Transient data

Background

Some recipes store transient data in a portion of the Fluo table. Transient data is data thats continually being added and deleted. Also these transient data ranges contain no long term data. The way Fluo works, when data is deleted a delete marker is inserted but the data is actually still there. Over time these transient ranges of the table will have a lot more delete markers than actual data if nothing is done. If nothing is done, then processing transient data will get increasingly slower over time.

These delete markers can be cleaned up by forcing Accumulo to compact the Fluo table, which will run Fluos garbage collection iterator. However, compacting the entire table to clean up these ranges within a table is overkill. Alternatively, Accumulo supports compacting ranges of a table. So a good solution to the delete marker problem is to periodically compact just the transient ranges.

Fluo Recipes provides helper code to deal with transient data ranges in a standard way.

Registering Transient Ranges

Recipes like Export Queue will automatically register transient ranges when configured. If you would like to register your own transient ranges, use TransientRegistry. Below is a simple example of using this.

FluoConfiguration fluoConfig = ...;
TransientRegistry transientRegistry = new TransientRegistry(fluoConfig.getAppConfiguration());
transientRegistry.addTransientRange(new RowRange(startRow, endRow));

//Initialize Fluo using fluoConfig. This will store the registered ranges in
//zookeeper making them availiable on any node later.

Compacting Transient Ranges

Although you may never need to register transient ranges directly, you will need to periodically compact transient ranges if using a recipe that registers them. Using TableOperations this can be done with one line of Java code like the following.

FluoConfiguration fluoConfig = ...;
TableOperations.compactTransient(fluoConfig);

Fluo recipes provides an easy way to compact transient ranges from the command line using the fluo exec command as follows:

fluo exec <app name> org.apache.fluo.recipes.accumulo.cmds.CompactTransient [<interval> [<multiplier>]]

If no arguments are specified the command will call compactTransient() once. If <interval> is specified the command will run forever compacting transient ranges sleeping <interval> seconds between compacting each transient ranges.

In the case where Fluo is backed up in processing data a transient range could have a lot of data queued and compacting it too frequently would be counterproductive. To avoid this the CompactTransient command will consider the time it took to compact a range when deciding when to compact that range next. This is where the <multiplier> argument comes in, the time to sleep between compactions of a range is determined as follows. If not specified, the multiplier defaults to 3.

   sleepTime = Math.max(compactTime * multiplier, interval);

For example assume a Fluo application has two transient ranges. Also assume CompactTransient is run with an interval of 600 and a multiplier of 10. If the first range takes 20 seconds to compact, then it will be compacted again in 600 seconds. If the second range takes 80 seconds to compact, then it will be compacted again in 800 seconds.