What is PostgreSQL Recovery Mode?
PostgreSQL Recovery Mode is a special mode used to preserve the contents of data files on disk and prevent any further changes to the opened database. It is activated when the database is started in a safe recovery mode, for instance after the instance has crashed, is about to be upgraded, or has been put in read-only mode for maintenance.
In this mode, Postgres is checking and verifying the transaction log files of the database, and is safe-guarding the data from any possible data loss. In addition, changes made during the "read-only" recovery mode can be rolled back, so that only the changes made prior to this point are left intact. For example, if you create a new table in the database during the recovery mode, the changes won't be kept, and the database will be restored to the previous state.
How To Use PostgreSQL Recovery Mode
Using PostgreSQL Recovery Mode is not complicated, you just need to follow these few steps, and you'll be able to safely upgrade your database, or recover from a crash.
Start the PostgreSQL instance in safe recovery mode. To do so, execute the command “pg_ctl -D -m fast”, where “datadir” is the full path of the Postgres’ data directory.
During the recovery process, Postgres will check and repair all the transaction logs, and then proceed to replay the data.
As soon as the recovery process is finished, you can re-enable the database.
- Once the database is running without issues, you can perform the necessary updates and maintenance, before finally turning the instance back to normal mode.
FAQ
What type of data is contained in a PostgreSQL transaction log?
The transaction log contains all the data operations (insert, delete, update, etc.) made by the database, including data structure changes and constraints. The logs are also used to ensure data integrity and crash recovery.
What happens if PostgreSQL encounters an error during the recovery mode?
PostgreSQL will halt the recovery process and report the error, but the instance won't stop or crash. This log will then provide you with useful information about why the error occurred, and what could have caused it.
Is it possible to make changes to the database during the recovery mode?
Yes, but those changes won't be saved, as the instance is in read-only mode. This means that when the recovery process is complete, any changes made during this period will be discarded.
What is the best way to implement PostgreSQL Recovery Mode?
The best way to implement Recovery Mode is to regularly practice running it on a test environment. That way, you can familiarize yourself with the process and make sure that the instance is working as expected when it is enabled.
Is PostgreSQL Recovery Mode compatible with all versions of the database?
Yes, the Recovery Mode is compatible with all recent versions of PostgreSQL.