At first, the journal articles available via ADS were scanned bitmaps created from the paper journals, but from 1995 onwards, the "Astrophysical Journal" began to publish an on-line edition, soon followed by the other main journals such as "Astronomy and Astrophysics" and the "Monthly Notices of the Royal Astronomical Society". ADS provided links to these electronic editions from their first appearance. Since about 1995, the number of ADS users has doubled roughly every two years. ADS now has agreements with almost all astronomical journals, who supply abstracts. Scanned articles from as far back as the early 19th century are available via the service, which now contains over eight million documents. The service is distributed worldwide, with twelve mirror sites in twelve countries on five continents, with the database synchronized by means of weekly updates using rsync, a mirroring utility which allows updates to only the portions of the database which have changed. All updates are triggered centrally, but they initiate scripts at the mirror sites which "pull" updated data from the main ADS servers.