CFEngine Architecture and Design
CFEngine operates autonomously in a network, under your guidance. While CFEngine supports anything from 1 servers to 100,000+ servers, the essence of any CFEngine deployment is the same.
CFEngine supports networks of any size, from a handful of nodes to hundreds of thousands of computers. It is built to scale. If your site is very large (many thousands of servers) you should spend some time discussing your requirements with CFEngine experts. They will know how to tune promises and configurations to your environment as scale requires you to have more infrastructure, and a potentially more complicated configuration. No matter the scale, the essence of any CFEngine deployment is the same, but with great power comes great responsibility (a.k.a. don't break things before the weekend, on the weekend, or in fact on any other day).
CFEngine was designed to enable scalable configuration management in any kind of environment, with an emphasis on supporting large, Unix-like systems that are connected via TCP/IP.
CFEngine doesn't depend on or assume the presence of reliable infrastructure. It works opportunistically in any environment, using the fewest possible resources, and it has a limited set of software dependencies. It can run anywhere and this lean approach to CFEngine's architecture makes it possible to support both traditional server-based approaches to configuration as well as more novel platforms for configuration including embedded and mobile systems.
CFEngine's design allows you to create fault-tolerant, available systems which are independent of external requirements. CFEngine works in all the places you think it should, and all the new places you haven't even thought of yet.
Managing Expectations with Promises
CFEngine works on a simple notion of promises. A promise is the documentation of an intention to act or behave in some manner. When you make a promise, it is an effort to improve trust. Trust is an economic time-saver. If you can't trust you have to verify everything, and that is expensive.
Everything in CFEngine can be thought of as a promise to be kept by different
resources in the system. In a system that delivers a web site with Apache
httpd, an important promise may be to make sure that the
apache package is installed,
running, and accessible on port 80. In a system which needs to satisfy mid-day
traffic on a busy web site, a promise may be to ensure that there are 200
application servers running during normal business hours.
These promises are not top-down directives for a central authority to push through the system. A large organization can't run on top-down authority alone. A group of people can't be managed without empowering and trusting them to make independent decisions.
CFEngine is a system that emphasizes the promises a client makes to the overall CFEngine network. They are the rules which clients are responsible for implementing. We can create large systems of scale because we don't create a bulky centralized authority. There should be no single point-of-failure when managing machines and people.
Combining promises with patterns to describe where and when promises should apply is what CFEngine is all about.
Automation with CFEngine
Users are good at researching solutions and making design decisions, but awful at repeated execution. Machines are pitiful at making decisions, but very good at reliable implementation at very large scale. It makes sense to let each side do the job that they are good at. With CFEngine, users make decisions and write promises for machines to implement and satisfy.
A CFEngine user will declare a promise in CFEngine, and CFEngine will then translate this promise into a series of actions to implement. For the most part, CFEngine understands how to deliver on promises, and they don't need to be given explicit instructions for completing tasks. It is your job to make decisions about the systems you are managing and to describe those in suitable promises. It is CFEngine's job to automate and deliver a promise.
CFEngine is a distributed solution that is completely independent of host operating systems, network topology or system processes. You describe the ideal state of a given system by creating promises and the CFEngine agents ensures that the necessary steps are taken to achieve this state. Automation in CFEngine is executed through a series of components that run locally on hosts.
Phases of System Management
There are four commonly cited phases in managing systems with CFEngine: Build, Deploy, Manage, and Audit.
A system is based on a number of decisions and resources that need to be `built' before they can be implemented. You don't need to decide every detail, just enough to build trust and predictability into your system. In CFEngine, what you build is a template of proposed promises for the machines being managed. If the machines in a system all make and keep these promises, the system will function seamlessly as planned.
Deploying really means implementing the policy that was already decided. In transaction systems, one tries to push out changes one-by-one, hence `deploying' the decision. In CFEngine you simply publish your policy (in CFEngine parlance these are "promise proposals") and the machines see the new proposals and can adjust accordingly. Each machine runs an agent that is capable of implementing policies and maintaining them over time without further assistance.
Once a decision is made, unplanned events will occur. Such incidents traditionally set off alarms and humans rush to make new transactions to repair them. In CFEngine, the autonomous agent manages the system, and you only have to deal with rare events that cannot be dealt with automatically. This is the key difference of CFEngine, a focus on autonomy and creating agents that are smart enough to adapt to changing situations.
In traditional configuration systems, the outcome is far from clear after a one-shot transaction, so one audits the system to determine what actually happened. In CFEngine, changes are not just initiated once, but locally audited and maintained. Decision outcomes are assured by design in CFEngine and maintained automatically, so the main worry is managing conflicting. Users can sit back and examine regular reports of compliance generated by the agents, without having to arrange for new transactions to roll-out changes.
You should not think of CFEngine as a roll-out system, i.e. one that attempts to force out absolute changes and perhaps reverse them in case of error. Roll-out and roll-back are theoretically flawed concepts that only sometimes work in practice. With CFEngine, you publish a sequence of policy revisions, always moving forward (because like it or not, time only goes in one direction). All of the desired-state changes are managed locally by each individual host, and continuously repaired to ensure on-going compliance with policy.
See Also: Client server communication