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From Erik Weber <terbol...@gmail.com>
Subject Re: Let’s discuss database upgrades
Date Tue, 29 Dec 2015 21:45:09 GMT
On Mon, Dec 28, 2015 at 2:16 PM, Rafael Weingärtner <
rafaelweingartner@gmail.com> wrote:

> Hi all devs,
> First of all, sorry the long text, but I hope we can start a discussion
> here and improve that part of ACS.
>
> A while ago I have faced the code that Apache CloudStack (ACS) uses to
> upgrade from a version to newer one and that did not seem to be a good way
> to execute our upgrades. Therefore, I decided to use some time to search
> for alternatives.
>
> I have read some material about versioning of scripts used to upgrade a
> database (DB) of a system and went through some frameworks that could help
> us.
>
> In the literature of software engineering, it is firmly stated that we have
> to version DB scripts as we do with the source code of the application,
> using the baseline approach. Gladly, we were not that bad at this point, we
> already versioned our routines for DB upgrade (.sql and .java). Therefore,
> it seemed that we just did not have used a practical approach to help us
> during DB upgrades.
>
> From my readings and looking at the ACS source code I raised the following
> requirement:
> •    We should be able to write more than one routine to upgrade to a
> version; those routines can be written in Java and SQL. We might have more
> than a routine to be executed for each version and we should be able to
> define an order of execution. Additionally, to go to an upper version, we
> have to run all of the routines from smaller versions first, until we
> achieve the desired version.
>
> We could also add another requirement that is the downgrade from a version,
> which we currently do not support. With that comes my first question for
> discussion:
> •    Do we want/need a method to downgrade from a version to a previous
> one?
>
> I found an explanation for not supporting downgrades, and I liked it:
> http://flywaydb.org/documentation/faq.html#downgrade
>
> So, what I devised for us:
> First the bureaucracy part  - our migrations occur basically in three (3)
> steps, first we have a "prepare script", then a cleanup script and finally
> the migration per se that is written in Java, at least, that is what we can
> expect when reading the interface “com.cloud.upgrade.dao.DbUpgrade”.
>
> Additionally, our scripts have the following naming convention:
> schema-<currentVersion>to<desiredVersion>, which in IMHO may cause some
> confusion because at first sight we may think that from the same version we
> could have different paths to an upper version, which in practice is not
> happening. Instead of a <currentVersion>to<version> we could simply use
> V_<numberOfVersion>_<sequencial>.<fileExtension>, giving that, we have
to
> execute all of the V_<version> scripts that are smaller than the version we
> want to upgrade.
>
> To clarify what I am saying, I will use an example. Let’s say we have just
> installed ACS and ran the cloudstack-setup-database. That command will
> create a database schema in version 4.0.0. To upgrade that schema to
> version 4.3.0 (it is just an example, it could be any other version), ACS
> will use the following mapping:
>
> _upgradeMap.put("4.0.0", new DbUpgrade[] {new Upgrade40to41(), new
> Upgrade410to420(), new Upgrade420to421(), new Upgrade421to430())
>
> After loading the mapping, ACS will execute the scripts defined in each one
> of the Upgrade path classes and the migration code per se.
>
> Now, let’s say we change the “.sql” scripts name to the pattern I
> mentioned, we would have the following scripts; those are the scripts found
> that aim to upgrade to versions between the interval 4.0.0 – 4.3.0
> (considering 4.3.0, since that is the goal version):
>
>
>    - schema-40to410, can be named to:  V_410_A.sql
>    - schema-40to410-cleanup, can be named to:  V_410_B.sql
>    - schema-410to420, can be named to:  V_420_A.sql
>    - schema-410to420-cleanup , can be named to:  V_420_b.sql
>    - schema-420to421, can be named to:  V_421_A.sql
>    - schema-421to430, can be named to:  V_430_A.sql
>    - schema-421to430-cleanup, can be named to:  V_430_B.sql
>
>
> Additionally, all of the java code would have to follow the same
> convention. For instance, we have “com.cloud.upgrade.dao.Upgrade40to41”,
> which has some java code to migrate from 4.0.0 to 4.1.0. The idea is to
> extract that migration code to a Java class named:  V_410_C.java, giving
> that it has to execute the SQL scripts before the java code.
>
> In order to go from a smaller version (4.0.0) to an upper one (4.3.0), we
> have to run all of the migration routines from intermediate versions. That
> is what we are already doing, but we do all of that manually.
>
> Bottom line, I think we could simple use the convention
> V_<numberOfVersion>_<sequencial>.<fileExtension> to name upgrade routines.
> That would facilitate us to use a framework to help us with that process.
> Additionally, I believe that we should always assume that to go from a
> smaller version to a higher one, we should run all of the scripts that
> exist between them. What do you guys think of that?
>
> After the bureaucracy, we can discuss tools. If we use that convention to
> name migration (upgrade) routines, we can start thinking on tools to
> support our migration process. I found two (2) promising ones: Liquibase
> and Flywaydb (both seem to be under Apache license, but the first one has
> an enterprise version?!). After reading the documentation and some usage
> examples I found the flywaydb easier and simpler to use.
>
> What are the options of tools that we can use to help us manage the
> database upgrade, without needing to code the upgrade path that you know?
>
> After that, I think we should decide if we should create another
> project/component to take care of migrations, or we can just  add the
> dependency of the tool to a project such as “cloud-framework-db” and start
> using it.
>
> The “cloud-framework-db” project seems to have a focus on other things such
> as managing transactions and generating SQLs from annotations (?!? That
> should be a topic for another discussion). Therefore, I would rather create
> a new project that has the specific goal of managing ACS DB upgrades.  I
> would also move all of the routines (SQL and Java) to this new project.
> This project would be a module of the CloudStack project and it would
> execute the upgrade routines at the startup of ACS.
>
> I believe that going from a homemade solution to one that is more
> consolidated and used by other communities would be the way to go.
>
> I can volunteer myself to create a PR with the aforementioned changes and
> using flywaydb to manage our upgrades. However, I prefer to have a good
> discussion with other devs first, before starting coding.
>
> Do you have suggestions or points that should be raised before we start
> working on that?
>


This isn't my field of work, so forgive me if this is self explanatory or
something, but is there no tool like terraform/puppet or similar for
database work?
I mean, where you state you desired state and the tool handles it.

To me it sounds like a good way would be if you could specify what you want
to exist (or not), and how it should look like.

"I want table XYZ to exist with THESE columns having THIS type(s) and THIS
default value bla bla bla"

Rather than handling a bunch of sql scripts that has to handle different
mysql versions (come to think about an issue with a mariadb version
crashing recently), a variety of cloudstack versions and a whole lot more.

Disclaimer: i have no idea if this is what flywaydb does, if it is, then
just ignore this.

-- 
Erik

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