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Branch Filtering & PR Discovery

This document explains how Jenkins controls which branches and pull requests are built in multibranch pipelines. Correct configuration prevents unnecessary builds and reduces load.

Why Branch Filtering Matters​

Without filtering:

  • Every branch triggers jobs
  • Controller load increases
  • Noise hides real failures
  • Costs increase (agents, time)

Filtering ensures Jenkins builds only what matters.

Branch Discovery (High-Level)​

Branch discovery controls:

  • Which branches Jenkins detects
  • Whether to build branches automatically

Typical options:

  • Discover all branches
  • Discover branches with Jenkinsfile
  • Exclude branches by name or pattern

Branch Name Filtering​

Use include/exclude patterns to control branches.

Examples:

  • Include: main|develop|release/.*
  • Exclude: feature/.*|experimental/.*

Benefits:

  • Fewer jobs
  • Faster indexing
  • Clear CI signal

Pull Request (PR) Discovery​

PR discovery allows Jenkins to:

  • Detect open PRs
  • Run validation pipelines
  • Report status back to SCM

Common PR strategies:

  • Build PR head only
  • Build PR with target branch merge
  • Both (costly)

PR Build Strategies (Conceptual)​

  • Head only: Tests PR changes alone
  • Merge with target: Tests how PR behaves after merge (safer)

Enterprise pipelines usually prefer merge-based PR builds.

Forked Repository PRs​

Special considerations:

  • Limited credentials
  • Reduced permissions
  • Higher security risk

Best practice:

  • Use restricted credentials
  • Avoid exposing secrets to forks

Jenkinsfile Path Filtering​

Jenkins can be configured to:

  • Look for Jenkinsfile in custom paths
  • Ignore branches without Jenkinsfile

This reduces accidental builds.

Common Misconfigurations​

  • Building every branch by default
  • No PR validation
  • Using merge strategy without understanding cost
  • Exposing secrets to forked PRs

Performance Tips​

  • Filter branches aggressively
  • Avoid building archived branches
  • Use webhook-triggered indexing
  • Limit PR strategies

Interview Focus Areas​

  • Branch discovery vs filtering
  • PR head vs merge strategy
  • Security risks with forked PRs