feat: add separate keyspace stats for write operations

[andymai]

Description

This PR adds separate statistics tracking for write operations in Redis keyspace lookups. Previously, write operations were completely excluded from keyspace statistics. This change provides better visibility into Redis workload patterns by tracking read and write operations separately.

Motivation

Currently, Redis only tracks keyspace hits/misses for read operations. When the LOOKUP_WRITE flag is set (commands like SET, INCR, APPEND, etc.), statistics are not tracked at all. This makes it impossible to understand the write workload characteristics of a Redis instance.

Before this change:

# INFO stats shows only read operations
keyspace_hits:1000       # Only from GET, etc.
keyspace_misses:50       # Only from GET on nonexistent keys

After this change:

# INFO stats shows both read and write operations
keyspace_hits:1000            # GET existing keys
keyspace_misses:50            # GET nonexistent keys
keyspace_write_hits:500       # SET existing keys
keyspace_write_misses:200     # SET new keys

This allows operators to:

• Understand the ratio of reads vs writes
• Identify cache efficiency for write operations
• Monitor workload patterns more accurately
• Debug performance issues related to write patterns

Changes

Code Changes

1. Added new stats in src/server.h:

   • stat_keyspace_write_hits - Successful write lookups (key exists)
   • stat_keyspace_write_misses - Failed write lookups (key doesn't exist)

2. Updated tracking logic in src/db.c:

   • Modified lookupKey() to check LOOKUP_WRITE flag
   • Write operations now increment write-specific counters
   • Read operations continue using existing counters
   • Removed TODO comments at lines 283 and 289

3. Initialized stats in src/server.c:

   • Added initialization in resetServerStats()
   • Added display in genRedisInfoString() for INFO command

Implementation Details

The implementation uses the existing LOOKUP_WRITE flag to differentiate operations:

Before (excluded writes):

if (!(flags & (LOOKUP_NOSTATS | LOOKUP_WRITE)))
    server.stat_keyspace_hits++;

After (separate tracking):

if (!(flags & LOOKUP_NOSTATS)) {
    if (flags & LOOKUP_WRITE)
        server.stat_keyspace_write_hits++;
    else
        server.stat_keyspace_hits++;
}

Testing

Manual Testing Performed

# Start fresh Redis instance
./src/redis-server

# Test write miss (SET new key)
SET key1 "value"
# Result: keyspace_write_misses increments

# Test write hit (SET existing key)
SET key1 "value2"
# Result: keyspace_write_hits increments

# Test read hit (GET existing key)
GET key1
# Result: keyspace_hits increments

# Test read miss (GET nonexistent key)
GET nonexistent
# Result: keyspace_misses increments

Test Results

All scenarios passed with expected behavior:

• ✅ Write operations on new keys increment write_misses
• ✅ Write operations on existing keys increment write_hits
• ✅ Read operations on existing keys increment keyspace_hits
• ✅ Read operations on nonexistent keys increment keyspace_misses
• ✅ All stats display correctly in INFO command
• ✅ Stats reset properly on server restart

Type of Change

• [x] Enhancement (adds new functionality without breaking existing behavior)
• [x] Feature (improves observability and monitoring capabilities)

Backward Compatibility

✅ Fully backward compatible

• Existing stats (keyspace_hits, keyspace_misses) remain unchanged in behavior
• New stats are additive - no existing functionality is modified
• INFO command output includes new fields without breaking parsers (new fields at end)
• No configuration changes required
• No behavioral changes for Redis commands

Note: The existing keyspace_hits/misses will now only count read operations, whereas previously they counted reads (writes were excluded entirely). This is technically a behavior change but represents a bug fix - write operations are now properly accounted for in separate counters.

Performance Impact

✅ Minimal performance impact

• One additional conditional check per lookup operation
• One counter increment per operation (same as before)
• No memory allocations or complex computations
• Stats are simple long long variables

Estimated overhead: < 1 nanosecond per operation on modern hardware.

Additional Notes

Resolved TODOs

This PR resolves two long-standing TODO comments:

• src/db.c:283 - "Use separate hits stats for WRITE"
• src/db.c:289 - "Use separate misses stats and notify event for WRITE"

Note: The second TODO also mentions "notify event for WRITE" which was not implemented in this PR. Investigation showed that write operations already have appropriate notification handling via the LOOKUP_NONOTIFY flag check. If separate write notifications are needed, that can be addressed in a future PR.

Commands Affected

Commands using lookupKeyWrite() now have stats tracked:

• SET, SETEX, SETNX
• INCR, DECR, INCRBY, DECRBY
• APPEND, SETRANGE
• LPUSH, RPUSH, LPOP, RPOP
• SADD, SREM
• ZADD, ZREM
• HSET, HDEL
• And all other write operations

Future Enhancements

Possible future work building on this change:

• Add write stats to redis-cli --stat output
• Add historical tracking (moving averages)
• Add per-database write statistics
• Add documentation to redis.io

---

Ready for review! 🚀

Resolves: TODOs at src/db.c:283 and src/db.c:289

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