Multiple SCM repositories in Jenkins - Deep Dive

Overview - Multiple SCM repositories

What is it?

Multiple SCM repositories means using more than one source code storage location in a single Jenkins job. Each repository holds different parts of a project or different projects. Jenkins can fetch code from all these places to build or test together. This helps when code is split across several teams or tools.

Why it matters

Without multiple SCM support, Jenkins jobs would be limited to one code source, making it hard to build projects that depend on code from different places. This would slow down development and cause manual work to combine code. Multiple SCM repositories let Jenkins automate complex builds smoothly, saving time and reducing errors.

Where it fits

Before learning this, you should understand basic Jenkins jobs and how to connect a single SCM repository. After this, you can explore advanced pipeline scripting and multi-branch pipelines that handle complex workflows.

Mental Model

Core Idea

Multiple SCM repositories in Jenkins let one job gather code from several places to build or test together seamlessly.

Think of it like...

Imagine cooking a meal where ingredients come from different stores. Instead of going to one store only, you visit several to get everything you need before cooking. Jenkins does the same by fetching code from multiple repositories before building.

┌─────────────────────────────┐
│        Jenkins Job           │
├─────────────┬───────────────┤
│ SCM Repo 1  │ SCM Repo 2    │
│ (Code A)    │ (Code B)      │
└─────┬───────┴─────┬─────────┘
      │             │
      ▼             ▼
  Checkout       Checkout
      │             │
      └─────┬───────┘
            ▼
       Build & Test

Build-Up - 7 Steps

1

Foundation

Understanding SCM Basics in Jenkins

Concept: Learn what SCM means and how Jenkins connects to one repository.

SCM stands for Source Code Management. Jenkins uses SCM plugins to connect to code repositories like Git or SVN. A simple Jenkins job can be set to pull code from one repository to build or test it.

Result

Jenkins fetches code from a single repository and runs the job.

Knowing how Jenkins connects to one SCM repository is the base for understanding how it can handle multiple repositories.

2

Foundation

Single Repository Checkout Configuration

Concept: Learn how to configure a Jenkins job to checkout code from one SCM repository.

In Jenkins job configuration, under 'Source Code Management', select Git and enter the repository URL. Jenkins will clone this repo during the build.

Result

Jenkins clones the specified repository when the job runs.

This step shows the simple process Jenkins uses to get code, which will be extended to multiple repositories.

3

Intermediate

Adding Multiple SCM Repositories Plugin

🤔Before reading on: do you think Jenkins supports multiple SCMs natively or needs a plugin? Commit to your answer.

Concept: Jenkins needs a plugin to handle multiple SCM repositories in one job.

Install the 'Multiple SCMs' plugin in Jenkins. This plugin allows you to add several SCM configurations in one job. After installation, the job configuration shows a new option to add multiple SCMs.

Result

Jenkins job can now list multiple repositories to checkout during build.

Understanding that Jenkins requires a plugin for multiple SCMs clarifies why this feature is not in basic jobs.

4

Intermediate

Configuring Multiple SCMs in One Job

🤔Before reading on: do you think Jenkins checks out all repositories in parallel or sequentially? Commit to your answer.

Concept: Learn how to add and configure multiple repositories in one Jenkins job.

In the job configuration, select 'Multiple SCMs' and add each repository with its URL and credentials. Jenkins will checkout each repository in order during the build.

Result

Jenkins clones all specified repositories before running build steps.

Knowing the checkout order helps avoid conflicts and manage dependencies between repositories.

5

Intermediate

Using Multiple SCMs in Pipeline Jobs

🤔Before reading on: do you think pipeline syntax supports multiple SCMs directly or requires workarounds? Commit to your answer.

Concept: Explore how to handle multiple SCM repositories in Jenkins pipeline scripts.

Pipeline jobs do not support the 'Multiple SCMs' plugin directly. Instead, use multiple 'checkout' steps with different SCM configurations in the pipeline script to fetch code from multiple repositories.

Result

Pipeline fetches code from multiple repositories during execution.

Understanding pipeline scripting for multiple SCMs enables flexible and powerful build workflows.

6

Advanced

Managing Workspace and Conflicts

🤔Before reading on: do you think Jenkins merges all repositories into one folder or keeps them separate? Commit to your answer.

Concept: Learn how Jenkins handles multiple repositories in the workspace and avoids file conflicts.

By default, Jenkins checks out each repository into separate subfolders inside the workspace. This prevents files from different repos overwriting each other. You can specify folder names for each checkout.

Result

Workspace contains separate folders for each repository's code.

Knowing workspace management prevents build failures due to file overwrites and helps organize code.

7

Expert

Optimizing Multiple SCM Checkouts for Performance

🤔Before reading on: do you think Jenkins fetches all repositories fully every build or can it optimize? Commit to your answer.

Concept: Explore advanced techniques to speed up builds with multiple SCM repositories.

Use shallow clones and reference repositories to reduce checkout time. Configure Jenkins to fetch only changed commits. Also, parallelize checkouts in pipeline scripts to save time.

Result

Builds run faster with optimized SCM fetches and parallel checkouts.

Understanding optimization techniques improves build efficiency and resource use in complex projects.

Under the Hood

Jenkins uses SCM plugins to interact with version control systems. The 'Multiple SCMs' plugin wraps multiple SCM configurations and executes their checkout steps sequentially in the workspace. Each checkout creates a separate folder to isolate code. Pipeline scripts use the 'checkout' step multiple times with different SCM definitions. Internally, Jenkins calls Git or other SCM commands to clone or fetch code. Workspace management ensures no file conflicts. Plugins and pipeline steps handle credentials, branches, and shallow clones to optimize performance.

