How do you handle parallelism in Jenkins pipelines?
Avinash Tietler
DevOps Architect at TELUS International, driving cloud automation and innovation
Parallelism in Jenkins pipelines is a powerful tool for improving build efficiency and speeding up the delivery pipeline. It helps scale processes, reduce execution time, and optimize resource usage but requires careful handling of challenges like resource management, debugging, and dependencies.
Jenkins provides native support for parallel execution through its Pipeline DSL (Domain Specific Language).
Here's a detailed explanation of how parallelism works in Jenkins, along with its benefits and challenges.
How to Handle Parallelism in Jenkins Pipelines
Declarative Pipeline:
The declarative pipeline syntax makes it easier to manage parallel execution by using the parallel directive. Here's an example:
pipeline {
agent any
stages {
stage('Parallel Execution') {
parallel {
stage('Unit Tests') {
steps {
echo 'Running Unit Tests...'
}
}
stage('Integration Tests') {
steps {
echo 'Running Integration Tests...'
}
}
stage('UI Tests') {
steps {
echo 'Running UI Tests...'
}
}
}
}
}
}
In this example, the stages "Unit Tests," "Integration Tests," and "UI Tests" are executed in parallel. Jenkins will spawn separate executors (within available resources) for each parallel block.
Scripted Pipeline:
You can achieve parallelism in a scripted pipeline using the parallel function. Here's an example:
node {
stage('Parallel Tasks') {
parallel(
'Unit Tests': {
echo 'Running Unit Tests...'
},
'Integration Tests': {
echo 'Running Integration Tests...'
},
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'UI Tests': {
echo 'Running UI Tests...'
}
)
}
}
This will execute the different tasks ("Unit Tests," "Integration Tests," "UI Tests") in parallel.
Benefits of Parallelism in Jenkins Pipelines
By executing multiple stages or tasks concurrently, you significantly reduce the total pipeline runtime. For example, running tests for different components or environments in parallel instead of sequentially cuts down the wait time.
Jenkins distributes tasks across multiple agents and executors, which optimizes resource use and ensures that available computing resources are fully leveraged.
Parallel pipelines scale better as they can run complex workflows involving multiple tasks simultaneously, especially in larger, distributed systems or microservices architectures.
Running multiple tasks (e.g., testing, building, and deploying) simultaneously helps you detect issues early, allowing you to act faster and reduce the feedback loop in Continuous Integration (CI).
Challenges of Parallelism in Jenkins Pipelines
When running parallel stages, the Jenkins master and the agent nodes may face resource contention if the number of tasks exceeds available executors or hardware capacity. This can lead to longer wait times or unstable builds.
In cases where one stage depends on another, parallel execution can introduce race conditions if proper synchronization isn’t handled. For example, a deployment job might depend on the completion of all tests, requiring proper orchestration to avoid incomplete or inconsistent states.
With parallel stages, logs from different jobs can be interleaved, making it harder to debug failures. Jenkins provides a UI to visualize logs for individual stages, but debugging a pipeline with many parallel stages can still be complex.
Jenkins pipelines depend on available agents. If there are not enough agents to execute all parallel jobs, some jobs might be delayed until an agent becomes free. This affects the overall speed of execution.
As pipelines become more complex with many parallel stages, maintaining the pipeline script becomes harder. You may need additional logic to manage conditional execution, failure handling, or retry mechanisms for specific stages.
If multiple parallel stages access shared resources (e.g., databases or files), concurrency issues like locking or data corruption can arise. Ensuring proper isolation or synchronization for these shared resources is important.
Best Practices for Parallelism in Jenkins Pipelines
Distribute tasks evenly across parallel stages to avoid overloading agents with a few heavy tasks while others are lightweight.
If you have limited resources, you can throttle the number of parallel jobs that can run concurrently using Jenkins plugins like Throttle Concurrent Builds Plugin.
Continuously monitor the load on Jenkins agents and executors. Adjust the parallelism based on the available resources and expected pipeline load.
Use the failFast: true option in the parallel directive to abort the pipeline quickly if one of the parallel stages fails. This saves time by not continuing tasks if something goes wrong early on.
If you need to run the same job in parallel but with different parameters (e.g., different environments or configurations), consider using Jenkins’ Matrix functionality. This enables the same stage to run across different axes in parallel.