流数据结构与命令

In the previous lesson, we learned what Redis Streams are. Now, let's dive into their core structure and how we interact with them.

Redis Streams are powerful, append-only data structures that act like a log, storing a sequence of entries. Each entry has a unique ID and a set of field-value pairs.

Each entry in a Redis Stream is like a small record. It consists of:

• Entry ID: A unique identifier, typically a timestamp and a sequence number (e.g., 1678886400000-0).
• Field-Value Pairs: One or more key-value pairs representing the actual data of the entry (e.g., sensor-id: 123, temperature: 25.5).

Think of it like a mini-hash map stored chronologically within the stream.

To add a new entry to a Redis Stream, we use the XADD command. This command appends a new entry to the end of the stream, generating a unique ID.

The basic syntax is:

For the ID argument, you'll typically use * to let Redis automatically assign a unique ID based on the current time.

Let's add some sensor data to a stream named sensor_readings. We'll use * to let Redis automatically assign a unique ID.

Once data is in a stream, you'll want to read it. The XREAD command is used for this purpose. It allows you to read entries from one or more streams, starting from a specific ID.

A common way to read from the very beginning of a stream is to use 0-0 as the starting ID for that stream.

Let's read the entries we just added. We can specify COUNT to limit the number of entries returned. The $ ID means 'read only new entries added since the last time I read' or 'from now' if it's the first read. It's great for tailing a stream!

Streams can grow infinitely, consuming a lot of memory. To prevent this, Redis allows you to limit the maximum length of a stream using the MAXLEN option with XADD.

When the stream exceeds the specified length, older entries are automatically evicted from the head of the stream.

We can add entries and ensure our sensor_readings stream never exceeds a certain number of entries. Using ~ before the count makes the trimming approximate, which is faster and often sufficient.

You've learned about adding and reading stream entries, as well as managing stream length. Let's test your understanding.

Great job! In this lesson, you mastered the fundamental commands for interacting with Redis Streams:

• XADD: To append new entries with unique IDs and field-value pairs.
• XREAD: To retrieve entries from streams, starting from a specific ID or reading new entries with $.
• MAXLEN: To control stream size during XADD operations, preventing unbounded memory growth.

Next, we'll explore how Streams provide message persistence and ordered delivery, differentiating them from Pub/Sub.