Storage Already Registered Error in KSP with Room Type Converters

Problem Statement

When adding new type converters to Room databases using Kotlin Symbol Processing (KSP) in Android projects, developers often encounter this compilation-stopping error:

java.lang.IllegalStateException: Storage for [C:\...\symbolLookups\id-to-file.tab] is already registered

This error occurs when:

1. You add or modify a Room type converter (especially for complex types like List<String>)
2. Use KSP for annotation processing
3. Work on projects where previous compilation artifacts exist

The root cause is a caching conflict in KSP's incremental processing. When new type converters are introduced, KSP fails to properly invalidate cached symbol lookup files, causing the "already registered" conflict during subsequent compilations.

Recommended Solutions

Permanent Solution: Disable KSP Incremental Processing

Add this line to your project's gradle.properties file:

# Disable incremental processing to prevent cache conflicts
ksp.incremental=false

How it works:
This configuration disables KSP's incremental compilation feature, forcing a full reprocessing of symbols each build. The trade-off is slightly longer compilation times, but it reliably prevents the storage registration conflict.

Implementation steps:

1. Open your project's root gradle.properties file
2. Add ksp.incremental=false
3. Sync Gradle and rebuild your project

TIP

If you're using multiple KSP processors, configure this per module:

// In module-level build.gradle.kts
ksp {
    incremental = false
}

Temporary Workarounds

When immediate fixes are needed (without modifying build properties):

1. Stop Gradle Daemon
   Run in terminal:

   ./gradlew --stop

2. Clean Project
   In Android Studio: Build > Clean Project
3. Invalidate Caches
   File > Invalidate Caches / Restart > Invalidate and Restart

Limitations

These are temporary solutions that resolve the current cache conflict but don't prevent recurrence when adding new type converters later.

Why These Solutions Work

The KSP Cache Conflict Explained

When you declare a Room type converter:

class Converters {
    @TypeConverter
    fun fromListToString(list: List<String>): String = list.joinToString(",")
    
    @TypeConverter
    fun fromStringToList(data: String): List<String> = data.split(",")
}

KSP generates symbol tables to process these annotations. With incremental processing enabled (ksp.incremental=true by default), KSP reuses partial caches between compilations. When you add new converters:

1. Existing cache files remain registered
2. New symbol files attempt to register at same paths
3. Conflict occurs: Storage [...] is already registered

Disabling incremental processing forces KSP to:

• Regenerate all symbols from scratch
• Avoid reuse of previous storage registrations
• Eliminate path conflicts entirely

Best Practices Prevention

1. Consistent Environment Setup
   Ensure all team members use compatible KSP and Room versions:

   // Recommended versions in build.gradle
   plugins {
       id("com.google.devtools.ksp") version "1.9.22-1.0.17"
   }
   
   dependencies {
       implementation("androidx.room:room-runtime:2.6.1")
       ksp("androidx.room:room-compiler:2.6.1")
   }

2. Repository Hygiene
   Add these to .gitignore to prevent sharing problematic caches:

   # KSP cache files
   .ksp/
   build/generated/ksp/
   
   # Gradle caches
   .gradle/
   build/
   !build.gradle

3. Continuous Integration Configuration
   For CI pipelines (like GitHub Actions), add initial clean steps:

   - name: Build project
     run: |
       ./gradlew --stop
       ./gradlew clean
       ./gradlew :app:assembleDebug

When to Re-enable Incremental Processing

If KSP fixes this cache invalidation issue in future updates, you can safely re-enable incremental processing for faster builds:

# Remove or comment in gradle.properties
# ksp.incremental=true

Monitor the KSP GitHub Issues for resolution updates regarding cached storage conflicts.

Key Takeaways

1. The "storage already registered" error stems from KSP cache conflicts during incremental compilation
2. Disable incremental processing provides a definitive fix via ksp.incremental=false
3. Temporary solutions like stopping Gradle or invalidating caches are helpful but not permanent
4. Keep Room and KSP versions synchronized across your development environment
5. Clean build systems regularly in continuous integration pipelines

By applying the permanent configuration change, you can seamlessly add new type converters without interrupting your development workflow with cache conflicts.