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The reliable pattern is cache-aside: Redis is checked first, MySQL remains the source of truth, and a miss loads the row from MySQL before storing it in Redis with an expiration. After a successful MySQL update, delete the corresponding Redis key so the next read reloads current data.
Install Redis privately on the VPS, secure it with authentication and firewall rules, choose either the Composer-based Predis client or the PhpRedis extension, then add namespaced keys, explicit invalidation, and a short lock for popular keys. The result is lower database load without pretending that Redis and MySQL are one synchronized database.
How the Redis–MySQL cache works
Redis is a separate in-memory service. It accelerates reads but does not replace MySQL as the durable source of truth. Your PHP application owns the cache policy:
- Construct a deterministic key for the requested record.
- Read Redis first.
- If the key exists, decode the cached value and return it.
- If it is absent, query MySQL, encode the result, store it in Redis with a TTL, and return it.
- When MySQL data is changed successfully, delete the related Redis key.
This is the cache-aside pattern. A TTL limits how long an invalid entry can survive, while deletion after writes handles normal update paths immediately. Redis documentation describes deletion followed by lazy repopulation as the simplest and safest way to avoid trying to synchronize two data stores directly.
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What a request looks like
For a product with ID 42, the application might use cache:app:product:42. A hit avoids the MySQL query. A miss performs one query and writes the result. If the product is edited, the write transaction commits in MySQL and the application then issues DEL cache:app:product:42.
Prerequisites and planning decisions
- A Linux VPS with root or sudo access.
- A running MySQL or compatible server containing the authoritative data.
- PHP 7.4 or newer; PHP 8.x is the preferred baseline in Redis’ PHP guidance.
- Composer if you choose Predis, or PECL and permission to enable a PHP extension if you choose PhpRedis.
- Network access from the PHP runtime to Redis, normally on TCP port 6379.
- A plan for where Redis listens, how it authenticates clients, and which firewall rules permit access.
Keep Redis on the same VPS as PHP when practical. If it must run on another host, use an encrypted connection or a private tunnel; do not send an unauthenticated Redis protocol over an untrusted network.
Install Redis on the VPS
Use the package or container method maintained for your distribution. Package names and configuration paths can vary slightly by release, so confirm the service name shown by your package manager.
Ubuntu or Debian
sudo apt update
sudo apt install redis-server
sudo systemctl enable --now redis-serverThe first command refreshes package metadata, the second installs the daemon and command-line client, and the third starts Redis now and on subsequent boots.
Rocky Linux or AlmaLinux
sudo dnf install redis
sudo systemctl enable --now redisIf the service name differs on your image, list installed units with systemctl list-unit-files | grep -i redis and use the unit supplied by the distribution package.
Docker
docker run -d --name redis
-p 127.0.0.1:6379:6379
redisBinding the published port to 127.0.0.1 keeps the container reachable only from the VPS. Pin a tested image tag or digest rather than relying on an unchanging latest tag for production deployments, and configure persistent storage if your operational design requires Redis data to survive container replacement. A cache can be rebuilt, but persistence may still matter for warm-start behavior.
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Secure Redis before connecting PHP
Redis’ installation guidance warns that an internet-exposed instance without security is very easy to exploit. Treat a default installation as a service that still needs hardening.
Bind only the interfaces you need
In the Redis configuration file supplied by your distribution, set bind to loopback addresses when PHP runs on the same VPS, commonly 127.0.0.1 ::1. If Redis is on a separate private host, bind only to the private interface required by the application. Do not bind to every interface unless a protected network design specifically requires it.
Require authentication
Use the configuration mechanism supported by your Redis version: a password for a simple single-user deployment, or ACL users and least-privilege commands for a multi-application setup. Keep the secret outside source control, preferably in the VPS secret store or an environment variable readable by the PHP process. Restart Redis after changing its configuration and verify that PHP authenticates successfully.
Apply a firewall policy
Allow TCP 6379 only from the PHP host or private application network. Deny public inbound access. A loopback-only deployment normally needs no external Redis firewall rule at all.
Encrypt traffic that leaves the trusted host
When PHP and Redis are separated across hosts or networks, use Redis TLS support where available or place the connection inside a properly secured private tunnel. Authentication alone does not encrypt credentials or cached values in transit.
