XREAD command is used in Redis to read data from one or more streams, where data is organized as a series of messages. This is particularly useful when implementing systems like event sourcing, message queues, or analytics data pipelines.
Let's assume we have a Redis stream named "mystream" and we want to read new messages using Ruby.
In this code snippet, we're reading new messages from "mystream". The second argument
0 tells Redis to retrieve entries that were not yet delivered to any consumer. The
block: 1000 sets the number of milliseconds the command will block if there are no items to retrieve.
This example demonstrates how to read from multiple streams at once. We pass a Hash where keys are stream names and values are the IDs from which to start reading.
Always specify a reasonable blocking time for
XREAD to prevent indefinite blocking of your application if no messages appear in the stream.
"0"as the ID to read from an existing stream - this may lead to reading the entire stream for large streams.
nil(which signifies it timed out). Always make sure you handle this case gracefully.
Q: What happens if I use the special ID "$" with XREAD? A: The "$" symbol is used to only retrieve new messages that are added to the stream after executing the command. Old messages will not be read.
Q: How can I stop XREAD from blocking indefinitely?
A: You can prevent XREAD from blocking indefinitely by providing a timeout value using the
block option. For example,
block: 1000 would block for one second before returning
nil if no new messages have appeared.