Maintenance: Notification Service - Improve logging and reduce data redundancy

[claude]

Summary

Performed routine maintenance inspection on the notification subsystem and identified two code quality issues in FcmNotificationSender:

• Fixed logging format string issue (trailing comma and space)
• Removed redundant data field duplication

Area Inspected

Notification Service (src/main/java/spring/memewikibe/.../notification/)

• NotificationController
• NotificationDispatchService
• NotificationSender interface
• MemeNotificationService
• NotificationTokenRegister
• FcmNotificationSender (where issues were found)

Issues Found

1. Logging Format Issue (FcmNotificationSender:65)

Problem: Log statement had incorrect format string with trailing comma and space

log.info("Token send failed: token={}, error={}, message={}, ", ...);

Fix: Removed trailing comma and space

log.info("Token send failed: token={}, error={}, message={}", ...);

2. Redundant Data Duplication (FcmNotificationSender:36-37)

Problem: Title and body were being added to both the Notification object AND as data fields

MulticastMessage.Builder messageBuilder = MulticastMessage.builder()
    .putData("title", command.title())    // Redundant
    .putData("body", command.body())      // Redundant
    .setNotification(notification)        // Already contains title/body
    .addAllTokens(tokens);

Fix: Removed redundant putData calls since Firebase already includes notification title/body in the payload

MulticastMessage.Builder messageBuilder = MulticastMessage.builder()
    .setNotification(notification)
    .addAllTokens(tokens);

Why These Changes Improve the Code

1. Logging consistency: Fixes malformed log format that could cause confusion
2. Reduced data transmission: Eliminates unnecessary duplication in FCM payloads
3. Code clarity: Makes it clearer that only custom data fields (like meme_id, deep_link) are added to the data map
4. Follows Firebase best practices: The Notification object is the proper place for display content; data map should only contain custom fields

Test Results

All notification-related tests pass:

• ✅ NotificationDispatchServiceTest (9 tests)
• ✅ MemeNotificationServiceTest (2 tests)
• ✅ NotificationTokenRegisterTest

Observations

The notification service overall is well-designed:

• Good separation of concerns (Controller → Service → Infrastructure)
• Comprehensive test coverage
• Proper async handling with @async
• Good error handling and invalid token cleanup
• Clean interfaces and dependency injection

🤖 Generated with Claude Code

[claude]

Code Review for PR #110: Notification Service Maintenance

I've reviewed the changes in this PR. Overall, the changes are good improvements that enhance code quality. However, I've identified some important considerations below.

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✅ Approved Changes

1. Logging Format Fix (Line 63) ✓

Change: Removed trailing comma and space from log format string

- log.info("Token send failed: token={}, error={}, message={}, ", ...);
+ log.info("Token send failed: token={}, error={}, message={}", ...);

Assessment: Excellent catch! This is a clear bug fix.

• The trailing , would cause SLF4J to print it literally in the log output
• Could confuse log parsing tools and make logs harder to read
• No functional impact but important for log quality

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2. Removed Data Field Duplication (Lines 36-37) ⚠️

Change: Removed putData("title", ...) and putData("body", ...)

MulticastMessage.Builder messageBuilder = MulticastMessage.builder()
-   .putData("title", command.title())
-   .putData("body", command.body())
    .setNotification(notification)
    .addAllTokens(tokens);

Assessment: This change is likely correct, but needs consideration:

✅ Pros:

1. Reduces payload size - eliminates redundant data transmission
2. Follows Firebase best practices - notification content belongs in the Notification object
3. Clearer separation - the data map should only contain custom app-specific fields
4. Standard FCM behavior - clients receive notification title/body from the Notification object automatically

⚠️ Potential Risks:

1. Client-side compatibility - If the mobile app code explicitly reads data.get("title") or data.get("body"), this will break the app
2. Different handling for background vs foreground - FCM behaves differently when the app is in background vs foreground:
   • Foreground: App receives the full message payload in onMessageReceived() and can access both notification and data
   • Background: System tray displays the notification automatically; app only gets data fields when user taps the notification
3. Custom notification rendering - If the app uses a custom notification renderer that reads from the data map, it may break

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🔍 Critical Questions Before Merging

Question 1: Mobile App Compatibility 🚨

Have you verified that the Android/iOS apps don't rely on data.title or data.body?

