the Issue:
    • Run the test_migrate_external_table test in isolation to ensure that the failure is consistent and not due to transient factors.
  2. Examine Metadata Access:
    • Use SQL queries to verify the existence and accessibility of the hive_metastore.dummy_snb7p.tables view.
    • Check permissions and catalog configurations.
  3. Validate External Location:
    • Use Databricks SQL or CLI commands to verify the configuration of the external location TEST_MOUNT_CONTAINER/a/b/oWni-ra78b42523.
    • Check storage connectivity and bucket/container existence.
  4. Review Table Metadata:
    • Examine the metadata of hive_metastore.dummy_szgzf.dummy_tus2a to verify the external location path.
  5. Implement Fixes and Retest:
    • Apply the necessary fixes based on the identified issues.
    • Rerun the test_migrate_external_table test to confirm that the failure is resolved.

Conclusion

The test_migrate_external_table failure in the Databricks Labs UCX project highlights the complexities involved in managing and migrating external tables. The error logs point to a combination of metadata access issues and external storage location unavailability. By systematically analyzing the error messages, verifying configurations, and implementing the proposed solutions, it's possible to resolve the failure and ensure the successful migration of external tables. This detailed analysis not only addresses the immediate test failure but also provides valuable insights into best practices for managing external tables in Databricks environments. The key takeaways include the importance of proper metadata management, accurate external location configuration, and robust storage connectivity. Addressing these aspects will contribute to a more stable and reliable data migration process within the Databricks ecosystem. Understanding these errors and how to debug them is crucial for anyone working with Databricks and UCX, ensuring smoother data operations and more reliable data pipelines.