Lec-80:Shared Exclusive Locking Protocol with Example in Hindi | Concurrency Control | DBMS | Part-1

Aug 15, 2023

Lec-80:Shared Exclusive Locking Protocol with Example in Hindi | Concurrency Control | DBMS | Part-1

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🧑‍🎓Contributed by: Anurag Singhal

Lock Based Protocols in DBMS is a mechanism in which a transaction cannot Read or Write the data until it acquires an appropriate lock. Lock based protocols help to eliminate the concurrency problem in DBMS for simultaneous transactions by locking or isolating a particular transaction to a single user.

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Content

0.407 -> Hello friends, Welcome to Gate smashers

2.92 -> In this video we're going to talk about concurrency control protocol

6.652 -> concurrency control protocol basic aim

10.133 -> is to achieve serializabity

12.787 -> and achieving recover-ability

14.652 -> and what are serializabity and recover-ability we had already discussed

19.372 -> Conflict serializabity, view serializabity

21.88 -> and in that we've discussed that how to check

25.052 -> that which schedule is serializable or which one is not

28.212 -> But

29.067 -> Aim of concurrency control protocol

31.827 -> is not to check but how to make them

35.513 -> that how to make it serializable

37.167 -> or how to make it recoverable

38.987 -> Schedule are concurrent

40.4 -> but how to make them recoverable and serializable

43.933 -> that comes under concurrency control protocol

46.732 -> and how we achieve this

48.8 -> By using the locking protocols

50.772 -> We've various locking protocols

52.74 -> Simple locking protocol under which comes Shared Exclusive locking protocol

56.74 -> There are 2 PL

57.82 -> Rigorous 2 PL

58.9 -> Strict 2 PL

59.987 -> Time stamp protocol so there are various protocols

61.972 -> But among these the shared exclusive protocol

64.2 -> and time stamp

65.253 -> are the most important ones

66.667 -> So like the video

68.5 -> Subscribe the channel who has not done yet and press the bell button

72.173 -> So that you'll be getting all notifications

75.927 -> Will start with Shared-Exclusive locking

78.853 -> It is very basic and the first locking protocol

82.94 -> or we can call simplest locking protocol

84.853 -> that is shared-exclusive locking

86.78 -> and here if we discuss we're using two types of lock

89.852 -> