1)
Problems
1 through 3 are an exercise in running mongod's, replica sets, and an exercise
in testing of replica set rollbacks, which can occur when a former
primary rejoins a set after it has previously had a failure.
Get the
files from Download Handout link, and extract them. Use a.bat instead of a.sh
on Windows.
Start a 3
member replica set (with default options for each member, all are peers). (a.sh
will start the mongod's for you if you like.)
# if on unix:
chmod +x a.sh
./a.sh
You will
need to initiate the replica set next.
Run:
mongo --shell --port 27003 a.js
// ourinit() will initiate the set for us.
// to see its source code type without the
parentheses:
ourinit
// now execute it:
ourinit()
We will
now do a test of replica set rollbacks. This is the case where data never
reaches a majority of the set. We'll test a couple scenarios.
Now,
let's do some inserts. Run:
db.foo.insert( { _id : 1 }, { writeConcern : { w :
2 } } )
db.foo.insert( { _id : 2 }, { writeConcern : { w :
2 } } )
db.foo.insert( { _id : 3 }, { writeConcern : { w :
2 } } )
Note: if
27003 is not primary, make it primary -- using rs.stepDown() on the mongod on port 27001 (perhaps also rs.freeze()) for example.
Next,
let's shut down that server on port 27001:
var a = connect("localhost:27001/admin");
a.shutdownServer()
rs.status()
At this
point the mongod on 27001 is shut down. We now have only our 27003 mongod, and
our arbiter on 27002, running.
Let's
insert some more documents:
db.foo.insert( { _id : 4 } )
db.foo.insert( { _id : 5 } )
db.foo.insert( { _id : 6 } )
Now,
let's restart the mongod that is shut down. If you like you can cut and paste
the relevant mongod invocation from a.sh.
Now run ps again and verify three are up:
ps -A | grep mongod
Now, we
want to see if any data that we attempted to insert isn't there. Go into the
shell to any member of the set. Use rs.status() to check
state. Be sure the member is "caught up" to the latest optime (if
it's a secondary). Also on a secondary you might need to invoke rs.slaveOk() before doing a query.)
Now run:
db.foo.find()
to see
what data is there after the set recovered from the outage. How many documents
do you have?
|
Answer:
6
|
2)Let's
do that again with a slightly different crash/recover scenario for each
process. Start with the following:
With all
three members (mongod's) up and running, you should be fine; otherwise, delete
your data directory, and, once again:
./a.sh
mongo --shell --port 27003 a.js
ourinit() // you might need to wait a bit after
this.
// be sure 27003 is the primary.
// use rs.stepDown() elsewhere if it isn't.
db.foo.drop()
db.foo.insert( { _id : 1 }, { writeConcern : { w :
2 } } )
db.foo.insert( { _id : 2 }, { writeConcern : { w :
2 } } )
db.foo.insert( { _id : 3 }, { writeConcern : { w :
2 } } )
var a = connect("localhost:27001/admin");
a.shutdownServer()
rs.status()
db.foo.insert( { _id : 4 } )
db.foo.insert( { _id : 5 } )
db.foo.insert( { _id : 6 } )
Now this
time, shut down the mongod on port 27003 (in addition to
the other member being shut down already) before doing anything else. One way
of doing this in Unix would be:
ps -A | grep mongod
# should see the 27003 and 27002 ones running
(only)
ps ax | grep mongo | grep 27003 | awk '{print $1}'
| xargs kill
# wait a little for the shutdown perhaps...then:
ps -A | grep mongod
# should get that just the arbiter is present…
Now
restart just the 27001 member. Wait for it t
o get
healthy -- check this with rs.status() in the
shell. Then query
> db.foo.find()
Then add
another document:
> db.foo.insert( { _id : "last" } )
After
this, restart the third set member (mongod on port
27003). Wait for it to come online and enter a health state (secondary or
primary).
Run (on
any member -- try multiple if you like) :
> db.foo.find()
You
should see a difference from problem 1 in the result above.
Question:
Which of the following are true about mongodb's operation in these scenarios?
Check all that apply.
Check all
that apply:
3)
4)
Keep the
three member replica set from the above problems running. We've had a request
to make the third member never eligible to be primary. (The member should still
be visible as a secondary.)
Reconfigure
the replica set so that the third member can never be primary. Then run:
$ mongo --shell a.js --port 27003
And run:
> part4()
And enter
the result in the text box below (with no spaces or line feeds just the exact
value returned).
|
Answer:
133
|
5)
Suppose
we have blog posts in a (not sharded*) postings collection, of the form:
{
_id : ...,
author :
'joe',
title :
'Too big to fail',
text :
'...',
tags : [
'business', 'finance' ],
when :
ISODate("2008-11-03"),
views :
23002,
votes : 4,
voters :
['joe', 'jane', 'bob', 'somesh'],
comments :
[
{
commenter : 'allan',
comment
: 'Well, i don't think so…',
flagged
: false,
plus :
2
},
...
