The plot servlet requires some temporary disk storage for caching images and prepared coordinate data to improve performance when updating plots during visualisation sequences. Servlet configuration options are available to manage usage of these resources.
When first requested to make a plot, it works out what coordinate data will be required, reads this from the input table, and writes it to cache in an efficient binary storage format. This may be simply a copy of columns from the input table, or may require some computation, for instance if the plotted quantity is the result of an expression in the expression language, or if the coordinates are longitude and latitude (in which case they are converted to unit vector components). This coordinate cache preparation requires a scan of the input table which is in the current implementation single-threaded, so may be time-consuming for very large tables.
Once the coordinate data is in cache, the cache rather than the input table is used for subsequent plots; that includes both derived replots resulting from user navigation and completely separate plots that happen to require the same row/column data. Plots from cache are in most cases multi-threaded, so can be quite rapid even for large datasets depending on server configuration and load.
When the plot is first made, the initial (pre-navigation) plotted image is also stored in a cache, so that if other HTTP clients request the same plot they can receive the initial image without further data access or computation.
By default, the directory used for caching is the value of the
java.io.tmpdir system property, which is typically
/tmp on Unix systems.
The directory used for caching can be changed by modifying this
system property, or caching can be configured with
the servlet context parameters controlled by the
When cache storage gets full, older items are dropped in the usual way. There is no doubt scope for improvement of the existing cache management; the details may be refined in future releases.