gistfile1.md

$driver = $this->pdo->getAttribute(\PDO::ATTR_DRIVER_NAME);

$this->pdo->beginTransaction();

try {
    // SERIALIZABLE transactions fail if multiple of them
    // even try to touch the same rows at the same time
    if ($driver == "pgsql") {
        $isolation = $this->pdo->prepare(
            "SET TRANSACTION ISOLATION LEVEL SERIALIZABLE"
        );
        $isolation->execute();
    }

    $deleteStmt = $this->pdo->prepare(
        "DELETE FROM $this->table WHERE $this->idCol = :id"
    );
    $deleteStmt->bindParam(':id', $sessionId, \PDO::PARAM_STR);
    $deleteStmt->execute();

    $insertStmt = $this->pdo->prepare(
        "INSERT INTO $this->table ($this->idCol, $this->dataCol, $this->timeCol) VALUES (:id, :data, :time)"
    );
    $insertStmt->bindParam(':id', $sessionId, \PDO::PARAM_STR);
    $insertStmt->bindParam(':data', $encoded, \PDO::PARAM_STR);
    $insertStmt->bindValue(':time', time(), \PDO::PARAM_INT);
    $insertStmt->execute();

    $this->pdo->commit();
} catch (\PDOException $e) {
    // If the error code was 40001 (serialization_failure)
    // then RETRY.
    if ($driver == "pgsql" && $e->getCode() == "40001") {
        $this->pdo->commit();
    } else {
        $this->pdo->rollback();

        throw $e;
    }
}

TLDR

Postgres' SERIALIZABLE transaction mode makes things behave like Optimistic concurrency

    if ($driver == "pgsql") {
        $isolation = $this->pdo->prepare(
            "SET TRANSACTION ISOLATION LEVEL SERIALIZABLE"
        );
        $isolation->execute();
    }

If two (or more) transactions try touching the same rows, a SERIALIZATION_FAILURE error (code 40001) is thrown. In the exception handling:

    if ($driver == "pgsql" && $e->getCode() == "40001") {
        $this->pdo->commit();
    } else {
        $this->pdo->rollback();

        throw $e;
    }

You just retry the transaction. No harm done.

Btw, your retry-logic actually does nothing. Just commiting again, will commit nothing. You need to reexecute the statements.

Also normally you need to set transactional isoltation before starting a transaction (at least in mysql).