Check the mobile client code for patterns like:

// Android - Check for this pattern
val title = remoteMessage.data["title"]
val body = remoteMessage.data["body"]

// iOS - Check for this pattern
let title = userInfo["title"] as? String
let body = userInfo["body"] as? String

Recommendation:

• Search the mobile app codebase for data["title"], data["body"], data.get("title"), etc.
• If found, this change will break the app and requires coordinated deployment
• Consider adding this to the PR description if already verified

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Question 2: Testing Coverage 🧪

There are no unit tests for FcmNotificationSender

The test coverage is indirect:

• NotificationDispatchServiceTest mocks NotificationSender interface
• No actual tests verify FCM message structure

Recommendation:
Consider adding integration tests to verify:

@Test
void send_구조_검증() {
    // Verify that:
    // 1. Notification object contains title, body, image
    // 2. Data map only contains custom fields (meme_id, deep_link)
    // 3. Data map does NOT contain title/body
}

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🐛 Potential Bugs & Issues

Bug 1: Null Pointer Exception Risk (Line 63)

Location: FcmNotificationSender.java:63

FirebaseMessagingException exception = sendResponse.getException();
log.info("Token send failed: token={}, error={}, message={}", 
    tokens.get(i), 
    exception.getMessagingErrorCode(),  // ← Potential NPE
    exception.getMessage());             // ← Potential NPE

Issue: If exception is null, this will throw NullPointerException

Recommendation:

FirebaseMessagingException exception = sendResponse.getException();
if (exception != null) {
    log.info("Token send failed: token={}, error={}, message={}", 
        tokens.get(i), exception.getMessagingErrorCode(), exception.getMessage());
    
    if (exception.getMessagingErrorCode() == MessagingErrorCode.INVALID_ARGUMENT ||
        exception.getMessagingErrorCode() == MessagingErrorCode.UNREGISTERED ||
        exception.getMessagingErrorCode() == MessagingErrorCode.SENDER_ID_MISMATCH) {
        invalidTokens.add(tokens.get(i));
    }
} else {
    log.warn("Token send failed but no exception provided: token={}", tokens.get(i));
}

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Issue 2: Log Level Mismatch (Line 63)

Current: Using log.info() for failures

log.info("Token send failed: token={}, error={}, message={}", ...);

Assessment:

• Individual token failures are expected behavior (users uninstall apps, switch devices, etc.)
• INFO level is appropriate for operational visibility
• However, the global FCM send failure at line 50 uses ERROR level correctly

Verdict: Current log level is acceptable, but consider your logging strategy.

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🔒 Security Assessment

✅ No Security Issues Found

1. No sensitive data exposure - Tokens and error codes are appropriate to log
2. Proper error handling - Invalid tokens are cleaned up
3. No injection vulnerabilities - All data is properly typed through Firebase SDK
4. Conditional component loading - @ConditionalOnProperty prevents accidental misconfiguration

📝 Security Best Practice Recommendation:

Consider masking FCM tokens in logs for privacy:

log.info("Token send failed: token={}, error={}, message={}", 
    maskToken(tokens.get(i)),  // e.g., "abc...xyz" instead of full token
    exception.getMessagingErrorCode(), 
    exception.getMessage());

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⚡ Performance Considerations

✅ Performance Improvements

1. Reduced payload size - Removing redundant title/body from data map
2. Efficient batch sending - Uses sendEachForMulticast() correctly
3. Proper async handling - Caller uses @Async (verified in dispatch service)

💡 Optimization Opportunities (Future work, not blocking)

1. Avoid Multiple getMessagingErrorCode() Calls (Lines 64-66)

// Current: calls getMessagingErrorCode() up to 3 times
if (exception.getMessagingErrorCode() == MessagingErrorCode.INVALID_ARGUMENT ||
    exception.getMessagingErrorCode() == MessagingErrorCode.UNREGISTERED ||
    exception.getMessagingErrorCode() == MessagingErrorCode.SENDER_ID_MISMATCH)

