Enterprise Search in Japanese: Why We Switched from Kuromoji to N-gram

The Problem: “Search Results Are Wrong”

On a project, a client came to us with a complaint:

“The search results don’t make sense.”

Results were showing up that didn’t match the search terms.

Context

• Not a DB search — ElasticSearch
• Data volume too large for an RDBMS
• Japanese-language search

At this point, I already knew this would be tricky.

Root Cause: Kuromoji

After investigation, the cause was straightforward.

👉 They were using Kuromoji.

Kuromoji is a morphological analyzer, which means:

• It breaks text into word units
• It analyzes based on context

In other words:

👉 It’s built for “text comprehension,” not “search”

What Was Happening

For example, even when the search term was an exact match:

• Tokenization differed
• Tokens changed
• Unintended matches occurred

The result:

👉 “Why is this showing up?” 👉 “Why isn’t this showing up?”

Conclusion: Not Suited for General Search

Kuromoji is powerful, but:

👉 It’s not suited for general search (partial match, fuzzy search)

Especially in enterprise systems, users expect:

• Near-exact match behavior
• Partial matching
• Intuitive search results

Kuromoji doesn’t deliver that.

Solution: Switch to N-gram

The fix was simple:

👉 Switch to n-gram

This gave us:

• Working partial matches
• Search results that matched user expectations

The Real Challenge

Since this was ElasticSearch:

👉 Changing the analyzer = full reindex required

That means:

• Reloading all data
• Risk of downtime

Zero-Downtime Release with Aliases

The technique we used:

👉 Index aliases

Here’s the process:

1. Create a new index (with n-gram analyzer)
2. Reindex the data
3. Verify behavior in advance
4. Switch the alias

👉 The actual release takes a split second

Why This Matters

With this approach:

• You can test beforehand
• Zero production impact
• Easy rollback

Summary

• Kuromoji is for “text analysis”
• N-gram is for “search”
• Japanese search requires choosing the right tool for the job

One Takeaway

👉 Japanese enterprise search is 90% decided by your technology choice

Side Note (From Experience)

Kuromoji is powerful when you understand it properly.

But if you just default to “it’s Japanese, so use Kuromoji” — things will break.