Details
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New Feature
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Status: Closed
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Major
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Resolution: Done
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None
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None
Description
For certain query operations, ARQ needs to store a large number of tuples temporarily. Currently these are stored in Java Collections, however for large result sets the system can exhaust the available memory. There is a need for a set of generic data structures that can hold these tuples and spill to disk if they get too large.
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The design is inspired by Apache Pig's DataBag [1]:
A DataBag is a collection of tuples. A DataBag may or may not fit into memory. It proactively spills to disk when its size exceeds the threshold. When it spills, it takes whatever it has in memory, opens a spill file, and writes the contents out. This may happen multiple times. The bag tracks all of the files it's spilled to. The spill behavior is controlled by a ThresholdPolicy object. The most basic policy spills based on the number of tuples added. A more advanced policy is to estimate the size of all the tuples added to the DataBag and spill when it passes a byte threshold.
A DataBag provides an Iterator interface, that allows callers to read through the contents. The iterators are aware of the data spilling. They have to be able to handle reading from the spill files.
The DataBag interface assumes that all data is written before any is read. That is, a DataBag cannot be used as a queue. If data is written after data is read, the results are undefined.
DataBags come in several types, default, sorted, and distinct. The type must be chosen up front, there is no way to convert a bag on the fly. Default data bags do not guarantee any particular order of retrieval for the tuples and may contain duplicate tuples. Sorted data bags guarantee that tuples will be retrieved in order, where "in order" is defined either by the default comparator for the tuple or the comparator provided by the caller when the bag was created. Sorted bags may contain duplicates. Distinct bags do not guarantee any particular order of retrieval, but do guarantee that they will not contain duplicate tuples.
The DataBags are generic containers, and may store any item that can be serialized and deserialized. It accepts a SerializationFactory that handles this task.
[1] http://pig.apache.org/docs/r0.9.0/api/org/apache/pig/data/DataBag.html
