Merge pull request #4 from scottrigby/principles

For initial pre-release of GitOps Principles revised by the GitOps Working Group

PRINCIPLES.md

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+# GitOps Principles v0.1.0
+
+## Summary
+
+GitOps is a set of principles for operating and managing software systems.
+
+When using GitOps, the _Desired State_ of a system or subsystem is defined declaratively as versioned, immutable data, and the running system's configuration is continuously derived from this data.
+
+These principles were derived from modern software operations but are rooted in pre-existing and widely adopted best practices.
+
+## Principles
+
+1. **The principle of declarative desired state**
+
+    A system managed by GitOps must have its _Desired State_ expressed declaratively as data in a format writable and readable by both humans and machines.
+
+2. **The principle of immutable desired state versions**
+
+    _Desired State_ is stored in a way that supports versioning, immutability of versions, and retains a complete version history.
+
+3. **The principle of continuous state reconciliation**
+
+    Software agents continuously, and automatically, compare a system's _Actual State_ to its _Desired State_.
+    If the actual and desired states differ for any reason, automated actions to reconcile them are initiated.
+
+4. **The principle of operations through declaration**
+
+    The only mechanism through which the system is intentionally operated on is through these principles.
+
+## Notes
+
+### Principle 3 Notes
+
+- These differences could be due to the actual state drifting from the desired state, or the desired state changing intentionally.
+- The source of drift doesn't matter. Contrary to CIops, _any_ drift will trigger a reconciliation
+
+### Principle 4 Notes
+
+- We talk here about "regular operations." In an emergency, other modes of operations, e.g. manual intervention, should be considered - followed by a reconciliation of the "tainted" system with the declared state. → resolve the conflict between "GitOps principle" and "I need to deal with problems that GitOps doesn't cover"
+
+## Glossary
+
+- ### Continuous
+
+    By "continuous" we adopt the industry standard term to mean reconciliation continues to happen, not that it must be instantaneous.
+
+- ### Declarative Description
+
+    Describing the desired state or behavior of a system without specifying how that state will be achieved, thereby separating between configuration - the desired state - and implementation - the commands, API calls, scripts ... that actually achieve the desired state described in the declarative description.
+
+- ### Desired State
+
+    The aggregate of all configuration data for a system form its _Desired State_ which is defined as data sufficient to recreate the system so that instances of the system are behaviourally indistinguishable.
+
+- ### Software System
+
+    One or more Runtime environments consisting of resources under management.
+    In each Runtime, management Agents to act on resources according to security policies.
+    One or more software Repositories for storing deployable artifacts that may be loaded into the runtime environments, eg. configuration files, code, binaries and packages.
+    One or more Administrators who are responsible for operating the runtime environments ie. installing, starting, stopping and updating software, code, configuration, etc.
+    A set of policies controlling access and management of repositories, deployments, runtimes.
+
+- #### State Store
+
+    A system for storing versioned, immutable Desired States that provides access control and auditing on the changes to the Desired State. Git may be configured as a State Store, but [special precautions must be taken](recipes/SETTING_UP_GIT.md).