MongoDB Python官方驅動 PyMongo 的簡單封裝

HanseyLee發表於2019-01-07
MongoDBPython封裝

最近，需要使用 Python 對 MongodB 做一些簡單的操作，不想使用各種繁重的框架。出於可重用性的考慮，想對 MongoDB Python 官方驅動 PyMongo 做下簡單封裝，百度一如既往的未能給我一個滿意的結果，於是有了下文。

【正文】

PyMongo，MongoDB Python官方驅動

  • docs: https://api.mongodb.com/python/current/index.html
  • github: https://github.com/mongodb/mongo-python-driver

PyMongo 驅動幾乎支援 MongoDB 的全部特性，可以連線單個的 MongoDB 資料庫、副本集和分片叢集。從提供的API角度來看，pymongo package是其核心，包含對資料庫的各種操作。本文將介紹一個簡單封裝類 DBManager。主要特性：對資料庫和集合的操作確保其存在性；支援PyMongo的原生操作，包括基本的CRUD操作、批量操作、MapReduce、多執行緒和多程式等；支援因果一致性會話和事務的流水線操作，並給出簡單示例。

MongoClient

mongo_client 提供了連線 MongoDB 的MongoClient類：
class pymongo.mongo_client.MongoClient(host=`localhost`, port=27017, document_class=dict, tz_aware=False, connect=True, **kwargs)
每個 MongoClient 例項 client （下文簡稱 client）都維護一個內建的連線池，預設 maxPoolsize 大小100。對於多執行緒的操作，連線池會給每一個執行緒一個 socket 連線，直到達到最大的連線數，後續的執行緒會阻塞以等待有可用的連線被釋放。client 對 MongoDB 拓撲結構中的每個server 還維護一個額外的連線來監聽 server 的狀態。

下面的 new_mongo_client函式用於獲取一個資料庫連線的 client。其中，client.admin.command(`ismaster`) 用來檢查 server 的可用狀態，簡單省事不需要認證。

def new_mongo_client(uri, **kwargs):
    """Create new pymongo.mongo_client.MongoClient instance. DO NOT USE IT DIRECTLY."""

    try:
        client = MongoClient(uri, maxPoolSize=1024, **kwargs)
        client.admin.command(`ismaster`)  # The ismaster command is cheap and does not require auth.
    except ConnectionFailure:
        logging.error("new_mongo_client(): Server not available, Please check you uri: {}".format(uri))
        return None
    else:
        return client

PyMongo 不是程式（fork-safe）安全的，但在一個程式中是執行緒安全（thread-safe）的。因此常見的場景是，對於一個MongoDB 環境，為每一個程式中建立一個 client ，後面所有的資料庫操作都使用這一個例項，包括多執行緒操作。永遠不要為每一次操作都建立一個 MongoClient 例項，使用完後呼叫 MongoClient.close() 方法，這樣沒有必要而且會非常浪費效能。

鑑於以上原因，一般不宜直接使用new_mongo_client函式獲取 client，而是進一步封裝為get_mongo_client方法。 其中全域性常量 URI_CLIENT_DICT 保持著資料庫 URI 字串與對應 clinet 的字典，一個 URI 對應一個 client 。程式碼如下：

MONGO_URI_DEFAULT = `mongodb://localhost:27017/admin`
URI_CLIENT_DICT = {}    # a dictionary hold all client with uri as key
def get_mongo_client(uri=MONGO_URI_DEFAULT, fork=False, **kwargs):
    """Get pymongo.mongo_client.MongoClient instance. One mongodb uri, one client.