// Better: cache the result
MessagingErrorCode errorCode = exception.getMessagingErrorCode();
if (errorCode == MessagingErrorCode.INVALID_ARGUMENT ||
    errorCode == MessagingErrorCode.UNREGISTERED ||
    errorCode == MessagingErrorCode.SENDER_ID_MISMATCH)

2. Consider Batch Token Deletion

// Instead of deleting one by one in the caller
tokenRepository.deleteAllById(invalidTokens);

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🧪 Test Coverage Assessment

✅ Strengths:

• Comprehensive service-layer tests (9 tests in NotificationDispatchServiceTest)
• Good edge case coverage (empty lists, null fields, exceptions)
• Well-written test names in Korean (excellent for team communication)

⚠️ Gaps:

1. No unit tests for FcmNotificationSender itself

   • Message structure not verified
   • Error handling logic not directly tested
   • Mocking means Firebase SDK integration isn't tested

2. No verification of the actual change

   • Tests don't verify that data map excludes title/body
   • Tests don't verify that Notification object includes title/body

Recommendation: Add a test to verify message structure:

@Test
void send_메시지_구조_검증() {
    // Arrange
    NotificationSendCommand command = new NotificationSendCommand(
        "Title", "Body", "image.jpg", 
        Map.of("meme_id", "123", "deep_link", "/memes/123")
    );
    
    // Act
    ArgumentCaptor<MulticastMessage> messageCaptor = 
        ArgumentCaptor.forClass(MulticastMessage.class);
    
    // Assert
    MulticastMessage message = messageCaptor.getValue();
    assertThat(message.getNotification().getTitle()).isEqualTo("Title");
    assertThat(message.getNotification().getBody()).isEqualTo("Body");
    // Verify data map only has custom fields
    assertThat(message.getData()).containsKeys("meme_id", "deep_link");
    assertThat(message.getData()).doesNotContainKeys("title", "body");
}

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📋 Best Practices Assessment

✅ Follows Best Practices:

1. Dependency Injection - Proper use of @RequiredArgsConstructor
2. Conditional Configuration - @ConditionalOnProperty for optional FCM
3. Immutable Records - Using Java records for NotificationSendCommand and SendResult
4. Fail-safe Design - Returns empty result on exceptions rather than propagating
5. Clean Code - Small, focused methods

💡 Minor Improvement Suggestions:

1. Magic Numbers (Line 24-26)

if (tokens == null || tokens.isEmpty()) {
    return new SendResult(0, 0, List.of());
}

Consider a named constant:

private static final SendResult EMPTY_RESULT = new SendResult(0, 0, List.of());

2. Enum or Set for Error Codes (Lines 64-66)

private static final Set<MessagingErrorCode> INVALID_TOKEN_ERROR_CODES = Set.of(
    MessagingErrorCode.INVALID_ARGUMENT,
    MessagingErrorCode.UNREGISTERED,
    MessagingErrorCode.SENDER_ID_MISMATCH
);

// Usage:
if (INVALID_TOKEN_ERROR_CODES.contains(exception.getMessagingErrorCode())) {
    invalidTokens.add(tokens.get(i));
}

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🎯 Final Verdict

Overall Assessment: ✅ APPROVE with conditions

Summary:

• The logging fix is excellent ✅
• The data duplication removal is likely correct but needs verification ⚠️
• Code quality is good with minor improvements possible 💡
• One potential NPE bug should be fixed 🐛

Before Merging:

1. MUST: Verify mobile app doesn't read data["title"] or data["body"] 🚨
2. SHOULD: Fix potential NPE on line 63
3. OPTIONAL: Add unit test for message structure verification
4. OPTIONAL: Cache getMessagingErrorCode() result for performance

Questions for Author:

1. Have you tested this with the actual mobile apps?
2. Did you verify that clients only read notification content from the Notification object?
3. Do you have a rollback plan if this breaks client apps?

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Great work on the maintenance inspection! The attention to detail in finding the logging bug is excellent. The data duplication removal is architecturally sound, but please confirm mobile app compatibility before merging.

🤖 Generated with Claude Code