Why designed this way?

Jenkins was originally designed for single SCM jobs to keep things simple. As projects grew complex, the need to combine code from multiple repositories arose. Instead of redesigning Jenkins core, a plugin approach was chosen to add this feature flexibly. This avoids breaking existing jobs and allows users to opt-in. Pipeline scripting supports multiple SCMs by design to give maximum control. Separating checkouts into folders prevents conflicts and keeps builds reliable.

┌───────────────────────────────┐
│         Jenkins Job            │
├───────────────┬───────────────┤
│ Multiple SCMs │ Pipeline      │
│ Plugin        │ Script        │
├───────┬───────┴───────┬───────┤
│ SCM 1 │ SCM 2         │ SCM N │
│ Clone │ Clone         │ Clone │
├───────┴───────────────┴───────┤
│ Workspace with separate folders│
│ repo1/, repo2/, repoN/         │
└───────────────────────────────┘

Myth Busters - 4 Common Misconceptions

Quick: Does Jenkins natively support multiple SCM repositories in freestyle jobs without plugins? Commit yes or no.

Common Belief: Jenkins can handle multiple SCM repositories in any job type without extra plugins.

Reality: Jenkins freestyle jobs need the 'Multiple SCMs' plugin to support multiple repositories; otherwise, only one SCM is supported.

Why it matters: Assuming native support leads to failed builds or confusion when trying to add multiple repositories without the plugin.

Quick: When using multiple SCMs, does Jenkins merge all code into one folder by default? Commit yes or no.

Common Belief: Jenkins merges all repositories into the same workspace folder automatically.

Reality: Jenkins checks out each repository into separate folders inside the workspace to avoid file conflicts.

Why it matters: Expecting merged code causes confusion and build errors due to overwritten files.

Quick: Can pipeline jobs use the 'Multiple SCMs' plugin directly? Commit yes or no.

Common Belief: Pipeline jobs can use the 'Multiple SCMs' plugin just like freestyle jobs.

Reality: Pipeline jobs do not support the plugin directly; they require multiple 'checkout' steps scripted manually.

Why it matters: Trying to use the plugin in pipelines causes configuration errors and failed builds.

Quick: Does Jenkins always fetch the full history of all repositories every build? Commit yes or no.

Common Belief: Jenkins clones the entire repository history every time it builds.

Reality: Jenkins can use shallow clones and fetch only recent changes to speed up builds.

Why it matters: Not using shallow clones leads to slow builds and wasted resources.

Expert Zone

1

The order of SCM checkouts can affect build results if repositories depend on each other, so explicit ordering is important.

2

Credentials for each repository must be managed carefully to avoid access failures, especially when mixing public and private repos.

3

Parallelizing SCM checkouts in pipelines can improve build speed but requires careful workspace and resource management to avoid conflicts.

When NOT to use

Avoid using multiple SCM repositories in a single job if the repositories are unrelated or independent; instead, create separate jobs or use multi-branch pipelines. For very complex dependencies, consider using a monorepo or artifact repositories to simplify builds.

Production Patterns

In production, teams use multiple SCMs to combine shared libraries, microservices, and infrastructure code in one build. Pipelines script multiple checkouts with precise folder paths and credentials. They optimize with shallow clones and parallel steps. Jobs often trigger downstream jobs after checkout to handle deployment or testing separately.

Connections

Monorepo vs Multirepo

Alternative approaches to managing codebases

Understanding multiple SCMs helps compare the tradeoffs between splitting code into many repositories versus keeping it in one large repository.

Continuous Integration Pipelines

Builds depend on fetching code from SCMs

Knowing how Jenkins handles multiple SCMs clarifies how complex pipelines integrate diverse code sources smoothly.

Supply Chain Management (Logistics)

Both involve coordinating multiple sources to deliver a final product

Just like supply chains gather parts from different suppliers to build a product, Jenkins gathers code from multiple repositories to build software.

Common Pitfalls

#1 Trying to add multiple repositories in a freestyle job without the 'Multiple SCMs' plugin.

Wrong approach:

In Jenkins job configuration, under 'Source Code Management', add multiple Git URLs directly without plugin support.

Correct approach:

Install and enable the 'Multiple SCMs' plugin, then select 'Multiple SCMs' in job configuration and add each repository separately.

Root cause:

Misunderstanding Jenkins' native capabilities and plugin requirements.

#2 Checking out multiple repositories into the same folder causing file overwrites.

Wrong approach:

Configure multiple SCMs without specifying different local subfolders, so all code lands in the root workspace.

Correct approach:

Specify unique local subdirectory for each repository checkout to isolate files.

Root cause:

Not realizing Jenkins defaults to the workspace root and needs explicit folder separation.

#3 Using 'Multiple SCMs' plugin in a pipeline job expecting it to work.

Wrong approach:

In pipeline script, try to call 'multipleSCM' or similar plugin step directly.

Correct approach:

Use multiple 'checkout' steps with separate SCM configurations scripted in the pipeline.

Root cause:

Confusing freestyle job plugins with pipeline scripting capabilities.

Key Takeaways

Jenkins does not support multiple SCM repositories in freestyle jobs without the 'Multiple SCMs' plugin.

Each repository is checked out into its own folder to avoid file conflicts in the workspace.

Pipeline jobs handle multiple SCMs by scripting multiple checkout steps manually.

Optimizing checkouts with shallow clones and parallel steps improves build speed.

Understanding multiple SCMs helps manage complex projects with code spread across repositories.