## What changes were proposed in this pull request?

`sendRpcSync` should copy the response content because the underlying buffer will be recycled and reused.

## How was this patch tested?

Jenkins unit tests.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #11499 from zsxwing/SPARK-13652.

## What changes were proposed in this pull request?

After SPARK-6990, `dev/lint-java` keeps Java code healthy and helps PR review by saving much time.
This issue aims remove unused imports from Java/Scala code and add `UnusedImports` checkstyle rule to help developers.

## How was this patch tested?
```
./dev/lint-java
./build/sbt compile
```

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #11438 from dongjoon-hyun/SPARK-13583.

## What changes were proposed in this pull request?

Make some cross-cutting code improvements according to static analysis. These are individually up for discussion since they exist in separate commits that can be reverted. The changes are broadly:

- Inner class should be static
- Mismatched hashCode/equals
- Overflow in compareTo
- Unchecked warnings
- Misuse of assert, vs junit.assert
- get(a) + getOrElse(b) -> getOrElse(a,b)
- Array/String .size -> .length (occasionally, -> .isEmpty / .nonEmpty) to avoid implicit conversions
- Dead code
- tailrec
- exists(_ == ) -> contains find + nonEmpty -> exists filter + size -> count
- reduce(_+_) -> sum map + flatten -> map

The most controversial may be .size -> .length simply because of its size. It is intended to avoid implicits that might be expensive in some places.

## How was the this patch tested?

Existing Jenkins unit tests.

Author: Sean Owen <sowen@cloudera.com>

Closes #11292 from srowen/SPARK-13423.

## What changes were proposed in this pull request?
This patch moves tags and unsafe modules into common directory to remove 2 top level non-user-facing directories.

## How was this patch tested?
Jenkins should suffice.

Author: Reynold Xin <rxin@databricks.com>

Closes #11426 from rxin/SPARK-13548.

## What changes were proposed in this pull request?
As the title says, this moves the three modules currently in network/ into common/network-*. This removes one top level, non-user-facing folder.

## How was this patch tested?
Compilation and existing tests. We should run both SBT and Maven.

Author: Reynold Xin <rxin@databricks.com>

Closes #11409 from rxin/SPARK-13529.

## What changes were proposed in this pull request?

This PR fixes the order of modifier from `abstract public` into `public abstract`.
Currently, when we run `./dev/lint-java`, it shows the error.
```
Checkstyle checks failed at following occurrences:
[ERROR] src/main/java/org/apache/spark/util/sketch/CountMinSketch.java:[53,10] (modifier) ModifierOrder: 'public' modifier out of order with the JLS suggestions.
```

## How was this patch tested?

```
$ ./dev/lint-java
Checkstyle checks passed.
```

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #11390 from dongjoon-hyun/fix_modifier_order.

## What changes were proposed in this pull request?

This PR tries to fix all typos in all markdown files under `docs` module,
and fixes similar typos in other comments, too.

## How was the this patch tested?

manual tests.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #11300 from dongjoon-hyun/minor_fix_typos.

This patch changes Spark's build to make Scala 2.11 the default Scala version. To be clear, this does not mean that Spark will stop supporting Scala 2.10: users will still be able to compile Spark for Scala 2.10 by following the instructions on the "Building Spark" page; however, it does mean that Scala 2.11 will be the default Scala version used by our CI builds (including pull request builds).

The Scala 2.11 compiler is faster than 2.10, so I think we'll be able to look forward to a slight speedup in our CI builds (it looks like it's about 2X faster for the Maven compile-only builds, for instance).

After this patch is merged, I'll update Jenkins to add new compile-only jobs to ensure that Scala 2.10 compilation doesn't break.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #10608 from JoshRosen/SPARK-6363.

Fixes various minor code and Javadoc styling issues.

Author: Cheng Lian <lian@databricks.com>

Closes #10985 from liancheng/sketch-polishing.

A dependency on the spark test tags was left out of the sketch module pom file causing builds to fail when test tags were used. This dependency is found in the pom file for every other module in spark.

Author: Alex Bozarth <ajbozart@us.ibm.com>

Closes #10954 from ajbozarth/spark13050.

This PR is a follow-up of #10911. It adds specialized update methods for `CountMinSketch` so that we can avoid doing internal/external row format conversion in `DataFrame.countMinSketch()`.

Author: Cheng Lian <lian@databricks.com>

Closes #10968 from liancheng/cms-specialized.

This PR integrates Bloom filter from spark-sketch into DataFrame. This version resorts to RDD.aggregate for building the filter. A more performant UDAF version can be built in future follow-up PRs.

This PR also add 2 specify `put` version(`putBinary` and `putLong`) into `BloomFilter`, which makes it easier to build a Bloom filter over a `DataFrame`.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #10937 from cloud-fan/bloom-filter.

This PR integrates Count-Min Sketch from spark-sketch into DataFrame. This version resorts to `RDD.aggregate` for building the sketch. A more performant UDAF version can be built in future follow-up PRs.

Author: Cheng Lian <lian@databricks.com>

Closes #10911 from liancheng/cms-df-api.

This PR adds serialization support for BloomFilter.

A version number is added to version the serialized binary format.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #10920 from cloud-fan/bloom-filter.

liancheng please take a look

Author: tedyu <yuzhihong@gmail.com>

Closes #10906 from tedyu/master.

This PR adds an initial implementation of bloom filter in the newly added sketch module.  The implementation is based on the [`BloomFilter` class in guava](https://code.google.com/p/guava-libraries/source/browse/guava/src/com/google/common/hash/BloomFilter.java).

Some difference from the design doc:

* expose `bitSize` instead of `sizeInBytes` to user.
* always need the `expectedInsertions` parameter when create bloom filter.

Author: Wenchen Fan <wenchen@databricks.com>

Closes #10883 from cloud-fan/bloom-filter.

This PR adds serialization support for `CountMinSketch`.

A version number is added to version the serialized binary format.

Author: Cheng Lian <lian@databricks.com>

Closes #10893 from liancheng/cms-serialization.

This PR adds an initial implementation of count min sketch, contained in a new module spark-sketch under `common/sketch`. The implementation is based on the [`CountMinSketch` class in stream-lib][1].

As required by the [design doc][2], spark-sketch should have no external dependency.
Two classes, `Murmur3_x86_32` and `Platform` are copied to spark-sketch from spark-unsafe for hashing facilities. They'll also be used in the upcoming bloom filter implementation.

The following features will be added in future follow-up PRs:

- Serialization support
- DataFrame API integration

[1]: https://github.com/addthis/stream-lib/blob/aac6b4d23a8686b000f80baa447e0922ecac3bcb/src/main/java/com/clearspring/analytics/stream/frequency/CountMinSketch.java
[2]: https://issues.apache.org/jira/secure/attachment/12782378/BloomFilterandCount-MinSketchinSpark2.0.pdf

Author: Cheng Lian <lian@databricks.com>

Closes #10851 from liancheng/count-min-sketch.