Consistency Patterns Knowledge Base

Quick reference for consistency guarantees, locking strategies, idempotency, and conflict resolution in distributed PHP applications.

Strong vs Eventual Consistency

┌─────────────────────────────────────────────────────────────────────────────┐
│                     CONSISTENCY DECISION TREE                                │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                              │
│                   Is data safety-critical?                                    │
│                      (money, inventory)                                       │
│                        /          \                                           │
│                      YES           NO                                         │
│                      /              \                                         │
│              Single write      Can tolerate                                   │
│              location?         stale reads?                                   │
│               /      \           /       \                                    │
│             YES      NO        YES       NO                                  │
│              |        |         |          |                                  │
│              ▼        ▼         ▼          ▼                                  │
│          ┌───────┐ ┌────────┐ ┌────────┐ ┌──────────┐                        │
│          │Strong │ │Saga +  │ │Eventual│ │Lineariz- │                        │
│          │(ACID) │ │Compen- │ │Consist.│ │able      │                        │
│          │       │ │sation  │ │+ TTL   │ │Reads     │                        │
│          └───────┘ └────────┘ └────────┘ └──────────┘                        │
│                                                                              │
└─────────────────────────────────────────────────────────────────────────────┘

Property	Strong Consistency	Eventual Consistency
Guarantee	All reads return latest write	Reads may return stale data temporarily
Latency	Higher (synchronous coordination)	Lower (async propagation)
Availability	Lower (CAP theorem trade-off)	Higher (tolerates partitions)
Scalability	Limited by coordination	Highly scalable
Complexity	Simpler app logic, harder infra	Simpler infra, harder app logic
Use Cases	Financial transactions, inventory	Social feeds, analytics, caches
PHP Pattern	DB transactions, SELECT FOR UPDATE	Event-driven sync, CQRS read models

Idempotency Keys

Overview

Idempotency ensures that performing the same operation multiple times produces the same result. Critical for payment processing, message handling, and API retries.

Aspect	Details
What	Unique key identifying a specific operation attempt
Why	Safe retries, at-least-once delivery semantics, duplicate prevention
Format	UUIDv4 or {client-id}:{operation}:{unique-ref}
Storage	Redis (fast, TTL) or DB (durable, queryable)
TTL	24-72 hours depending on retry window

Delivery Guarantees

Guarantee	Description	Idempotency Needed?
At-most-once	Fire and forget, may lose messages	No
At-least-once	Retries until ACK, may duplicate	Yes
Exactly-once	Process exactly once (hard)	Built-in deduplication

Idempotency Middleware (PSR-15)

<?php

declare(strict_types=1);

namespace Infrastructure\Http\Middleware;

use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\MiddlewareInterface;
use Psr\Http\Server\RequestHandlerInterface;

final readonly class IdempotencyMiddleware implements MiddlewareInterface
{
    private const string HEADER = 'Idempotency-Key';

    public function __construct(
        private \Redis $redis,
        private int $ttlSeconds = 86400,
    ) {}

    public function process(
        ServerRequestInterface $request,
        RequestHandlerInterface $handler,
    ): ResponseInterface {
        if ($request->getMethod() === 'GET') {
            return $handler->handle($request);
        }

        $key = $request->getHeaderLine(self::HEADER);
        if ($key === '') {
            return $handler->handle($request);
        }

        $cacheKey = sprintf('idempotency:%s', $key);
        $cached = $this->redis->get($cacheKey);

        if ($cached !== false) {
            return unserialize($cached, ['allowed_classes' => true]);
        }

        $response = $handler->handle($request);

        $this->redis->setex($cacheKey, $this->ttlSeconds, serialize($response));

        return $response;
    }
}

Optimistic Locking

Assumes conflicts are rare. Reads a version, performs work, writes only if version unchanged.