    @:param uri: mongodb uri
    @:param fork: for fork-safe in multiprocess case, if fork=True, return a new MongoClient instance, default False.
    @:param kwargs: refer to pymongo.mongo_client.MongoClient kwargs
    """
    if fork:
        return new_mongo_client(uri, **kwargs)
    global URI_CLIENT_DICT
    matched_client = URI_CLIENT_DICT.get(uri)
    if matched_client is None:  # no matched client
        new_client = new_mongo_client(uri, **kwargs)
        if new_client is not None:
            URI_CLIENT_DICT[uri] = new_client
        return new_client
    return matched_client

確保 Database 和 Collection 的存在

PyMongo 有個特性：對於不存在的資料庫、集合上的查詢不會報錯。如下，Ipython中演示在不存在xxDB 資料庫和 xxCollection 集合上的操作：

In [1]: from pymongo import MongoClient
In [2]: client = MongoClient()          # default uri is `mongodb://localhost:27017/admin`
In [3]: db = client.get_database(`xxDB`)    # Database(MongoClient(host=[`localhost:27017`], document_class=dict, tz_aware=False, connect=True), u`xxDB`)
In [4]: coll = db.get_collection(`XXCollection`)  #  Collection(Database(MongoClient(host=[`localhost:27017`], document_class=dict, tz_aware=False, connect=True), u`xxDB`), u`XXCollection`)
In [5]: coll.find_one()                 # note: no tip, no error, no exception， return None
In [6]: coll.insert_one({`hello` : `what a fucking feature`})
Out[6]: <pymongo.results.InsertOneResult at 0x524ccc8>
In [7]: coll.find_one()
Out[7]: {u`_id`: ObjectId(`5c31c807bb048515b814d719`), u`hello`: u`what a fucking feature`}

這對於手誤寫錯資料庫或集合名字後進行的後續操作，簡直就是災難。鑑於此因，有必要對獲取資料庫或集合時加上確認保護。
下面對於獲取資料庫，使用 MongoClient.list_database_names() 獲取所有的資料庫名字，如果資料庫名稱不在其中，則返回None。同樣的道理，對於集合使用 Database.list_collection_names()。注：由於使用者許可權問題造成的獲取資料庫或集合列表的操作報錯的情況，預設不加確認保護。

def get_existing_db(client, db_name):
    """Get existing pymongo.database.Database instance.

    @:param client: pymongo.mongo_client.MongoClient instance
    @:param db_name: database name wanted
    """

    if client is None:
        logging.error(`client {} is None`.format(client))
        return None
    try:
        db_available_list = client.list_database_names()
    except PyMongoError as e:
        logging.error(`client: {}, db_name: {}, client.list_database_names() error: {}`.
                      format(client, db_name, repr(e)))
    else:
        if db_name not in db_available_list:
            logging.error(`client {} has no db named {}`.format(client, db_name))
            return None
    db = client.get_database(db_name)
    return db

def get_existing_coll(db, coll_name):
    """Get existing pymongo.collection.Collection instance.

    @:param client: pymongo.mongo_client.MongoClient instance
    @:param coll_name: collection name wanted
    """

    if db is None:
        logging.error(`db {} is None`.format(db))
        return None
    try:
        coll_available_list = db.list_collection_names()
    except PyMongoError as e:
        logging.error(`db: {}, coll_name: {}, db.list_collection_names() error: {}`.
                      format(db, coll_name, repr(e)))
    else:
        if coll_name not in coll_available_list:
            logging.error(`db {} has no collection named {}`.format(db, coll_name))
            return None
    coll = db.get_collection(coll_name)
    return coll

PyMongo 封裝類 DBManger

前文的冗長鋪墊主要是為了引入這個 PyMongo 驅動封裝類 DBManger。

DBManger 類的例項保持的狀態有MongoClient例項 self.client, 資料庫self.db 和 集合self.coll，並通過屬性（property）對外開放。PyMongo 原生的方法對這裡的 client, db 和 coll 同樣適用。client 由類的構造器呼叫上文的get_mongo_client方法獲取，db 和 coll 即可通過類的構造器獲取也可通過 self.db_name 和 self.coll_name 這些 setter 來切換。
DBManger 類的例項持有的方法 self.create_coll(self, db_name, coll_name), session_pipeline(self, pipeline) 和 transaction_pipeline(self, pipeline)。後兩種方法在下一節再具體解釋。

class DBManager:
    """A safe and simple pymongo packaging class ensuring existing database and collection.

