FileHandler vs DatabaseHandler for CodeIgniter 4 Session Persistence

Direct answer

For horizontal scaling across load-balanced nodes, DatabaseHandler is the most sustainable built-in CodeIgniter 4 session driver because session rows live in one table reachable by every node. FileHandler is only viable if all nodes share the same writable session directory; otherwise sessions fragment by node and a user authenticated on node A cannot resume on node B.

Database overhead outweighs persistence benefits when request volume is very high, sessions are large or regenerated on every request, many concurrent requests share one session ID, the sessions table lacks indexes on id and timestamp, or the database is already near CPU or connection limits. Write-heavy traffic and AJAX polling sessions are the most likely to turn the database into the bottleneck.

Likely explanation versus confirmed facts

Likely explanation: FileHandler performs local disk I/O and avoids a database round trip, so it is faster on a single server. DatabaseHandler adds at least one read and usually a write or garbage-collection operation per request, which introduces network latency and connection-pool pressure.

Confirmed scope: This answer covers only CodeIgniter 4's built-in FileHandler and DatabaseHandler. It excludes Redis, Memcached, and other third-party stores. Handler defaults, table schema, cookie behavior, and garbage-collection tuning can change between 4.x releases, so verify against your installed version.

Steps for this case

1. Open app/Config/Session.php and confirm driverName and savePath. For DatabaseHandler, savePath should name the sessions table.
2. Confirm whether every load-balanced node can reach the same database and whether all nodes share the same writable session directory if you consider FileHandler.
3. Ensure the sessions table has indexes on the session ID column and the timestamp column used by garbage collection.
4. Run a load test with realistic session size and concurrency. Compare request latency, database CPU, lock waits, connection-pool usage, and garbage-collection frequency between drivers.
5. If database latency or lock contention rises linearly with concurrency, keep sessions small or move session storage off the relational database; neither built-in driver is ideal at extreme scale.

Missing diagnostic

Do all load-balanced nodes share the same writable session directory? This single detail determines whether FileHandler is even a candidate; if the answer is no, DatabaseHandler is the only built-in option for shared session state.

Verification commands

# Inspect session driver configuration
grep -n 'driverName\|savePath' app/Config/Session.php

# Confirm sessions table indexes (adjust table name as needed)
SHOW INDEX FROM sessions;

Assumptions: CodeIgniter 4.x built-in handlers only; all nodes use the same database connection details; cookie settings such as secure, SameSite, expiration, and IP matching are handled separately and do not replace shared storage. FileHandler over a network filesystem can introduce stale locks and inconsistent garbage collection, so test node failover before production use.