Component	Description
Mechanism	Version column incremented on each update
Conflict Detection	WHERE version = :expected in UPDATE
On Conflict	Throw exception, retry or return error
Best For	Read-heavy workloads, low contention
Doctrine	#[ORM\Version] attribute on integer/datetime column

Doctrine Versioned Entity

<?php

declare(strict_types=1);

namespace Domain\Model;

use Doctrine\ORM\Mapping as ORM;

#[ORM\Entity]
#[ORM\Table(name: 'products')]
final class Product
{
    #[ORM\Id]
    #[ORM\Column(type: 'string', length: 36)]
    private string $id;

    #[ORM\Column(type: 'string', length: 255)]
    private string $name;

    #[ORM\Column(type: 'integer')]
    private int $stock;

    #[ORM\Version]
    #[ORM\Column(type: 'integer')]
    private int $version = 1;

    public function __construct(string $id, string $name, int $stock)
    {
        $this->id = $id;
        $this->name = $name;
        $this->stock = $stock;
    }

    public function decrementStock(int $quantity): void
    {
        if ($this->stock < $quantity) {
            throw new InsufficientStockException($this->id, $this->stock, $quantity);
        }

        $this->stock -= $quantity;
    }

    public function getVersion(): int
    {
        return $this->version;
    }
}

Handling Optimistic Lock Failures

<?php

declare(strict_types=1);

namespace Application\UseCase;

use Doctrine\ORM\OptimisticLockException;

final readonly class DecrementStockUseCase
{
    public function __construct(
        private ProductRepositoryInterface $repository,
        private int $maxRetries = 3,
    ) {}

    public function execute(string $productId, int $quantity): void
    {
        $attempt = 0;

        while ($attempt < $this->maxRetries) {
            try {
                $product = $this->repository->findById($productId);
                $product->decrementStock($quantity);
                $this->repository->save($product);

                return;
            } catch (OptimisticLockException) {
                $attempt++;
                if ($attempt >= $this->maxRetries) {
                    throw new ConcurrencyConflictException($productId);
                }
                usleep(random_int(10_000, 50_000));
            }
        }
    }
}

Pessimistic Locking

Assumes conflicts are likely. Acquires a lock before reading, blocks other transactions.

Lock Type	SQL	Behavior
Exclusive (FOR UPDATE)	SELECT ... FOR UPDATE	Blocks reads and writes
Shared (FOR SHARE)	SELECT ... FOR SHARE	Allows reads, blocks writes
NOWAIT	SELECT ... FOR UPDATE NOWAIT	Fails immediately if locked
SKIP LOCKED	SELECT ... FOR UPDATE SKIP LOCKED	Skips locked rows (queue pattern)

Deadlock Prevention Rules

1. Always acquire locks in consistent order (e.g., by entity ID ascending)
2. Use lock timeouts (SET innodb_lock_wait_timeout = 5)
3. Keep transactions short (< 1 second)
4. Avoid user interaction within transactions

Conflict Resolution Strategies

Strategy	Description	Consistency	Complexity
Last Write Wins (LWW)	Latest timestamp wins	Weak	Low
First Write Wins	First write preserved, reject subsequent	Strong	Low
Merge Function	Custom logic merges conflicting writes	Application-defined	High
CRDTs	Conflict-free Replicated Data Types	Eventual (mathematically guaranteed)	Medium
Manual Resolution	Human decides on conflict	Strongest	Variable

CRDT Basics

CRDT Type	Operations	Example Use
G-Counter	Increment only	Page view counter
PN-Counter	Increment and decrement	Inventory count
G-Set	Add only	Tag collection
OR-Set	Add and remove	Shopping cart items
LWW-Register	Last write wins register	User profile fields

Saga Compensation vs Distributed Transactions

Aspect	Distributed Transaction (2PC)	Saga Pattern
Coordination	Transaction coordinator	Choreography or orchestration
Locking	Holds locks across services	No cross-service locks
Consistency	Strong (ACID)	Eventual (compensating actions)
Availability	Lower (blocking)	Higher (non-blocking)
Complexity	Simpler logic, complex infra	Complex logic, simpler infra
Failure Handling	Rollback	Compensating transactions
Latency	Higher (2-phase commit)	Lower (async steps)
Scalability	Poor (locks span services)	Good (independent services)

