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Troubleshooting Postgres Sources

Connector Limitations​

General Limitations​

  • The Postgres source connector currently does not handle schemas larger than 4MB.
  • The Postgres source connector does not alter the schema present in your database. Depending on the destination connected to this source, however, the schema may be altered. See the destination's documentation for more details.
  • The following two schema evolution actions are currently supported:
    • Adding/removing tables without resetting the entire connection at the destination Caveat: In the CDC mode, adding a new table to a connection may become a temporary bottleneck. When a new table is added, the next sync job takes a full snapshot of the new table before it proceeds to handle any changes.
    • Resetting a single table within the connection without resetting the rest of the destination tables in that connection
  • Changing a column data type or removing a column might break connections.

Version Requirements​

  • For Airbyte Open Source users, upgrade your Airbyte platform to version v0.40.0-alpha or newer
  • Use Postgres v9.3.x or above for non-CDC workflows and Postgres v10 or above for CDC workflows
  • For Airbyte Cloud (and optionally for Airbyte Open Source), ensure SSL is enabled in your environment

CDC Requirements​

  • Incremental sync is only supported for tables with primary keys. For tables without primary keys, use Full Refresh sync.
  • Data must be in tables and not views. If you require data synchronization from a view, you would need to create a new connection with Standard as Replication Method.
  • The modifications you want to capture must be made using DELETE/INSERT/UPDATE. For example, changes made using TRUNCATE/ALTER will not appear in logs and therefore in your destination.
  • Schema changes are not supported automatically for CDC sources. Reset and resync data if you make a schema change.
  • The records produced by DELETE statements only contain primary keys. All other data fields are unset.
  • Log-based replication only works for master instances of Postgres. CDC cannot be run from a read-replica of your primary database.
  • An Airbyte database source using CDC replication can only be used with a single Airbyte destination. This is due to how Postgres CDC is implemented - each destination would recieve only part of the data available in the replication slot.
  • Using logical replication increases disk space used on the database server. The additional data is stored until it is consumed.
    • Set frequent syncs for CDC to ensure that the data doesn't fill up your disk space.
    • If you stop syncing a CDC-configured Postgres instance with Airbyte, delete the replication slot. Otherwise, it may fill up your disk space.

Supported cursors​

  • TIMESTAMP
  • TIMESTAMP_WITH_TIMEZONE
  • TIME
  • TIME_WITH_TIMEZONE
  • DATE
  • BIT
  • BOOLEAN
  • TINYINT/SMALLINT
  • INTEGER
  • BIGINT
  • FLOAT/DOUBLE
  • REAL
  • NUMERIC/DECIMAL
  • CHAR/NCHAR/NVARCHAR/VARCHAR/LONGVARCHAR
  • BINARY/BLOB

Troubleshooting​

Sync data from Postgres hot standby server​

When the connector is reading from a Postgres replica that is configured as a Hot Standby, any update from the primary server will terminate queries on the replica after a certain amount of time, default to 30 seconds. This default waiting time is not enough to sync any meaning amount of data. See the Handling Query Conflicts section in the Postgres documentation for detailed explanation.

Here is the typical exception:

Caused by: org.postgresql.util.PSQLException: FATAL: terminating connection due to conflict with recovery
    Detail: User query might have needed to see row versions that must be removed.
    Hint: In a moment you should be able to reconnect to the database and repeat your command.

Possible solutions include:

  • [Recommended] Set hot_standby_feedback to true on the replica server. This parameter will prevent the primary server from deleting the write-ahead logs when the replica is busy serving user queries. However, the downside is that the write-ahead log will increase in size.
  • [Recommended] Sync data when there is no update running in the primary server, or sync data from the primary server.
  • [Not Recommended] Increase max_standby_archive_delay and max_standby_streaming_delay to be larger than the amount of time needed to complete the data sync. However, it is usually hard to tell how much time it will take to sync all the data. This approach is not very practical.

Under CDC incremental mode, there are still full refresh syncs​

Normally under the CDC mode, the Postgres source will first run a full refresh sync to read the snapshot of all the existing data, and all subsequent runs will only be incremental syncs reading from the write-ahead logs (WAL). However, occasionally, you may see full refresh syncs after the initial run. When this happens, you will see the following log:

Saved offset is before Replication slot's confirmed_flush_lsn, Airbyte will trigger sync from scratch

The root causes is that the WALs needed for the incremental sync has been removed by Postgres. This can occur under the following scenarios:

  • When there are lots of database updates resulting in more WAL files than allowed in the pg_wal directory, Postgres will purge or archive the WAL files. This scenario is preventable. Possible solutions include:
    • Sync the data source more frequently. The downside is that more computation resources will be consumed, leading to a higher Airbyte bill.
    • Set a higher wal_keep_size. If no unit is provided, it is in megabytes, and the default is 0. See detailed documentation here. The downside of this approach is that more disk space will be needed.
  • When the Postgres connector successfully reads the WAL and acknowledges it to Postgres, but the destination connector fails to consume the data, the Postgres connector will try to read the same WAL again, which may have been removed by Postgres, since the WAL record is already acknowledged. This scenario is rare, because it can happen, and currently there is no way to prevent it. The correct behavior is to perform a full refresh.

Temporary File Size Limit​

Some larger tables may encounter an error related to the temporary file size limit such as temporary file size exceeds temp_file_limit. To correct this error increase the temp_file_limit.

(Advanced) Custom JDBC Connection Strings​

To customize the JDBC connection beyond common options, specify additional supported JDBC URL parameters as key-value pairs separated by the symbol & in the JDBC URL Parameters (Advanced) field.

Example: key1=value1&key2=value2&key3=value3

These parameters will be added at the end of the JDBC URL that the AirByte will use to connect to your Postgres database.

The connector now supports connectTimeout and defaults to 60 seconds. Setting connectTimeout to 0 seconds will set the timeout to the longest time available.

Note: Do not use the following keys in JDBC URL Params field as they will be overwritten by Airbyte: currentSchema, user, password, ssl, and sslmode.

(Advanced) Setting up initial CDC waiting time​

The Postgres connector may need some time to start processing the data in the CDC mode in the following scenarios:

  • When the connection is set up for the first time and a snapshot is needed
  • When the connector has a lot of change logs to process

The connector waits for the default initial wait time of 5 minutes (300 seconds). Setting the parameter to a longer duration will result in slower syncs, while setting it to a shorter duration may cause the connector to not have enough time to create the initial snapshot or read through the change logs. The valid range is 120 seconds to 1200 seconds.

If you know there are database changes to be synced, but the connector cannot read those changes, the root cause may be insufficient waiting time. In that case, you can increase the waiting time (example: set to 600 seconds) to test if it is indeed the root cause. On the other hand, if you know there are no database changes, you can decrease the wait time to speed up the zero record syncs.

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