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Using logs for message storage

A log is simply an append-only sequence of records on disk. We previously discussed logs in the context of log-structured storage engines and write-ahead logs in Chapter 3, and in the context of replication in Chapter 5.

The same structure can be used to implement a message broker: a producer sends a message by appending it to the end of the log, and a consumer receives messages by reading the log sequentially. If a consumer reaches the end of the log, it waits for a notification that a new message has been appended. The Unix tool tail -f, which watches a file for data being appended, essentially works like this.

In order to scale to higher throughput than a single disk can offer, the log can be partitioned (in the sense of Chapter 6). Different partitions can then be hosted on different machines, making each partition a separate log that can be read and written independently from other partitions. A topic can then be defined as a group of partitions that all carry messages of the same type. This approach is illustrated in Figure 11-3.

Within each partition, the broker assigns a monotonically increasing sequence number, or offset, to every message (in Figure 11-3, the numbers in boxes are message offsets). Such a sequence number makes sense because a partition is append-only, so the messages within a partition are totally ordered. There is no ordering guarantee across different partitions.

Producers send messages by appending them to a topic-partition file, and consumers read these files sequentially

Figure 11-3. Producers send messages by appending them to a topic-partition file, and consumers read these files sequentially.

Apache Kafka [17, 18], Amazon Kinesis Streams [19], and Twitter’s DistributedLog [20, 21] are log-based message brokers that work like this. Google Cloud Pub/Sub is architecturally similar but exposes a JMS-style API rather than a log abstraction [16]. Even though these message brokers write all messages to disk, they are able to achieve throughput of millions of messages per second by partitioning across multiple machines, and fault tolerance by replicating messages [22, 23].

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