    Operations:
    MongoClient level operations: https://api.mongodb.com/python/current/api/pymongo/mongo_client.html
    Database level operations: https://api.mongodb.com/python/current/api/pymongo/database.html
    Collection level operations: https://api.mongodb.com/python/current/api/pymongo/collection.html
    """
    __default_uri = `mongodb://localhost:27017/admin`
    __default_db_name = `test`
    __default_coll_name = `test`

    def __init__(self, uri=__default_uri, db_name=__default_db_name, coll_name=__default_coll_name, **kwargs):
        self.__uri = uri
        self.__db_name = db_name
        self.__coll_name = coll_name
        self.__client = get_mongo_client(uri, **kwargs)
        self.__db = get_existing_db(self.__client, db_name)
        self.__coll = get_existing_coll(self.__db, coll_name)

    def __str__(self):
        return u`uri: {}, db_name: {}, coll_name: {}, id_client: {}, client: {}, db: {}, coll: {}`.format(
            self.uri, self.db_name, self.coll_name, id(self.client), self.client, self.db, self.coll)

    @property
    def uri(self):
        return self.__uri

    @property
    def db_name(self):
        return self.__db_name

    @property
    def coll_name(self):
        return self.__coll_name

    @db_name.setter
    def db_name(self, db_name):
        self.__db_name = db_name
        self.__db = get_existing_db(self.__client, db_name)

    @coll_name.setter
    def coll_name(self, coll_name):
        self.__coll_name = coll_name
        self.__coll = get_existing_coll(self.__db, coll_name)

    @property
    def client(self):
        return self.__client

    @property
    def db(self):
        return self.__db

    @property
    def coll(self):
        # always use the current instance self.__db
        self.__coll = get_existing_coll(self.__db, self.__coll_name)
        return self.__coll

    def create_coll(self, db_name, coll_name):
        """Create new collection with new or existing database"""
        if self.__client is None:
            return None
        try:
            return self.__client.get_database(db_name).create_collection(coll_name)
        except CollectionInvalid:
            logging.error(`collection {} already exists in database {}`.format(coll_name, db_name))
            return None

    def session_pipeline(self, pipeline):
        if self.__client is None:
            logging.error(`client is None in session_pipeline: {}`.format(self.__client))
            return None
        with self.__client.start_session(causal_consistency=True) as session:
            result = []
            for operation in pipeline:
                try:
                    if operation.level == `client`:
                        target = self.__client
                    elif operation.level == `db`:
                        target = self.__db
                    elif operation.level == `coll`:
                        target = self.__coll

                    operation_name = operation.operation_name
                    args = operation.args
                    kwargs = operation.kwargs
                    operator = getattr(target, operation_name)
                    if type(args) == tuple:
                        ops_rst = operator(*args, session=session, **kwargs)
                    else:
                        ops_rst = operator(args, session=session, **kwargs)

                    if operation.callback is not None:
                        operation.out = operation.callback(ops_rst)
                    else:
                        operation.out = ops_rst

                except Exception as e:
                    logging.error(`{} {} Exception, session_pipeline args: {}, kwargs: {}`.format(
                        target, operation, args, kwargs))
                    logging.error(`session_pipeline Exception: {}`.format(repr(e)))
                result.append(operation)
            return result

    # https://api.mongodb.com/python/current/api/pymongo/client_session.html#transactions
    def transaction_pipeline(self, pipeline):
        if self.__client is None:
            logging.error(`client is None in transaction_pipeline: {}`.format(self.__client))
            return None
        with self.__client.start_session(causal_consistency=True) as session:
            with session.start_transaction():
                result = []
                for operation in pipeline:
                    try:
                        if operation.level == `client`:
                            target = self.__client
                        elif operation.level == `db`:
                            target = self.__db
                        elif operation.level == `coll`:
                            target = self.__coll
                        operation_name = operation.operation_name
                        args = operation.args
                        kwargs = operation.kwargs
                        operator = getattr(target, operation_name)
                        if type(args) == tuple:
                            ops_rst = operator(*args, session=session, **kwargs)
                        else:
                            ops_rst = operator(args, session=session, **kwargs)

