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https://github.com/aternosorg/php-lock

Distributed exclusive and shared resource locking based on etcd
https://github.com/aternosorg/php-lock

Last synced: about 1 year ago
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Distributed exclusive and shared resource locking based on etcd

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README 

          
# PHP library for distributed locking

Distributed exclusive and shared resource locking based on [etcd](https://github.com/etcd-io/etcd) (using [aternos/etcd](https://github.com/aternosorg/php-etcd)).



### About

This library was created to provide a highly available distributed locking solution for a file storage system,

where a failed or incorrect lock could lead to potential data loss or corruption. Therefore it tries to achieve

the best possible consistency while still providing simple usability, stability and scalability. There are lots 

of other locking libraries out there. Some of them don't allow shared locks, some of them implement complicated 

algorithms on the client-side which also didn't seem to fit our needs. Therefore we've decided to create our own solution 

and let etcd do most of the heavy lifting.



Etcd is a fast and reliable key-value store that allows transaction operations to ensure that no unexpected changes were made 

since the last read (see `putIf()` and `deleteIf()` in [`Aternos\Etcd\Client`](https://github.com/aternosorg/php-etcd/blob/master/src/Client.php)).

This locking library uses those methods to ensure consistency. If two processes try to lock the same resource at the same

time one of them will be denied by etcd and then retry its lock by either adding it to the other lock (shared) or waiting

for the other lock to finish (exclusive). If more processes are fighting over the same lock, they will start to delay their

retries in random intervals to find a consistent conclusion.



Timeouts (UnavailableExceptions) from etcd will also be detected and the operations retried after some delay to avoid

problems because of short availability problems.



This library was extracted from a different project to be used in different places and not only for a file storage.

Therefore we've already used this in production to create thousands of locks every minute in only a few milliseconds.



### Installation

The gRPC PHP extension has to be installed to use this library. See [aternos/etcd](https://github.com/aternosorg/php-etcd#installation).



```bash

composer require aternos/lock

```



## Usage

The most important class is the [`Lock`](src/Lock.php) class. Each instance of this class represents

one lock on a resource that is identified by a key. There are also several static functions to set options

for all locks, but all of them have default values, so you can just start by creating a lock:



```php

lock()) {

    // do something

} else {

    // error/exit

}

```



### Exclusive/shared locks

There can only be one exclusive lock at the same time, but there can be multiple shared locks. If there is

any shared lock an exclusive lock is not possible. Exclusive locks can be useful for write operations while

shared locks can be useful for read operations. By default all locks are shared, you can create an exclusive

lock like this:



```php

lock(true);

```



### Locking time(out) and refreshing

You always have to set a timeout for your locks. The lock will be released automatically if the time runs out.

This is necessary to avoid infinite dead locks. The timeout can be refreshed e.g. after writing or reading a

chunk of data. You can also define a threshold for the refresh time. The lock will only be refreshed if

the remaining locking time is below this threshold. This avoids spamming etcd with unnecessary queries.



```php

lock(true, 60); // 60 seconds timeout



// refresh the lock (with default values)

$lock->refresh();



// refresh with 120 seconds timeout

$lock->refresh(120);



// refresh with 120 seconds timeout, but only if remaining time is lower than 60 seconds

$lock->refresh(120, 60);

```



### Waiting for other locks

If the resource that you want to access is currently locked exclusively or you need an exclusive lock and there

are still shared locks, you might want to wait some time for the locks to be released. You can specify a maximum

time to wait for other locks:



```php

lock(true, 60, 300); // wait 300 seconds for other locks



// check if the lock was actually acquired or if the wait timeout was reached

if($lock->isLocked()) {

    // do something

} else {

    // error/exit

}

```



### Break the lock

Of course it's important to break the lock after finishing the operation to avoid running into timeouts. You can break

a lock like this:



```php

isLocked()) {

    // do something

    $lock->break();

} else {

    // error/exit

}

```



### Identifier

You can identify yourself (the current process/request) by providing and identifier string. By default this library

uses the `uniqid()` function to create a unique identifier. There aren't many reasons to provide a custom identifier.

The default identifier is the same for all locks in the current process. Therefore all those locks are able to take

over the other locks that were previously created in the same process. It also would be possible to see which other

process (represented by its identifier) is currently holding a lock. There is currently no such function, but that 

would be easy to add.



There are two ways to provide a custom identifier. Either by setting the default for all locks or by overwriting 

the default identifier on a specific lock:



```php

lock();



// overwrites the "default-identifier" with "different-identifier"

$lock->lock(true, 60, 300, "different-identifier");

```



### Settings

The default identifier above is already one of the static settings, which you can specify for all locks. All settings 

are stored in protected static fields in the [`Lock`](src/Lock.php) class (at the top) and have their own static

setter functions. Below are only a few important ones, but you can read the PHPDoc function comments for further explanations

of the other settings. Only change those if you know what you are doing.



#### Etcd client

You can set an etcd client object to specify the etcd connection details. See [this](https://github.com/aternosorg/php-etcd#client-class)

for more information.



```php