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sparksession:使用数据集或DataFrameAPI进行spark编程的入口点。
sparksession可以用来创建DateFrame,将DataFrame当作数据表数据，可以在这个数据表上执行sql语句，也可以缓存数据表，从本地读取文件等。
用下面的方法来创建一个sparksession
>>> spark = SparkSession.builder \\
... .master("local") \\
... .appName("Word Count") \\
... .config("spark.some.config.option", "some-value") \\
... .getOrCreate()
sparksession里面的内部类：Builder
config 函数：配置函数，通过该函数可以自定义配置项或使用SparkConf中的配置。
对于一个存在的SparkConf,可以使用conf参数进行配置
使用键值对进行配置，可以省略参数的名字
对于config函数里面配置的值到底是取SparkConf中的conf，还是用户自定义的键值对。
一个加锁操作内，判断conf参数是否为None,若是，则配置内容应该为用户自定义的内容，否则，则是sparkconf里面的配置。
最后返回一个实例对象
关于return self :返回的是这个函数所引用的实例对象。
Returning self from a method simply means that your method returns a reference to the instance object on which it was called. This can sometimes be seen in use with object oriented APIs that are designed as a fluent interface that encourages method cascading.
appName函数：
设置应用的名字，若没有设定应用的名字的话，会随机生成一个。
getOrCreate函数：获取已经 存在的sparksession，如果当前没有sparksession存在，则创建一个。
具体看源码
def getOrCreate(self):
"""Gets an existing :class:`SparkSession` or, if there is no existing one, creates a
new one based on the options set in this builder.
This method first checks whether there is a valid global default SparkSession, and if
yes, return that one. If no valid global default SparkSession exists, the method
creates a new SparkSession and assigns the newly created SparkSession as the global
default.
>>> s1 = SparkSession.builder.config("k1", "v1").getOrCreate()
>>> s1.conf.get("k1") == s1.sparkContext.getConf().get("k1") == "v1"
True
In case an existing SparkSession is returned, the config options specified
in this builder will be applied to the existing SparkSession.
>>> s2 = SparkSession.builder.config("k2", "v2").getOrCreate()
>>> s1.conf.get("k1") == s2.conf.get("k1")
True
>>> s1.conf.get("k2") == s2.conf.get("k2")
True
"""
with self._lock:
from pyspark.context import SparkContext
from pyspark.conf import SparkConf
session = SparkSession._instantiatedSession
if session is None or session._sc._jsc is None:
sparkConf = SparkConf()
for key, value in self._options.items():
sparkConf.set(key, value)
sc = SparkContext.getOrCreate(sparkConf)
# This SparkContext may be an existing one.
for key, value in self._options.items():
# we need to propagate the confs
# before we create the SparkSession. Otherwise, confs like
# warehouse path and metastore url will not be set correctly (
# these confs cannot be changed once the SparkSession is created).
sc._conf.set(key, value)
session = SparkSession(sc)
for key, value in self._options.items():
session._jsparkSession.sessionState().conf().setConfString(key, value)
for key, value in self._options.items():
session.sparkContext._conf.set(key, value)
return session
class Builder(object):
"""Builder for :class:`SparkSession`.
"""
_lock = RLock()#加锁处理
_options = {}
@since(2.0)
def config(self, key=None, value=None, conf=None):
"""Sets a config option. Options set using this method are automatically propagated to
both :class:`SparkConf` and :class:`SparkSession`'s own configuration.
For an existing SparkConf, use `conf` parameter.
>>> from pyspark.conf import SparkConf
>>> SparkSession.builder.config(conf=SparkConf())
<pyspark.sql.session...
For a (key, value) pair, you can omit parameter names.
>>> SparkSession.builder.config("spark.some.config.option", "some-value")
<pyspark.sql.session...
:param key: a key name string for configuration property
:param value: a value for configuration property
:param conf: an instance of :class:`SparkConf`
"""
with self._lock:
if conf is None:
self._options[key] = str(value)
else:
for (k, v) in conf.getAll():
self._options[k] = v
return self
@since(2.0)
def master(self, master):
"""Sets the Spark master URL to connect to, such as "local" to run locally, "local[4]"
to run locally with 4 cores, or "spark://master:7077" to run on a Spark standalone
cluster.
:param master: a url for spark master
"""
return self.config("spark.master", master)
@since(2.0)
def appName(self, name):
"""Sets a name for the application, which will be shown in the Spark web UI.
If no application name is set, a randomly generated name will be used.
:param name: an application name
"""
return self.config("spark.app.name", name)
@since(2.0)
def enableHiveSupport(self):
"""Enables Hive support, including connectivity to a persistent Hive metastore, support
for Hive serdes, and Hive user-defined functions.
"""
return self.config("spark.sql.catalogImplementation", "hive")
@since(2.0)
def getOrCreate(self):
"""Gets an existing :class:`SparkSession` or, if there is no existing one, creates a
new one based on the options set in this builder.
This method first checks whether there is a valid global default SparkSession, and if
yes, return that one. If no valid global default SparkSession exists, the method
creates a new SparkSession and assigns the newly created SparkSession as the global
default.
>>> s1 = SparkSession.builder.config("k1", "v1").getOrCreate()
>>> s1.conf.get("k1") == s1.sparkContext.getConf().get("k1") == "v1"
True
In case an existing SparkSession is returned, the config options specified
in this builder will be applied to the existing SparkSession.
>>> s2 = SparkSession.builder.config("k2", "v2").getOrCreate()
>>> s1.conf.get("k1") == s2.conf.get("k1")
True
>>> s1.conf.get("k2") == s2.conf.get("k2")
True
"""
with self._lock:
from pyspark.context import SparkContext
from pyspark.conf import SparkConf
session = SparkSession._instantiatedSession
if session is None or session._sc._jsc is None:
sparkConf = SparkConf()
for key, value in self._options.items():
sparkConf.set(key, value)
sc = SparkContext.getOrCreate(sparkConf)
# This SparkContext may be an existing one.
for key, value in self._options.items():
# we need to propagate the confs
# before we create the SparkSession. Otherwise, confs like
# warehouse path and metastore url will not be set correctly (
# these confs cannot be changed once the SparkSession is created).
sc._conf.set(key, value)
session = SparkSession(sc)
for key, value in self._options.items():
session._jsparkSession.sessionState().conf().setConfString(key, value)
for key, value in self._options.items():
session.sparkContext._conf.set(key, value)
return session