Verify the daemon and service startup
Run the Redis client locally:
redis-cli PINGA healthy unauthenticated local test returns PONG. If authentication is enabled, supply credentials using the method configured for your installation. Confirm that the service is active and enabled:
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systemctl is-active redis-server
systemctl is-enabled redis-serverOn distributions that call the unit redis, substitute that name. Check the service log if startup fails, especially after editing bind, authentication, TLS, or memory settings.
Choose a PHP Redis client
| Characteristic | Predis | PhpRedis |
|---|---|---|
| Installation | Composer package; no compiled PHP extension | PECL extension compiled for the PHP runtime |
| Basic install | composer require predis/predis |
pecl install redis, then enable the extension in php.ini |
| Operational control | Managed with the application dependency lockfile | Managed with PHP modules, package builds, and PHP-FPM or web-server restarts |
| Deployment portability | Convenient where Composer is available and extensions cannot be changed | Convenient when you control the VPS image and can standardize PHP extensions |
| Runtime model | PHP implementation of the Redis protocol | Native PHP extension with generally lower client overhead in many deployments; measure your own workload rather than assuming a benchmark |
Both clients connect to the same Redis server and support the cache-aside design. The choice is primarily an operational one: Predis simplifies deployment on managed PHP environments, while PhpRedis gives you a compiled extension when you control the server image.
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composer require predis/predisRun Composer as the deployment user in the application directory. Commit the resulting lockfile and deploy the vendor directory or run Composer during deployment.
Install PhpRedis
sudo pecl install redisEnable the resulting extension in the PHP configuration used by your web process, then restart PHP-FPM or the web server. The command-line PHP binary and PHP-FPM can load different php.ini files, so verify the module in the same execution context that serves requests:
php -m | grep -i redisConnect PHP to Redis
Predis connection
<?php
require __DIR__ . '/vendor/autoload.php';
$redis = new PredisClient([
'scheme' => 'tcp',
'host' => '127.0.0.1',
'port' => 6379,
'password' => getenv('REDIS_PASSWORD') ?: null,
]);
$redis->ping();If your Redis server does not use a password, omit the password option instead of passing an empty credential. For a remote secured endpoint, configure the client for the TLS or tunnel arrangement used by your infrastructure.
PhpRedis connection
<?php
$redis = new Redis();
$redis->connect('127.0.0.1', 6379, 1.0);
$password = getenv('REDIS_PASSWORD');
if ($password) {
$redis->auth($password);
}
$redis->ping();Set a finite connection timeout. A cache should not make every web request wait indefinitely when Redis is unavailable.
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Design keys and cached values
Use a namespace that identifies the application, environment, entity, and identifier. A practical form is:
cache:app:product:42- Environment separation: include a production or staging namespace when multiple environments can reach the same Redis instance.
- Collision avoidance: include the application name and entity type when Redis is shared.
- Versioning: add a schema suffix when the serialized representation changes, such as
cache:app:product:v2:42. - Bounded size: cache only fields needed by the read path, not an entire object graph by default.
JSON is easy to inspect and language-neutral. PHP serialization preserves more PHP types but couples the value to PHP implementation details. Whichever format you choose, handle decoding failures as misses and repopulate from MySQL.
Use a string value for one complete record. Use a Redis hash with HSET when you need to read or inspect individual fields; keep the key and expiration policy explicit because a hash still needs a key-level TTL.
Implement cache-aside around MySQL
The following example uses PDO, JSON, and Predis. The same flow applies with PhpRedis; only client method syntax differs.
<?php
function productKey(int $id): string
{
return 'cache:app:product:' . $id;
}
function getProduct(PDO $db, PredisClient $redis, int $id): ?array
{
$key = productKey($id);
$cached = $redis->get($key);
if ($cached !== null) {
$value = json_decode($cached, true);
if (is_array($value)) {
$redis->incr('cache:app:metrics:hits');
return $value;
}
// Treat corrupt data as a miss and replace it below.
