Ouray Software SCM Articles Series: The ACID Database Properties and OurayCM's Durable Versions

In this article we explain how OurayCM's Durable Versions are a natural consequence of applying the ACID database properties to SCM.

Review of ACID

The ACID properties are the core properties of a database that are required for database reliability.

• Atomicity means that when a group of changes is committed to the database, either all of them will be applied or none of them will. SCM example: When you save the changes you made to a set of files, either all of the changes are recorded or none of them are.
• Consistency means that the database will always be in a proper state relative to your application. SCM example: When you submit your changes to a shared area, they are not mixed in with someone else's changes (see below).
• Isolation means that different transactions will not interfere with one another, particularly, they won't lead to an inconsistent database state. SCM example: When you update your workspace from a shared area, you don't get someone's partially-submitted changes.
• Durability means that completed transactions will not be lost due to a software fault or power outage - "complete" means that the transaction actually made it to long-term storage (the disk). SCM example: When your server goes down, you never lose the information related to previously completed SCM operations.

How other SCM tools fail the ACID Test

There are various ways in which other version control tools fail to comply with the ACID properties. Some common instances of failure:

• A tool that provides no support for any kind of database transaction at all.
• A tool that allows arbitrary blending of files from different users' workspaces (this is typical of file-locking tools). This violates Consistency, because the standard of Consistency for software projects is the full set of working files from a given workspace, not single files from randomly selected workspaces.
• A tool that requires explicit tagging operations in order to form a version. This violates Durability: the workspace state as it was a few minutes ago is erased when the current workspace is saved.

Note that while ACID is important, it is not the only standard that an SCM system should comply with. Our article 10 Qualities You Need in an SCM System discusses other important SCM qualities.

ACID and OurayCM

The key idea behind OurayCM's Durable Versions is simple: Given that you had saved the state of your workspace, you ought to be able to get back to it - with no exceptions or special steps required on your part. A violation of any of the above ACID properties means that you will not be able to reliably perform this very basic recovery operation.

To see how we have taken this basic idea and expanded it into a powerful system, see The Basics of OurayCM.

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