MongoDB course for developers. unit 6/8. Application Engineering. Homeworks

Homework 6.1

Which of the following statements are true about MongoDB replication. Check all that apply.

The minimum sensible number of voting nodes to a replica set is three.MongoDB replication is synchronous.The Mongo shell is capable of attaching to a replica set and automatically failing over.By default, using the new MongoClient connection class, w=1 and j=1.The oplog utilizes a capped collection.

Homework 6.2

Let's suppose you have a five member replica set and want to assure that writes are committed to the journal and are acknowledged by at least 3 nodes before you proceed forward. What would be the appropriate settings for w and j?

w=1, j=1w="majority", j=1w=3, j=0w=5, j=1w=1,j=3

Homework 6.3

Which of the following statements are true about choosing and using a shard key:

The shard key must be uniqueThere must be a index on the collection that starts with the shard key.Mongo can not enforce unique indexes on a sharded collection other than the shard key itself.Any update that does not contain the shard key will be sent to all shards.You can change the shard key on a collection if you desire.

Homework 6.4

You have a sharded system with three shards and have sharded the collections "grades" in the "test" database across those shards. The output of sh.status() when connected to mongos looks like this:

mongos> sh.status()
--- Sharding Status --- 
  sharding version: { "_id" : 1, "version" : 3 }
  shards:
 {  "_id" : "s0",  "host" : "s0/localhost:37017,localhost:37018,localhost:37019" }
 {  "_id" : "s1",  "host" : "s1/localhost:47017,localhost:47018,localhost:47019" }
 {  "_id" : "s2",  "host" : "s2/localhost:57017,localhost:57018,localhost:57019" }
  databases:
 {  "_id" : "admin",  "partitioned" : false,  "primary" : "config" }
 {  "_id" : "test",  "partitioned" : true,  "primary" : "s0" }
  test.grades chunks:
    s1 4
    s0 4
    s2 4
   { "student_id" : { $minKey : 1 } } -->> { "student_id" : 0 } on : s1 Timestamp(12000, 0) 
   { "student_id" : 0 } -->> { "student_id" : 2640 } on : s0 Timestamp(11000, 1) 
   { "student_id" : 2640 } -->> { "student_id" : 91918 } on : s1 Timestamp(10000, 1) 
   { "student_id" : 91918 } -->> { "student_id" : 176201 } on : s0 Timestamp(4000, 2) 
   { "student_id" : 176201 } -->> { "student_id" : 256639 } on : s2 Timestamp(12000, 1) 
   { "student_id" : 256639 } -->> { "student_id" : 344351 } on : s2 Timestamp(6000, 2) 
   { "student_id" : 344351 } -->> { "student_id" : 424983 } on : s0 Timestamp(7000, 2) 
   { "student_id" : 424983 } -->> { "student_id" : 509266 } on : s1 Timestamp(8000, 2) 
   { "student_id" : 509266 } -->> { "student_id" : 596849 } on : s1 Timestamp(9000, 2) 
   { "student_id" : 596849 } -->> { "student_id" : 772260 } on : s0 Timestamp(10000, 2) 
   { "student_id" : 772260 } -->> { "student_id" : 945802 } on : s2 Timestamp(11000, 2) 
   { "student_id" : 945802 } -->> { "student_id" : { $maxKey : 1 } } on : s2 Timestamp(11000, 3) 

If you ran the query

use test
db.grades.find({'student_id':530289})

Which shards would be involved in answering the query?

s0,s1 and s2s0s1s2

Homework 6.5

Create three directories for the three mongod processes. On unix, this could be done as follows:

mkdir -p /data/rs1 /data/rs2 /data/rs3

Now start three mongo instances as follows. Note that are three commands. The browser is probably wrapping them visually.

./mongod --replSet m101 --logpath "1.log" --dbpath /data/rs1 --port 27017 --smallfiles --fork

./mongod --replSet m101 --logpath "2.log" --dbpath /data/rs2 --port 27018 --smallfiles --fork

./mongod --replSet m101 --logpath "3.log" --dbpath /data/rs3 --port 27019 --smallfiles --fork

Now connect to a mongo shell and make sure it comes up

./mongo --port 27017

Now you will create the replica set. Type the following commands into the mongo shell:

config = { _id: "m101", members:[
          { _id : 0, host : "localhost:27017"},
          { _id : 1, host : "localhost:27018"},
          { _id : 2, host : "localhost:27019"} ]
};
rs.initiate(config);

At this point, the replica set should be coming up. You can type

rs.status()

to see the state of replication. 

m101:PRIMARY> rs.status()

{

"set" : "m101",

"date" : ISODate("2014-12-04T18:50:17Z"),

"myState" : 1,

"members" : [

{

"_id" : 0,

"name" : "localhost:27017",

"health" : 1,

"state" : 1,

"stateStr" : "PRIMARY",

"uptime" : 234,

"optime" : Timestamp(1417718826, 1),

"optimeDate" : ISODate("2014-12-04T18:47:06Z"),

"electionTime" : Timestamp(1417718835, 1),

"electionDate" : ISODate("2014-12-04T18:47:15Z"),

"self" : true

},

{

"_id" : 1,

"name" : "localhost:27018",

"health" : 1,

"state" : 2,

"stateStr" : "SECONDARY",

"uptime" : 190,

"optime" : Timestamp(1417718826, 1),

"optimeDate" : ISODate("2014-12-04T18:47:06Z"),

"lastHeartbeat" : ISODate("2014-12-04T18:50:17Z"),

"lastHeartbeatRecv" : ISODate("2014-12-04T18:50:17Z"),

"pingMs" : 0,

"syncingTo" : "localhost:27017"

},

{

"_id" : 2,

"name" : "localhost:27019",

"health" : 1,

"state" : 2,

"stateStr" : "SECONDARY",

"uptime" : 190,

"optime" : Timestamp(1417718826, 1),

"optimeDate" : ISODate("2014-12-04T18:47:06Z"),

"lastHeartbeat" : ISODate("2014-12-04T18:50:17Z"),

"lastHeartbeatRecv" : ISODate("2014-12-04T18:50:17Z"),

"pingMs" : 0,

"syncingTo" : "localhost:27017"

}

],

"ok" : 1

}