$redis->del([$key]);
}
$redis->incr('cache:app:metrics:misses');
$statement = $db->prepare(
'SELECT id, name, price, updated_at FROM products WHERE id = :id'
);
$statement->execute(['id' => $id]);
$product = $statement->fetch(PDO::FETCH_ASSOC);
$redis->incr('cache:app:metrics:mysql_reads');
if ($product === false) {
return null;
}
$payload = json_encode($product, JSON_THROW_ON_ERROR);
$redis->set($key, $payload, 'EX', 300);
return $product;
}The five-minute TTL in this example is a policy choice, not a universal value. Choose it from the acceptable staleness window and update frequency of the data. Shorter TTLs reduce stale exposure but increase MySQL reads; longer TTLs improve hit rate but keep unchanged values around longer.
PhpRedis method equivalents
$cached = $redis->get($key);
if ($cached === false) {
// Cache miss
}
$redis->set($key, $payload, ['ex' => 300]);
$redis->del($key);
$redis->incr('cache:app:metrics:hits');Check the client version’s method signature when upgrading. Keep the application-level behavior identical: read, miss-load, write with expiration, and return.
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Invalidate Redis when MySQL changes
For a successful update, commit MySQL first and then delete the cache key. This preserves MySQL as the source of truth and ensures the next read obtains the committed row.
<?php
$db->beginTransaction();
try {
$statement = $db->prepare(
'UPDATE products SET name = :name, price = :price WHERE id = :id'
);
$statement->execute([
'name' => $name,
'price' => $price,
'id' => $id,
]);
$db->commit();
$redis->del([productKey($id)]);
} catch (Throwable $exception) {
if ($db->inTransaction()) {
$db->rollBack();
}
throw $exception;
}Do not delete the key before a transaction that may roll back; that creates an unnecessary miss and can allow an old reader to repopulate data while the failed write is being handled. If the database commit succeeds but Redis deletion fails, the old value remains only until its TTL or a successful retry. Log deletion failures and alert on them rather than silently treating them as successful writes.
Deletes and bulk changes
For a deleted row, commit the MySQL delete and then remove its key. For a bulk operation, invalidate every affected key if the set is known; otherwise use a namespace version, such as changing v1 to v2 in generated keys, and let old keys expire. Versioning avoids an expensive key scan but temporarily leaves old entries occupying memory.
Prevent cache stampedes on hot keys
A stampede occurs when a popular key expires and many PHP requests all query MySQL simultaneously. A short single-flight lock allows one request to rebuild the value while others wait briefly and retry.
Lock requirements
- Use a distinct lock key, for example
lock:cache:app:product:42. - Acquire it atomically with
SET,NX, and an expiration. - Set the lock TTL longer than the worst-case MySQL read and serialization time.
- Store a random ownership token so a request cannot delete a lock acquired by another request.
- Always release only when the token still matches.
- Use a short wait and retry limit; falling back to MySQL is safer than waiting indefinitely.
Predis single-flight example
<?php
function releaseLock(PredisClient $redis, string $lockKey, string $token): void
{
$script = <<<'LUA'
if redis.call('get', KEYS[1]) == ARGV[1] then
return redis.call('del', KEYS[1])
end
return 0
LUA;
$redis->eval($script, 1, $lockKey, $token);
}
function loadWithLock(PDO $db, PredisClient $redis, int $id): ?array
{
$key = productKey($id);
$lockKey = 'lock:' . $key;
$token = bin2hex(random_bytes(16));
$lockTtl = 10;
for ($attempt = 0; $attempt < 20; $attempt++) {
$cached = $redis->get($key);
if ($cached !== null) {
return json_decode($cached, true);
}
$acquired = $redis->set($lockKey, $token, 'NX', 'EX', $lockTtl);
if ($acquired) {
try {
// Recheck after acquiring the lock: another request may have filled it.
$cached = $redis->get($key);
if ($cached !== null) {
return json_decode($cached, true);
}
$statement = $db->prepare(
'SELECT id, name, price, updated_at FROM products WHERE id = :id'
);
$statement->execute(['id' => $id]);
$product = $statement->fetch(PDO::FETCH_ASSOC);
if ($product !== false) {
$redis->set(
$key,
json_encode($product, JSON_THROW_ON_ERROR),
'EX',
300
);
return $product;
}
return null;
} finally {
releaseLock($redis, $lockKey, $token);
}
}
usleep(50000);
}
throw new RuntimeException('Cache rebuild lock timeout');
}The second read after lock acquisition is essential. It avoids a duplicate database query when another request filled the key just before this request obtained the lock. In a high-traffic service, add jitter to expirations so many related keys do not expire at exactly the same second.