                        if operation.callback is not None:
                            operation.out = operation.callback(ops_rst)
                        else:
                            operation.out = ops_rst
                    except Exception as e:
                        logging.error(`{} {} Exception, transaction_pipeline args: {}, kwargs: {}`.format(
                            target, operation, args, kwargs))
                        logging.error(`transaction_pipeline Exception: {}`.format(repr(e)))
                        raise Exception(repr(e))
                    result.append(operation)
                return result

這裡給出一些例子來說明 DBManager的使用方法。

  • 建立集合、切換資料庫或集合：
# get DBManger instance
var dbm = DBManager(`mongodb://localhost:27017/admin`) # db_name, coll_name default `test`
dbm.create_coll(`testDB`, `testCollection`)
# change db or coll
dbm.db_name = `testDB` # dbm.db (test -> testDB) and dbm.coll (test.testCollection-> testDB.testCollection) will be changed at the same time
dbm.coll_nmae = `testCollection` # dbm.coll (test.test-> test.testCollection) will be change at the same time
  • 基本的操作，CRUD:
# simple manipulation operation
dbm.coll.insert_one({`hello`: `world`})
print(dbm.coll.find_one())   # {`_id`: ObjectId(`...`), `hello`: `world`}
dbm.coll.update_one({`hello`: `world`}, {`hello`: `hell`})

# bulk operation
from pymongo import InsertOne, DeleteOne, ReplaceOne, ReplaceOne
dbm.coll.bulk_write([InsertOne({`y`:1}), DeleteOne({`x`:1}), ReplaceOne({{`w`:1}, {`z`:1}, upsert=True})])

# simple managing operation
import pymongo
dbm.coll.create_index([(`hello`, pymongo.DESCENDING)], background=True)
dbm.client.list_database_names()
dbm.db.list_collection_names()
  • 執行緒併發，程式並行：
# thread concurrent
import threading
def fun(uri, db_name, coll_name):
    # new DBManager instance avoid variable competition
    dbm = DBManager(uri, db_name, coll_name)
    pass
t = threading.Thread(target=func, args=(uri, db_name, coll_name))
t.start()

# multiprocess parallel
import multiprocessing
def func(uri, db_name, coll_name):
    # new process, new client with fork=True parameter, and new DBManager instance.
    dbm = DBManager(uri, db_name, coll_name, fork=True)
    # Do something with db.
    pass
proc = multiprocessing.Process(target=func, args=(uri, db_name, coll_name))
proc.start()
  • MapReduce ：
# MapReduce
from bson.code import Code
mapper = Code(```
function () {...}
```)
reducer = Code(```
function (key, value) {...}
```)
rst = dbm.coll.inline_map_reduce(mapper, reducer)

對 MongoDB 一致性會話（session）和 事務（transaction）的支援

MongoDB Reference

  • docs: https://docs.mongodb.com/manual/
  • causal-consistency session: https://docs.mongodb.com/manual/core/read-isolation-consistency-recency/#causal-consistency
  • transation: https://docs.mongodb.com/manual/core/transactions/#transactions

會話（session），是對資料庫連線的一種邏輯表示。從MongoDB 3.6開始，MongoDB引入了客戶端會話（client session），並在其中加入了對操作的因果一致性(causal-consistency)的支援。因此，更準確地說，這裡 DBManger 類封裝的其實是因果一致性的會話，即client.start_session(causal_consistency=True)。不過，一致效能夠保證的前提是客戶端的應用應保證在一個會話中只有一個執行緒（thread）在做這些操作。在一個客戶端會話中，多個順序的讀寫操作得到的結果與它們的執行順序將是因果一致的，讀寫的設定都自動設為 “majority”。應用場景：先寫後讀，先讀後寫，一致性的寫，一致性的讀（Read your writes，Writes follow reads，Monotonic writes， Monotonic reads）。客戶端會話與服務端會話（server session）進行互動。從3.6版本開始，MongoDB驅動將所有的操作都關聯到服務端會話。服務端會話是客戶端會話順序操作因果一致性和重試寫操作的得以支援的底層框架。

