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If your Continua environment includes multiple concurrent builds then additional agents can be installed on your network. An unlimited number of agents can be installed on your network once your Continua CI environment has been licensed for multiple concurrent builds.
How Does Continua Decide Which Agent Will Run a Particular Build?
Every stage that is executed as part of the build process must be build by an agent, however each stage within a build process are self contained and are treated individually. This means that as a build executes, its stages may be built on different agents. The Distributed Architecture concepts page has more information on how each stage is processed by the server and agent.
When a stage is ready to execute, the Continua server must pick an agent to run the current stage. When picking an agent the server will always pick an agent that is capable of running every single action in that stage. Continua uses Agent Properties (which are automatically created by Property Collectors) to determine whether an agent can execute each action.
For example, say you have a stage that runs an NUnit action which is set to use NUnit version 2.6.2. Once this stage is ready to execute, the server will find all agents that have NUnit 2.6.2 installed by checking whether the agent has the nunit.2.6.2.path property (Check out the Agent Properties page for more information). This property provides the path to the installed NUnit executable on the agent. If an agent does not have this property then NUnit 2.6.2 cannot be found on the agent. Any agent that does not have this property will never be able to run this stage until NUnit 2.6.2 is installed.
Once the server has all the agents that can run the current stage, it will then try to assign the stage to an agent that is currently available. If all your agents are available then it will arbitrarily pick an agent to run that stage.
Concurrent Stages
Concurrent stages allow you to specify how many stages a particular agent can run concurrently. More information on concurrent stages can be found at the bottom of this page.
If you have multiple builds running over multiple agents, Continua will attempt to send the current stage to the agent that is running the least amount of stages.
Additional Information
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Viewing an Agent
All Continua agents can be viewed through the administration section on the Agents page (as shown below). This page shows you all agents that are currently registered with Continua CI and each agent can be viewed individually to provide more detailed information regarding that agent.
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The agent is running an older version than the Continua Server and it is no longer compatible with the server. Builds will no longer work on this agent until it is upgraded to the same version as the server. An agent will only become incompatible if a later release includes changes to the agent code as well as the server.
Agent Conditions
Agent Properties
Concurrent Stages
Agent Compatibility
Conditions allow you to define rules that restrict when an agent can be used to execute a build. Check out the Agent Conditions page for more information.
Agent Properties
Agent properties store information about the agent that can then be used in builds, finding build tools, etc. Most properties are automatically generated with Property Collectors however custom properties can also be created. Check out the Agent Properties page for more information.
Concurrent Stages
The number of concurrent stages on an agent define how many stages an agent can process at any one time. For example, if you have 5 builds queued and an agent set to 3 concurrent builds then 3 builds will be sent to the agent while the remaining 2 builds will remain on the queue.
Reducing the number of concurrent stages on an agent can help reduce the strain on a particular agent and can force builds to run on other agents instead.
You can set the concurrent stages limit to unlimited by specifying 0 stages.
Note that concurrent stages can only be used once Continua CI has been licensed and concurrent builds have been enabled. If you are running the free edition of Continua then the local agent can only ever run 1 build at a time. Also, the number of concurrent builds as defined by your license takes precedence over an agent's concurrent stages. For example, if your license grants you 5 concurrent builds then each agent will be automatically capped at 5 concurrent stages. Note that your concurrent builds, as specified by your license, cap the total number of builds running across all agents at any one time. So given that your license grants you 5 concurrent builds and you have 2 agents, both with their concurrent stages set to 5, you will never be able to run more than 5 builds at any one time between both agents, regardless of the agent's concurrent stages limit.
Agent Compatibility
Each Agent comes with a compatibility list that shows whether the agent is capable of running each stage. Check out the Agent Compatibility Matrix page for more information.