State of the World: The Message Database

This section describes the AMPS State of the World, which functions as a last value cache or database.

State of the World (SOW): The Message Database

One of the core features of AMPS is the ability to persist the most recent update for each distinct message published to a topic. To enable this for a topic, you add the topic to the State of the World (SOW).

You can think of the SOW as a database that maintains a specific set of topics, equivalent to tables. Each distinct message published to that topic is equivalent to updating a row in the table. AMPS allows applications to query the table for the current state of the topic.

SOW topics also provide full support for pub/sub messaging. Applications can use a combination of queries and subscriptions as necessary. AMPS also includes a set of commands that perform an atomic query and subscribe, allowing an application to query a SOW topic and register for updates to the topic in a single operation, without risk of missing messages or receiving duplicates.

The most common uses of SOW topics include:

  • Quickly loading initial state for an application. For example, an application that tracks open orders can quickly retrieve a snapshot of all of the orders that are currently open, without having to wait for updates to the orders to be published.

  • Queryable snapshots of data flows. For example, an application that monitors telemetry data may need to quickly determine if any telemetry source has not provided an update within a given period of time. With a SOW topic, the application can run a simple query over the current state of the topic.

  • NoSQL document stores. SOW topics are frequently used as high-performance key/value stores: an application can choose to explicitly provide a key and store a document in the SOW. Documents can be efficiently retrieved by key, queried over the full content of the document, or any combination. As mentioned above, a consumer can retrieve the document and be automatically notified when the content of the document changes.

SOW topics are also the foundation of many of the more advanced capabilities of AMPS, including out-of-focus tracking, aggregation, and delta messaging. These are described later in this chapter.

For applications that are transitioning from topic-based routing and that, therefore, need to maintain the last value per topic for a large number of topics (hundreds, thousands, or more), AMPS provides the ability to reduce the overhead in creating a large number of identical topics that contain a single message. More details on the State of the World are available in the AMPS User Guide.

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