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Spaces are object stores with several important properties that contribute to making JavaSpaces a powerful, expressive tool. Let's take a closer look:

• Spaces are shared: Many remote processes can interact with a space concurrently -- the space itself handles the details of concurrent access, leaving you to focus on the design of the high-level protocols between your processes.

• Spaces are persistent: Spaces provide reliable storage for objects. When you store an object in a space, it will remain there indefinitely until it is removed. You can also request a lease time during which an object should be stored. Once stored in the space, an object will remain there until its lease time (which can be renewed) is over, or until a process explicitly removes it. We will discuss leases in more depth later in this series.

• Spaces are associative: Objects in a space are located via associative lookup, not by memory location or by identifier. Associative lookup provides a simple means of finding the objects in which you're interested according to their content, without having to know what the object is called, who created it, or where it is stored. To look up an object, you create a template (an object with some or all of its fields set to specific values, and the others left as null to act as wildcards). An object in the space matches a template if it matches the template's specified fields exactly. You'll see that, with associative lookup, you can easily express queries for objects such as "Are there any tasks to compute?" or "Are there any answers to the prime factor I asked for?"

• Spaces are transactionally secure: JavaSpaces makes use of Jini's transaction service to ensure that an operation on a space is atomic (either the operation is applied, or it isn't). Transactions are supported for single operations on a single space, as well as multiple operations over one or more spaces (either all the operations are applied, or none are). As you will see later in the series, transactions are an important way to deal with partial failure.

• Spaces let you exchange executable content: While in a space, objects are just passive data -- you can't modify them or invoke their methods. However, when you read or take an object from a space, a local copy of the object is created. As with any other local object, you can modify its public fields and invoke its methods, even if you've never seen an object like it before. This capability gives you a powerful mechanism for extending the behavior of your applications through a space.

As this series progresses, we will show you how these properties play a key part in letting you create distributed applications that work well in the Jini environment, where networking is often spontaneous, and processes join and leave the computation dynamically, sometimes because of device or network failure.