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vFabric™ GemFire®
cacheOverflow
Java Caching API Programming Example
The cacheOverflow example program demonstrates the "overflow"
feature of GemFire cache regions. The example is located in the GemFire
installation under examples/dist/cacheOverflow.
Before running this application, you need to start a terminal session and configure its environment
according to the instructions provided under the GemFire installation
in examples/EnvSetup.html.
After the environment has been set, change directory to examples/dist/cacheOverflow in the SampleCode distribution directory.
You can run the cacheOverflow example
with no command line arguments to obtain usage information.
usage: java cacheOverflow.CacheOverflow [options] args
Where [options] are
-synchronous Writes to disk are synchronous
-backup Backup the region data on disk (data will be written
to disk as soon as it is added to the region). Restarting
the VM with the -backup flag will recover it's contents.
-validate Instead of populating a region validates its contents.
-disableCompaction Save the old backup files instead of compacting them
Where args are
threads Number of threads adding to region
arrays Number of 8 kilobyte arrays added by each thread
overflowThreshold Number of megabytes of region data that can reside
in the VM before overflowing to disk
maxOplogSize Number of megabytes of region data that can be written
to a backup file before rolling over to a new file
dir+ One or more directories in which to write region data
The cacheOverflow example launches one or more threads that
populate a GemFire region with 8 kilobyte chunks of data (long
arrays of size 1024). Once the amount of region data exceeds the specified
threshold, the data is overflowed to disk, thus allowing the
logical size of the region to exceed the amount of heap space in the
VM.
Enter the following command exactly as shown. The call puts a total of 20*100*8K = 16MB of data into
a region with a size limit of 1 megabyte, so 15 megabytes are
overflowed (into a 30 megabyte file in the current working directory)
as the threads run.
java cacheOverflow.CacheOverflow 20 100 1 30 .
When the threads finish, the program exits. This forces a cache close and
region destruction.
If you run the same command with the -backup flag, it will store all of the entries
on disk. It will still keep 1 megabyte in the in memory cache.
java cacheOverflow.CacheOverflow -backup 20 100 1 30 .
Running the program with the -backup flag again will recover the region contents from disk.
Run it with both the -backup and -verify flags to make sure the data was recovered.
java cacheOverflow.CacheOverflow -backup -validate 20 100 1 30 .
While the program itself does not have very interesting output,
there are a number of GemFire statistics that are worth observing. The
example program creates it's own statistics in WorkerStats.java. It keeps
track of the number of bytes it has added to the cache (the
bytesAdded statistic of the WorkerStats). GemFire
itself tracks a several of stats related to cache operations. You can see
the number of region entry values that have been overflowed to disk
(the lruEvictions statistic of
MemLRUStatistics, and the amount of time that is spent
writing data to disk (the writeTime statistic of
DiskRegionStatistics).
The program's statistics are archived in a file named statArchive.gfs. The optional VSD tool
can be used to view these statistics (available through the
vFabric GemFire Product Page). Run this command to the view the statistics:
vsd statArchive.gfs
By default, the cacheOverflow program runs as a standalone GemFire application,
meaning it does not join a distributed system, but only creates a
cache.
The CacheOverflow program can be
configured to join a distributed system by setting certain
distribution-related system properties as outlined in the
online documentation for the DistributedSystem class and
in the GemFire User's Guide.