]
}
Which of
these statements is true?
Note: to
get a multiple answer question right in this final you must get all the
components right, so even if some parts are simple, take your time.
*Certain
restrictions apply to unique constraints on indexes when sharded, so I mention
this to be clear.
Check all
that apply:
6)
7)
8) We have been asked by our users to pull some data from a previous database backup of a sharded cluster. They'd like us to set up a temporary data mart for this purpose, in addition to answering some questions from the data. The next few questions involve this user request.
First we
will restore the backup. Download gene_backup.zip from the Download Handout
link and unzip this to a temp location on your computer.
The
original cluster that was backed up consisted of two shards, each of which was
a three member replica set. The first one named "s1" and the second
"s2". We have one mongodump (backup) for each shard, plus the config
database. After you unzip you will see something like this:
$ ls -la
total 0
drwxr-xr-x
5 dwight staff 170 Dec 11 13:47 .
drwxr-xr-x
17 dwight staff 578 Dec 11 13:49 ..
drwxr-xr-x
4 dwight staff 136 Dec 11 13:45 config_server
drwxr-xr-x
5 dwight staff 170 Dec 11 13:46 s1
drwxr-xr-x
5 dwight staff 170 Dec 11 13:46 s2
Our data
mart will be temporary, so we won't need more than one mongod per shard, nor
more than one config server (we are not worried about downtime, the mart is
temporary).
As a
first step, restore the config server backup and run a mongod config server
instance with that restored data. The backups were made with mongodump. Thus
you will use the mongorestore utility to restore.
Once you
have the config server running, confirm the restore of the config server data
by running the last javascript line below in the mongo shell, and entering the
5 character result it returns.
$ mongo localhost:27019/config
configsvr>
configsvr> db
config
configsvr>
db.chunks.find().sort({_id:1}).next().lastmodEpoch.getTimestamp().toUTCString().substr(20,6)
Notes:
- You must do this with
MongoDB 3.0. The mongorestore may not work with prior versions of MongoDB.
- If you do not see the prompt
with 'configsvr' before the '>', then you are not running as a config
server.
|
Answer:
39:15
|
9) Now
that the config server from question #8 is up and running, we will restore the
two shards ("s1" and "s2").
If we
inspect our restored config db, we see this in db.shards:
~/dba/final $ mongo localhost:27019/config
MongoDB shell version: 3.0.0
connecting to: localhost:27019/config
configsvr> db.shards.find()
{ "_id" : "s1", "host"
: "s1/genome_svr1:27501,genome_svr2:27502,genome_svr2:27503" }
{ "_id" : "s2",
"host" :
"s2/genome_svr4:27601,genome_svr5:27602,genome_svr5:27603" }
From this
we know when we run a mongos for the cluster, it will expect
the first shard to be a replica set named "s1", and the second to be
a replica set named "s2", and also to be able to be able to resolve
and connect to at least one of the seed hostnames for each shard.
If we
were restoring this cluster as "itself", it would be best to assign
the hostnames "genome_svr1" etc. to the appropriate IP addresses in
DNS, and not change config.shard. However, for this problem, our
job is not to restore the cluster, but rather to create a new temporary data
mart initialized with this dataset.
Thus
instead we will update the config.shards metadata to point to the locations of
our new shard servers. Update the config.shards collection such that your
output is:
configsvr> db.shards.find()
{ "_id" : "s1",
"host" : "localhost:27501" }
{ "_id" : "s2",
"host" : "localhost:27601" }
configsvr>
Be sure
when you do this nothing is running except the single config server. mongod and
mongos processes cache metadata, so this is important. After the update restart
the config server itself for the same reason.
Now start
a mongod for each shard -- one on port 27501 for shard "s1" and on
port 27601 for shard "s2". At this point if you run ps you should see
three mongod's -- one for each shard, and one for our config server. Note they
need not be replica sets, but just regular mongod's, as we did not begin our host string in config.shards with setname/. Finally, use mongorestore to
restore the data for each shard.
The next
step is to start a mongos for the cluster.
Connect
to the mongos with a mongo shell. Run this:
use snps
var x = db.elegans.aggregate( [ { $match : { N2 :
"T" } } , { $group : { _id:"$N2" , n : { $sum : 1 } } } ]
).next(); print( x.n )
Enter the
number output for n.
Notes:
- You must do this with
MongoDB 3.0. The mongoimport tool may not work with prior versions of
MongoDB.
|
Answer: 47664
|
10
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