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Chapter 2: Resource Acquisition

"I find that a great part of the information I have was acquired by looking up something and finding something else on the way."

Franklin P. Adams

Overview

The lifecycle of a resource begins with its acquisition. How and when resources are acquired can play a critical role in the functioning of a software system. By optimizing the time it takes to acquire resources, system performance can be significantly improved.

Before a resource can be acquired, the most fundamental problem to be solved is how to find a resource. The Lookup (21) pattern addresses this problem and describes how resources can be made available by resource providers, and how these resources can be found by resource users.

Once a resource has been found, it can be acquired. The timing of resource acquisition is important, and is mainly addressed by Lazy Acquisition (38) and Eager Acquisition (53). While Lazy Acquisition defers acquisition of resources to the latest possible point in time, Eager Acquisition instead strives to acquire resources as early as possible. Both extremes are important and are heavily dependent on the use of the resources.

If resources that are acquired are not used immediately it can lead to their wastage. Lazy Acquisition addresses this problem and consequently leads to better system scalability. On the other hand, some systems have real-time constraints and stringent requirements for the timing of resource acquisition. Eager Acquisition addresses this problem, and consequently leads to better system performance and predictability.

A resource of large or unknown size may not be needed completely, and therefore it might be sufficient to acquire only part of the resource. The Partial Acquisition (81) pattern describes how the acquisition of a resource can be partitioned into steps to allow only part of the resource to be acquired. Partial Acquisition can serve as a bridge between Lazy Acquisition and Eager Acquisition. The first step of Partial Acquisition typically acquires part of the resource eagerly, while subsequent steps defer further resource acquisition to a later stage.

The Lookup pattern can be used with both reusable and non-reusable resources. Furthermore, it can support both concurrently accessible resources and exclusive resources. This is because Lookup only focuses on providing access to resources, and does not deal with how the resources are actually used.

Lazy Acquisition, Eager Acquisition and Partial Acquisition can also be used with both reusable and non-reusable resources. However, if a reusable resource is concurrently accessible, the three patterns can provide certain optimizations such that the resource is only acquired once and can then be shared by the concurrent users. Such an optimization requires special handling in the case of Partial Acquisition. This is because the amount of a resource that is partially acquired can vary among the concurrent users.

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