as concurrently. Delivery of messages in cc:Mail is time invariant meaning that many database changes, such as message deliveries and deletions, can be under way at the same time without conflicting. Fundamentally, time invariance is made possible in OSI model network operating systems by the combination of the ability to write data to a file system past the end of a file and the ability to lock a record within a file. Advantages The shared file access architecture of cc:Mail offered significant performance benefits and made it possible for cc:Mail to implement a single instance message store years in advance of other products. The file-based nature of the message store also made the system very flexible and in some respects, e.g., moving a database to a new server, easy to manage. Criticisms The architectural approach of cc:Mail had drawbacks both in terms of scalability and in terms of vulnerability of cc:Mail databases to data corruption due to network errors or network operating system software defects. The cc:Mail system became notorious for its tendency to suffer database corruptions. Additionally, the technology was originally developed in a 1980s environment comprising disconnected LANs linked by dial-up connections. While the technology adapted well to WAN environments due to the robust nature of the Router, the system was best suited to a highly distributed deployment model. Client access over a WAN was not recommended because of poor performance related to the network traffic overhead of file I/O redirection and because of increased risk of database corruption. Although automation was possible, maintenance of large numbers of databases, each with relatively few users, was undesirable compared to highly centralized client/server systems where client access could be reliably provided over a WAN. Client/Server cc:Mail cc:Mail developed a native cc:Mail server, cc:Guardian, which would allow superior scalability, reliable client access over a WAN, and virtually eliminate database corruptions by removing file I/O access to the database. At the same time the development of POP3 and IMAP4 servers provided integration with Internet standards-based client/server technologies. With the development of cc:Guardian and with support for POP3 and IMAP4, cc:Mail evolved into a true client/server platform. However, customers never deployed cc:Mail as a client/server solution in large numbers. Acquisition by Lotus Lotus Development acquired cc:Mail, Inc. (formerly PCC Systems), which was a Silicon Valley startup, in 1991 and used the cc:Mail technology to enhance Lotus Notes. Lotus Notes features derived from cc:Mail