Redis Atomic Operations

<?php

declare(strict_types=1);

namespace Infrastructure\Lock;

final readonly class RedisAtomicOperations
{
    public function __construct(
        private \Redis $redis,
    ) {}

    public function acquireLock(string $resource, string $token, int $ttlMs): bool
    {
        return $this->redis->set(
            sprintf('lock:%s', $resource),
            $token,
            ['NX', 'PX' => $ttlMs],
        ) === true;
    }

    public function releaseLock(string $resource, string $token): bool
    {
        $script = <<<'LUA'
            if redis.call("get", KEYS[1]) == ARGV[1] then
                return redis.call("del", KEYS[1])
            else
                return 0
            end
        LUA;

        return $this->redis->eval(
            $script,
            [sprintf('lock:%s', $resource), $token],
            1,
        ) === 1;
    }

    public function compareAndSwap(string $key, string $expected, string $newValue, int $ttl): bool
    {
        $script = <<<'LUA'
            if redis.call("get", KEYS[1]) == ARGV[1] then
                return redis.call("setex", KEYS[1], ARGV[3], ARGV[2])
            else
                return 0
            end
        LUA;

        return $this->redis->eval(
            $script,
            [$key, $expected, $newValue, (string) $ttl],
            1,
        ) !== 0;
    }
}

Common Violations Quick Reference

Violation	Where to Look	Severity
No locking on shared mutable state	Repositories, aggregates	Critical
Missing idempotency on payment endpoints	API controllers, handlers	Critical
Optimistic lock exception swallowed silently	Catch blocks, use cases	Critical
Long-running pessimistic locks	Transaction scopes	Warning
No retry on optimistic lock failure	Use cases, command handlers	Warning
Distributed transaction across services	Service-to-service calls	Critical
Missing version field on aggregates	Doctrine entities	Warning
Idempotency key without TTL	Redis/cache stores	Warning

Detection Patterns

# Optimistic locking
Grep: "@Version|ORM\\\\Version|version" --glob "**/Entity/**/*.php"
Grep: "OptimisticLockException|StaleObjectStateException" --glob "**/*.php"
Grep: "LOCK_OPTIMISTIC|LockMode::OPTIMISTIC" --glob "**/*.php"

# Pessimistic locking
Grep: "FOR UPDATE|FOR SHARE|LOCK_PESSIMISTIC" --glob "**/*.php"
Grep: "LockMode::PESSIMISTIC|SKIP LOCKED|NOWAIT" --glob "**/*.php"

# Idempotency patterns
Grep: "Idempotency|idempotency.key|IdempotencyKey" --glob "**/*.php"
Grep: "deduplication|dedup|duplicate.check" --glob "**/*.php"

# Distributed locks
Grep: "SETNX|NX.*PX|redis.*lock|Lock::acquire" --glob "**/*.php"
Grep: "Redlock|RedisLock|LockFactory" --glob "**/*.php"

# Saga patterns
Grep: "compensat|SagaStep|SagaOrchestrator" --glob "**/*.php"
Grep: "CompensatingAction|rollback|undo" --glob "**/Saga/**/*.php"

# Consistency indicators
Grep: "EventualConsistency|ReadModel|Projection" --glob "**/*.php"
Grep: "transaction|beginTransaction|commit|rollback" --glob "**/*.php"

References

For detailed information, load these reference files:

• references/locking-patterns.md — Optimistic locking, pessimistic locking, distributed locks, Redlock algorithm, Symfony Lock component
• references/idempotency-patterns.md — Idempotency key generation, deduplication store, PSR-15 middleware, delivery guarantees, non-idempotent operations