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Compare cache designs and their trade-offs
| Design | Staleness window | Write correctness | Stampede behavior | Operational notes |
|---|---|---|---|---|
| No cache | None beyond MySQL | MySQL alone defines truth | No cache stampede | Every read consumes database capacity |
| Cache-aside with TTL only | Up to the configured TTL | Writes do not immediately remove old values | Many misses can coincide at expiry | Simplest implementation; monitor miss and database-read counts |
| Cache-aside plus delete-on-write | Normally until deletion completes; TTL remains a safety bound | Explicit invalidation after successful MySQL writes | Still vulnerable when a hot key expires | Recommended baseline for mutable records |
| Cache-aside plus delete-on-write and lock | Same invalidation policy | Same source-of-truth model | One loader rebuilds a hot key while peers wait briefly | More code, lock tuning, and token-safe release are required |
Observe hits, misses, expiry, and contents
Inspect a known key from the VPS:
redis-cli HGETALL cache:app:product:42
redis-cli TTL cache:app:product:42HGETALL is useful when the value is a Redis hash. For a string containing JSON, use GET instead. TTL reports the remaining lifetime in seconds; a negative result indicates no expiration or that the key does not exist, depending on the returned code.
Track counters separately for cache hits, cache misses, and MySQL reads. A rising miss rate can indicate an overly short TTL, a deployment that changed key names, invalidation that is too aggressive, or a Redis restart. Compare those counters with MySQL query latency and connection utilization. Do not infer a guaranteed performance number from Redis’ qualitative description of sub-millisecond reads for a hot working set; actual PHP, network, serialization, VPS, and query conditions determine the result.
Quick Recap
Troubleshoot common failures
| Symptom | Likely cause | Action |
|---|---|---|
Connection refused |
Redis is stopped, listening on another address, or blocked by a firewall | Check systemctl status, the bind setting, listening sockets, and local firewall rules |
| Authentication errors | PHP uses a missing, stale, or wrong password or ACL user | Test with redis-cli using the same credentials and reload the PHP process environment |
| PHP reports an unknown class | Predis is absent from Composer autoloading | Run Composer in the deployed release and require vendor/autoload.php |
Class Redis not found |
PhpRedis is not enabled for the PHP SAPI serving requests | Enable the extension in the correct php.ini and restart PHP-FPM or the web server |
| Keys never expire | The write omitted an expiration or used the wrong client syntax | Inspect with TTL and use the client’s documented EX option |
| Old data persists after an edit | The write path did not delete the exact namespaced key, or deletion failed | Log the generated key, verify it with GET or HGETALL, and alert on invalidation errors |
| MySQL is overloaded when a key expires | Concurrent requests rebuild the same hot key | Add the token-owned single-flight lock and set its TTL above worst-case read time |
| Cache values cannot be decoded | Serializer or schema changed, or a partial value was written | Treat decode failure as a miss, delete the bad key, and repopulate using a versioned key when the format changes |
Production deployment checklist
- Redis is bound to loopback or a private interface, never an unnecessarily public address.
- Authentication is enabled and secrets are not committed to the repository.
- Firewall rules allow only required application hosts.
- Traffic leaving the trusted host uses TLS or a secure tunnel.
- The Redis service is enabled through the operating system’s service manager.
redis-cli PINGreturnsPONGfrom the intended execution context.- The selected PHP client is installed in the same PHP environment used by web requests.
- Every cache key includes an application namespace and, where needed, an environment or schema version.
- Every cache write sets a deliberate TTL.
- Successful MySQL writes delete affected keys.
- Hot-key rebuilds use a token-owned lock with a bounded TTL.
- Hit, miss, MySQL-read, invalidation-failure, and lock-timeout metrics are recorded.
- Applications have a controlled behavior for Redis outages, such as logging the error and reading MySQL directly when the request can tolerate the added latency.
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