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Diffstat (limited to 'docs')
| -rw-r--r-- | docs/programming-guide.md | 6 |
1 files changed, 6 insertions, 0 deletions
diff --git a/docs/programming-guide.md b/docs/programming-guide.md index c011a8404f..eda3a95426 100644 --- a/docs/programming-guide.md +++ b/docs/programming-guide.md @@ -1207,6 +1207,12 @@ than shipping a copy of it with tasks. They can be used, for example, to give ev large input dataset in an efficient manner. Spark also attempts to distribute broadcast variables using efficient broadcast algorithms to reduce communication cost. +Spark actions are executed through a set of stages, separated by distributed "shuffle" operations. +Spark automatically broadcasts the common data needed by tasks within each stage. The data +broadcasted this way is cached in serialized form and deserialized before running each task. This +means that explicitly creating broadcast variables is only useful when tasks across multiple stages +need the same data or when caching the data in deserialized form is important. + Broadcast variables are created from a variable `v` by calling `SparkContext.broadcast(v)`. The broadcast variable is a wrapper around `v`, and its value can be accessed by calling the `value` method. The code below shows this: |