Connecting to a Replica Set from Pymongo
read_pref = pymongo.read_preferences.ReadPreference.SECONDARY
connection = pymongo.MongoClient() # for default localhost:port
connection = pymongo.MongoClient(host=['mongodb://localhost:27001', 'mongodb://localhost:27002'], replicaSet="setName", w=1, j=False, read_preference = read_pref)
host参数只要列出可用的一个node即可，pymongo会自动查找到其余的成员。
w设置保证成功写入的副本个数。可以为数字也可以为”majority”
j设置是否等待（Primary）日志写入。
read_preferenceOne of primary, primaryPreferred, secondary, secondaryPreferred, or nearest. Defaults to primary.
Catching FailoverPrimary离线时，pymongo会抛出pymong.errors.AutoReconnect异常，捕获异常等待几秒重试，会自动连接到其它节点。但抛出AutoReconnect异常时无法保证数据成功写入磁盘也无法保证数据未写入，所以如果再次插入带相同_id字段文档有可能会抛出pymongo.errors.DuplicateKeyError。
for retry in range(3): try: # insert something break except pymongo.errors.AutoReconnect as e: print(e) time.sleep(5) except pymong.errors.DuplicateKeyError as e: # 如果是读取则不需要捕获DuplicateKeyError print(e) break对于update， $inc、$push 不应该被重复执行，所以重试的时候需要验证上次操作是否成功，或者使用 $set 则无需验证。
Shardingsharding 用于把一个较大的集合拆分存储。拆分的方法有 range based 和 hash based 。
Building a Sharded Environment以下是mongodb university公开课提供的shell代码，创建shards，注意 shardsvr shardsvr 选项和 mongos 命令。创建2个shards，每个shard包含3个replicate set，则一共创建需要9个mognod进程。
# Andrew Erlichson # MongoDB # script to start a sharded environment on localhost # clean everything up echo "killing mongod and mongos" killall mongod killall mongos echo "removing data files" rm -rf /data/config rm -rf /data/shard* # start a replica set and tell it that it will be shard0 echo "starting servers for shard 0" mkdir -p /data/shard0/rs0 /data/shard0/rs1 /data/shard0/rs2 mongod --replSet s0 --logpath "s0-r0.log" --dbpath /data/shard0/rs0 --port 37017 --fork --shardsvr --smallfiles mongod --replSet s0 --logpath "s0-r1.log" --dbpath /data/shard0/rs1 --port 37018 --fork --shardsvr --smallfiles mongod --replSet s0 --logpath "s0-r2.log" --dbpath /data/shard0/rs2 --port 37019 --fork --shardsvr --smallfiles sleep 5 # connect to one server and initiate the set echo "Configuring s0 replica set" mongo --port 37017 << 'EOF' config = { _id: "s0", members:[ { _id : 0, host : "localhost:37017" }, { _id : 1, host : "localhost:37018" }, { _id : 2, host : "localhost:37019" }]}; rs.initiate(config) EOF # start a replicate set and tell it that it will be a shard1 echo "starting servers for shard 1" mkdir -p /data/shard1/rs0 /data/shard1/rs1 /data/shard1/rs2 mongod --replSet s1 --logpath "s1-r0.log" --dbpath /data/shard1/rs0 --port 47017 --fork --shardsvr --smallfiles mongod --replSet s1 --logpath "s1-r1.log" --dbpath /data/shard1/rs1 --port 47018 --fork --shardsvr --smallfiles mongod --replSet s1 --logpath "s1-r2.log" --dbpath /data/shard1/rs2 --port 47019 --fork --shardsvr --smallfiles sleep 5 echo "Configuring s1 replica set" mongo --port 47017 << 'EOF' config = { _id: "s1", members:[ { _id : 0, host : "localhost:47017" }, { _id : 1, host : "localhost:47018" }, { _id : 2, host : "localhost:47019" }]}; rs.initiate(config) EOF # start a replicate set and tell it that it will be a shard2 echo "starting servers for shard 2" mkdir -p /data/shard2/rs0 /data/shard2/rs1 /data/shard2/rs2 mongod --replSet s2 --logpath "s2-r0.log" --dbpath /data/shard2/rs0 --port 57017 --fork --shardsvr --smallfiles mongod --replSet s2 --logpath "s2-r1.log" --dbpath /data/shard2/rs1 --port 57018 --fork --shardsvr --smallfiles mongod --replSet s2 --logpath "s2-r2.log" --dbpath /data/shard2/rs2 --port 57019 --fork --shardsvr --smallfiles sleep 5 echo "Configuring s2 replica set" mongo --port 57017 << 'EOF' config = { _id: "s2", members:[ { _id : 0, host : "localhost:57017" }, { _id : 1, host : "localhost:57018" }, { _id : 2, host : "localhost:57019" }]}; rs.initiate(config) EOF # now start 3 config servers echo "Starting config servers" mkdir -p /data/config/config-a /data/config/config-b /data/config/config-c mongod --logpath "cfg-a.log" --dbpath /data/config/config-a --port 57040 --fork --configsvr --smallfiles mongod --logpath "cfg-b.log" --dbpath /data/config/config-b --port 57041 --fork --configsvr --smallfiles mongod --logpath "cfg-c.log" --dbpath /data/config/config-c --port 57042 --fork --configsvr --smallfiles # now start the mongos on a standard port mongos --logpath "mongos-1.log" --configdb localhost:57040,localhost:57041,localhost:57042 --fork echo "Waiting 60 seconds for the replica sets to fully come online" sleep 60 echo "Connnecting to mongos and enabling sharding" # add shards and enable sharding on the test db mongo <<'EOF' db.adminCommand( { addshard : "s0/"+"localhost:37017" } ); db.adminCommand( { addshard : "s1/"+"localhost:47017" } ); db.adminCommand( { addshard : "s2/"+"localhost:57017" } ); db.adminCommand({enableSharding: "school"}) db.adminCommand({shardCollection: "school.students", key: {student_id:1}}); EOF