# Streams

Completed

Distributed microservices may often wish to leave data or messages for other microservices to consume. Many different consumer microservices may wish to consume the same messages in a large enough solution. This pattern is commonly referred to as competing consumers architecture.

In the retail company scenario, microservices must leave messages asynchronously for many clients to consume. Ideally, these clients would consume this data at their schedule without forcing the message producer to wait. Today, these microservices are designed to send data and wait for a reply directly, or acknowledgment, to continue processing. As the retail solution scales, it becomes increasingly important to design and implement a solution where message producers and consumers can asynchronously store and parse messages without waiting for external stimuli.

Diagram illustrating three pairs of services communicating with each other and waiting for acknowledgment (ack) as part of a replay payload.

## Message brokering using Streams

A message broker is a middleware solution that manages the distribution of message data to multiple consumers that would typically compete for the same data.

Here, you will learn how the Streams feature of Azure Cache for Redis can serve as middleware to simplify the storage of messages from multiple producers and the distribution of messages to many clients. Streams can help your application components manage messaging in a scalable, fault-tolerant, and distributed manner.

The Streams feature of Azure Cache for Redis stores messages in a stream that clients can consume. Microservices can use this feature to distribute messages to many clients that the microservice may not know about at the time of message production. Additionally, the number of message consumers and producers can be scaled up or down separately in patterns that make sense for a cloud solution.

Diagram illustrating a message broker implementation that sits between message producers and consumers, implemented using Azure Cache for Redis.

This feature makes the brokering of messages between microservices significantly simpler.

## Adding entries to a stream

Clients can produce messages for a stream using a set of field-value pairs. The client can specify a unique identifier when the message is created or have it auto-generated by Redis. In Redis terminology, the message is referred to as an entry, and the unique identifier is the key.

Note

An auto-generated key is composed of the Unix time of the local machine in milliseconds and a sequence number if multiple entries occur within the exact millisecond. For example, if the current date and time on the local device is midnight (UTC) on December 1, 2010, and the entry is the third that occurred in the current millisecond, the auto-generated key would be 1291161600000-3.

Redis includes a XADD command used to add new entries to a stream. The stream does not need to be created before issuing the command. For example, use the XADD command to add a new entry to the media.photos.genthumb stream using an auto-generated key and the following field-value pair data:

Field Value
location sd7f9sd7.png
width 300
height 300
XADD media.photos.genthumb * location sd7f9sd7.png width 300 height 300


Alternatively, you can specify a specific key to use with the XADD command. In this example, the XADD command is used to add a new entry to the media.photos.genthumb stream with a key of 1596514316945-2 and the following field-value pair data:

Field Value
location xczv897.png
filter grayscale
XADD media.photos.genthumb 1596514316945-2 location xczv897.png filter grayscale


## Querying entries in a stream

Once there are entries in a stream, clients can manually count the number of entries or retrieve entries directly.

Redis includes an XRANGE command to directly get all or a subset of the entries in a stream. Redis also contains an XLEN command to count the number of entries in a specific stream.

### Querying ranges of entries

The XRANGE command is used with three core parameters:

• The name of the stream to query
• The starting point of the result set
• The endpoint point of the result set

Diagram illustrating a query that returns a subset of entries between two distinct time values.

The XRANGE command supports the use of special operators if you do not know or do not wish to specify the start or end of a range. The - operator is used to indicate the range should start from the beginning, chronologically, of the stream. The + operator is used to indicate that the range should continue to the end, again chronologically, of the stream.

For a first example, the XRANGE command could be used with two keys to retrieve all chronologically occurring entries between the streams. In this code example, the XRANGE command is used to get all entries in the media.photos.genthumb stream that occurs starting with the 1596514316945-2 key and ending with the 1609476184275-0 key.

XRANGE media.photos.genthumb 1596514316945-2 1609476184275-0


As a second example, the + and - operators can be used together to retrieve all entries from a stream. This example queries the same stream, but retrieves all entries through the use of the special operators.

XRANGE media.photos.genthumb - +


Next, the special operators can be used separately. In this example, use the - operator to get all entries from a stream up to the 1596514316945-2 entry.

XRANGE media.photos.genthumb - 1596514316945-2


This example illustrates getting all entries starting with the 1596514316945-2 entry to the end of the stream with the + operator.

XRANGE media.photos.genthumb 1596514316945-2 +


Alternatively, the XREVRANGE command is available with the same syntax as the XRANGE command to get all entries in reverse order.

XREVRANGE media.photos.genthumb - +
XREVRANGE media.photos.genthumb - 1596514316945-2
XREVRANGE media.photos.genthumb 1596514316945-2 +


### Counting entries in a stream

The XLEN command is a simple command that takes the name of a stream as an argument, and returns an integer with the count of entries within the stream.

XLEN media.photos.genthumb


## Reading data from a stream

While you can query data from Redis as a time series, it’s preferable to listen for new items from a stream. A stream can have many listeners waiting for data stored in the stream. If a new listener is created after the data has started, the listener can still “start over” and process data from the beginning of the stream.

Data is consumed from a stream using the XREAD command. This command includes a suite of options that will not all be covered in this module. For now, we will focus on the XREAD command’s ability to read one or more streams, starting from specific points.

### Reading data from a specific starting point in a stream

The XREAD command, in its simplest and most commonly used form, can subscribe to entries from a stream starting from the very first entry. The consumer will then process the entries, in order, before reaching the current end of the stream. At this point, the consumer can wait for new data in the stream.

To subscribe to data in a stream, the XREAD command should be used with the STREAMS argument and a list of streams to read along with their corresponding starting points.

In this example, the XREAD command is used to read data from the media.photos.genthumb stream starting from the very beginning by using the 0 key. In most situations, the 0 key will effectively be less than the current time-based key.

XREAD STREAMS media.photos.genthumb 0


If a stream consumer has to pause consuming messages for any reason, a key can be passed into the XREAD command to start consuming messages from a specific point. In this example, messages are consumed, starting with the 1639714145947-2 key.

XREAD STREAMS media.photos.genthumb 1639714145947-2


Tip

This technique is convenient if a stream consumer crashes or fails for any reason. The XREAD command can resume reading from the last known good read.

Another option is to use the $ operator. This particular operator indicates to start reading the stream from the end. Effectively, this example will only receive new entries instead of historical entries. XREAD STREAMS media.photos.genthumb$


### Reading data from multiple streams

All of the previous examples read data from a single stream. You may remember that the STREAMS argument supports a list of streams along with their corresponding starting points.

In this example, the XREAD command will read from both the media.photos.genthumb and the media.photos.delete streams simultaneously while starting both streams from their chronologically first entries.

XREAD STREAMS media.photos.genthumb media.photos.delete 0 0


## Deleting a stream and its data

When data is consumed from a stream, it remains in the stream until it is manually deleted. This implementation allows multiple clients to consume data from the same stream without missing any data.

To manually delete a specific entry from a stream, use the XDEL command with the stream's name and the entry's key to delete. In this example, the XDEL command is used to delete the entry with a key of 1596514316945-2 from the media.photos.genthumb stream.

XDEL media.photos.genthumb 1596514316945-2


Removing all items from a stream will not delete the stream as an empty stream is an entirely legal construct in Redis.

To delete a stream altogether, use the DEL command with the stream's name.

DEL media.photos.genthumb