MongoDB 對單個文件的操作時是原子性的（atomic）。原子性是指一個操作的結果要麼有要麼沒有，不可再切割，換句話說叫 “all or nothing”。從MongoDB 4.0開始，副本集（Replica set）開始支援多個文件級別的原子性，即多文件事務（muti-document transaction）。在同一個事務中，對跨越不同資料庫或集合下的多個文件操作，如果全部操作成功，則該事務被成功提交（commit）；如果某些操作出現失敗，則整個事務會終止（abort），操作中對資料庫的改動會被丟棄。只有在事務被成功提交之後，操作的結果才能被事務外看到，事務正在進行或者事務失敗，其中的操作對外都不可見。單個mongod服務和分片叢集（sharded cluster）暫不支援事務。MongoDB官方預計在4.2版本左右對分片叢集加入對事務的支援。另外，需要注意的是，多文件事務會引入更大的效能開銷，在場景允許的情況下，儘可能考慮用巢狀文件或陣列的單文件操作方式來解決問題。

會話和事務的主要應用場景其實都是多個的時序性操作，即流水線形式。因此 DBManager 加入了session_pipeline(self, pipeline) 和 transaction_pipeline(self, pipeline)的操作方法。首先引入表徵操作的類Operation，描述一個操作作用的層次（client, db或coll）、操作方法、引數和操作結果需要呼叫的回撥函式，見名知意，不再贅解。多個操作 Operation 類的例項構成的list 為pipeline, 作為session_pipeline(self, pipeline) 和 transaction_pipeline(self, pipeline) 的輸入引數。pipeline 操作的每一步的輸出會寫入到對應Operation 類的例項的out屬性中。

class Operation:
    """Operation for constructing sequential pipeline. Only used in DBManager.session_pipeline() or transaction_pipeline().

    Constructor parameters:
    level: <`client` | `db` | `coll`> indicating different operation level, MongoClient, Database, Collection
    operation_name: Literally, the name of operation on specific level
    args: position arguments the operation need. Require the first parameter or a tuple of parameters of the operation.
    kwargs: key word arguments the operation need.
    callback: callback function for operation result
    
    Examples:
    # pymongo.collection.Collection.find(filter, projection, skip=None, limit=None,...)
    Operation(`coll`, `find`, {`x`: 5}) only filter parameter, equivalent to:
    Operation(`coll`, `find`, args={`x`: 5}) or Operation(`coll`, `find`, kwargs={filter: {`x`: 5}})

    Operation(`coll`, `find`, ({`x`: 5},{`_id`: 0}) {`limit`:100}), equivalent to:
    Operation(`coll`, `find`, args=({`x`: 5},{`_id`: 0}, None, {`limit`:100}) ), OR
    Operation(`coll`, `find`, kwargs={`filter`:{`x`: 5}, `projection`: {`_id`: 0},`limit`:100})

    def cursor_callback(cursor):
        return cursor.distinct(`hello`)
    Operation(`coll`, `find`, kwargs={`limit`: 2}, callback=cursor_callback)
    """
    def __init__(self, level, operation_name, args=(), kwargs={}, callback=None):
        self.level = level
        self.operation_name = operation_name
        self.args = args
        if kwargs is None:
            self.kwargs = None
        else:
            self.kwargs = kwargs
        self.callback = callback
        self.out = None

基於 DBManager 和 Operation 的因果一致性的會話和事務的簡單示例如下：

# causal-consistency session or transaction pipeline operation
def cursor_callback(cursor):
    return cursor.distinct(`hello`)
op_1 = Operation(`coll`, `insert_one`, {`hello`: `heaven`})
op_2 = Operation(`coll`, `insert_one`, {`hello`: `hell`})
op_3 = Operation(`coll`, `insert_one`, {`hello`: `god`})
op_4 = Operation(`coll`, `find`, kwargs={`limit`: 2}, callback=cursor_callback)
op_5 = Operation(`coll`, `find_one`, {`hello`: `god`})
pipeline = [op_1, op_2, op_3, op_4, op_5]
ops = dbm.transaction_pipeline(pipeline) # only on replica set deployment
# ops = dbm.session_pipeline(pipeline) # can be standalone, replica set or sharded cluster.
for op in ops:
    print(op.out)

【正文完】

注：內容同步自同名CSDN部落格：https://blog.csdn.net/fzlulee/article/details/85944967

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