1. A method for caching customized metadata documents for a metadata-driven software application, the method comprising: receiving, by a computer system, a request for a customized version of a base metadata document, wherein the base metadata document defines one or more characteristics of at least a portion of the software application, and wherein the base metadata document is to be customized for a first user among a plurality of users of the software application; identifying, by the computer system, a first customization for the base metadata document, wherein the first customization defines a customized version of the base metadata document for the plurality of users; including in the first customization a first cache hint indicating that the first customization is applicable to the plurality of users; identifying, by the computer system, a second customization for the base metadata document, wherein the second customization defines a customized version of the base metadata document for only the first user; including in the second customization a second cache hint indicating that the second customization is applicable only to the first user; generating, by the computer system, a first customized metadata document based on the first customization and the base metadata document; generating, by the computer system, a second customized metadata document based on the first customized metadata document and the second customization; storing, by the computer system, the first customized metadata document in a shared cache applicable to application sessions of the plurality of users, wherein the first customized metadata document is stored in the shared cache based on the first cache hint included in the first customization and is stored in a working memory as a first tree memory structure, the first tree memory structure including an allocated memory segment for a first element of the first customized metadata document and an allocated memory segment for a second element of the first customized metadata document; storing, by the computer system, the second customized metadata document in a session cache applicable only to an application session of the first user, wherein the second customized metadata document is stored in the session cache based on the second cache hint included in the second customization and is stored in the working memory as a second tree memory structure, the second tree memory structure including a pointer to the allocated memory segment for the first element in the first tree memory structure and an allocated memory segment for a modified version of the second element; and returning, by the computer system, the second customized metadata document.