As a data architect I always ensure that for any database schema change there a fully recoverable execution path.
I have generally advised to create a patch/revert process for every change. For example, if a change adds a new column or index to a table, a revert script would remove the respective column or index.
The goal is to always have a defensive position for any changes. The concept is that simple, it is not complex.
In its simplest form I use the following directory and file structure.
/schema schema.sql /patch YYYYMMDDXX.sql where XX,ZZ are sequential 2 digit numbers, e.g. 01,02 YYYYMMDDZZ.sql /revert YYYYMMDDXX.sql This is the same file name in the revert sub-directory. YYYYMMDDZZ.sql
At any commit or tag in configuration management it is possible to create a current copy of the schema, i.e. use schema.sql.
It is also possible to take the first version of schema.sql and apply chronologically all the patch scripts to arrive at the same consistent structure of the schema that is in schema.sql. You can also run a validation process to confirm these are equivalent.
For each tagged version or commit of this directory structure and files in version control, this should always hold true.
While not the desired execution path, every revert script can be applied in a reverse chronological order and return to the first version of the schema.
If you want to maintain a first_schema.sql file within the directory structure, you can always create any version of the schema from a given commit in a roll-forward or roll-back scenario.
In reality however this is rarely implemented. There is always divergence or drift. Drift occurs for several primary reasons. The first is non-adherence to the defined process. The second and more critical is the lack of adequate testing and verification at each and every step. A Test Driven Design (TDD) that validates the given approach would enable a verification of end state of the schema and enable the verification at each accumulated
In addition to each patch/revert there needs to be a state that is maintained of what has been applied. Generally for RDBMS storing this metadata within a table is recommended.
The above example shows files of .sql extension. Any schema management process needs to cater for .sh, .py or other extensions to cater for more complex operations.
What about data changes? I would recommend that for all configuration information you follow the same management principles as for schema objects, that is you have a patch to insert/update/delete data, and you have a revert script that can restore that data. Generally the complexity of the rollback process is a hurdle for developers/engineers. Having a framework is important to manage how data consistency is maintained. This framework could generate a statement to restore the data (e.g. a selective mysqldump), require a hand-crafted statement, or leverage the benefit of the RDBMS by storing the data into intermediate shallow tables.
Using a least privileged model complicates an applicable framework approach. Does the user applying the change now require the FILE privilege, or CREATE/DROP privilege to create tables for the ability to restore data.
If there is strict referential integrity at the database level, those protections will defend against unintended consequences. For example, deleting a row that is dependent on a foreign key relationship. In a normal operating system accommodations are made generally for the sake of performance, but also for supporting poor data cleansing requirements. If the application maintains a level of referential integrity, the schema management process also needs to support this, adding a further complexity. Ensuring data integrity is an important separate topic. If there is a dangling row, what is the impact? The data still exists, it is just not presented in a user interface or included in calculations. This generally leads to greater unintended consequences that are generally never obvious at the time of execution, but rather days, weeks or months later.
When it comes to objects within the structure of an RDBMS the situation is more complex. A classic example in MySQL is a user. A user in MySQL is actually the user definition which is just the username, password and host. A user contains one or more grants. The user may be the owner of additional objects. Using default and legacy MySQL, it is simply not possible to determine if a user is actually being used. Percona and other variances support INFORMATION_SCHEMA.USER_STATISTICS which is a better method of evaluating the use of a user. This does however require the intervention of time-based data collection, as this table is the accumulative statistics since an instance restart or flush.
With this type of object, or meta object several defensive techniques exist.
If you had the user `blargie` and that user had grants to read data from several schemas, is the user used? I don’t think so, let’s just delete it is not a fact-based approach to avoiding a subsequent problem.
Is the user used? Let’s revoke the users privileges and monitor for errors or user feedback? Or let’s change the user’s password? With each of these strategies it is important to always have a defensive process to rollback.
A different approach is to use a common data technique of marking information as deleted before it’s physically deleted (think trash can before you empty the trash). For MySQL users there is no default functionality (in the most recent versions of MySQL you can DISABLE a user). One implementation to apply this pattern is to rename the user, which has the benefit of keeping the user’s password and privileges intack, therefore reducing the amount of complexity in restoring.
Regardless of the technique, it is important there is always a recovery path. In a subsequent post I will discuss this approach towards cloud metadata, for example an AWS KMS policy, IAM Rule or ASG setting and the impact of Infrastructure as a Service (IaaS) such as Terraform.