Redis XREAD in PHP (Detailed Guide w/ Code Examples)

Use Case(s)

The XREAD command in Redis is used when working with streams. It allows you to read data from one or multiple streams, where every entry has a unique ID. In PHP Redis, this can be utilised for various real-time applications such as chat systems, data ingestion pipelines, activity tracking and more.

Code Examples

Simple XREAD

Reading from a single stream with the $stream key and 0 as the ID.

$redis = new Redis();
$redis->connect('127.0.0.1', 6379);

$result = $redis->xRead(['stream' => 0]);
print_r($result);

This code connects to the local Redis server, reads from the 'stream' starting at ID '0', then prints the result.

XREAD with Count and Block

Reading with COUNT gives a maximum number of elements returned, while BLOCK specifies the maximum waiting time.

$redis = new Redis();
$redis->connect('127.0.0.1', 6379);

$options = [
    'count' => 100,
    'block' => 2000,
];
$result = $redis->xRead(['stream' => '$'], $options);
print_r($result);

Here, we're reading from the 'stream' starting at the latest entry ($). We also specify to return at most 100 elements and wait for at most 2000 milliseconds.

Best Practices

• Always handle connection failures. In production environments, your Redis instance may not always be available or reachable.
• Be cautious when using BLOCK option with a high value or without a timeout as it may lead to your application hanging.
• Use meaningful stream keys. Stream keys should be descriptive enough for another developer to understand their purpose.

Common Mistakes

• Using XREAD without understanding that it's blocking. If no 'COUNT' option is provided, Redis will block until there are available items in the stream.
• Not connecting to the Redis server before trying to read from the stream. Always confirm that the Redis connection is active and working correctly.

FAQs

Q: What does the $ symbol mean when reading from a stream?

A: In Redis streams, $ represents the ID of the latest message in the stream. This is useful when you want to start reading only new messages that arrive after you run the command.

Q: What happens if I use XREAD on an empty stream?

A: By default, XREAD will block until some data is available. However, you can control this using the 'BLOCK' option in the command.