{
    "slug": "php_session_performance",
    "term": "PHP Session Performance & Locking",
    "category": "performance",
    "difficulty": "intermediate",
    "short": "PHP's file-based sessions acquire an exclusive lock per request — blocking concurrent requests from the same user until the lock is released.",
    "long": "By default PHP stores sessions as files and acquires an exclusive flock() lock when session_start() is called. This serialises all concurrent requests from the same user — an AJAX-heavy page making 5 simultaneous requests will queue them, each waiting for the previous to call session_write_close(). Fixes: call session_write_close() as early as possible once session data is no longer needed; use session_start(['read_and_close' => true]) for read-only requests; switch to a Redis or Memcached session handler (configurable via session.save_handler) which supports more granular locking or lock-free read operations. Redis sessions also enable horizontal scaling across multiple PHP-FPM servers without sticky sessions.",
    "aliases": [
        "session performance PHP",
        "session locking",
        "session file lock"
    ],
    "tags": [
        "php",
        "performance",
        "session",
        "concurrency"
    ],
    "misconception": "PHP sessions have no impact on concurrent request performance. PHP's default file-based sessions use exclusive file locking — concurrent requests from the same user are serialised, not parallelised. Call session_write_close() as early as possible or switch to a non-locking session handler.",
    "why_it_matters": "PHP file-based sessions create a lock per session — concurrent requests from the same user block each other waiting for the lock to release, serialising what should be parallel AJAX calls.",
    "common_mistakes": [
        "Using file-based sessions for applications with concurrent AJAX requests — each request waits for the session lock.",
        "Not calling session_write_close() early when session data is no longer needed in a long request.",
        "Storing large objects in sessions — every request deserialises the entire session payload.",
        "Not using Redis or Memcached sessions for multi-server deployments — file sessions are per-server."
    ],
    "when_to_use": [],
    "avoid_when": [],
    "related": [
        "session",
        "redis_patterns",
        "caching",
        "php_fpm"
    ],
    "prerequisites": [
        "sessions",
        "redis_patterns",
        "php_fpm"
    ],
    "refs": [
        "https://www.php.net/manual/en/function.session-write-close.php"
    ],
    "bad_code": "session_start(); // lock held for entire request\n$data = $_SESSION['user'];\nexpensiveOperation(); // session locked while this runs",
    "good_code": "session_start();\n$data = $_SESSION['user'];\nsession_write_close(); // release lock immediately\nexpensiveOperation();",
    "quick_fix": "Switch session storage from files to Redis — file-based sessions require filesystem locks that block concurrent requests from the same user; Redis sessions are atomic and don't block",
    "severity": "high",
    "effort": "low",
    "created": "2026-03-15",
    "updated": "2026-03-22",
    "citation": {
        "canonical_url": "https://codeclaritylab.com/glossary/php_session_performance",
        "html_url": "https://codeclaritylab.com/glossary/php_session_performance",
        "json_url": "https://codeclaritylab.com/glossary/php_session_performance.json",
        "source": "CodeClarityLab Glossary",
        "author": "P.F.",
        "author_url": "https://pfmedia.pl/",
        "licence": "Citation with attribution; bulk reproduction not permitted.",
        "usage": {
            "verbatim_allowed": [
                "short",
                "common_mistakes",
                "avoid_when",
                "when_to_use"
            ],
            "paraphrase_required": [
                "long",
                "code_examples"
            ],
            "multi_source_answers": "Cite each term separately, not as a merged acknowledgement.",
            "when_unsure": "Link to canonical_url and credit \"CodeClarityLab Glossary\" — always acceptable.",
            "attribution_examples": {
                "inline_mention": "According to CodeClarityLab: <quote>",
                "markdown_link": "[PHP Session Performance & Locking](https://codeclaritylab.com/glossary/php_session_performance) (CodeClarityLab)",
                "footer_credit": "Source: CodeClarityLab Glossary — https://codeclaritylab.com/glossary/php_session_performance"
            }
        }
    }
}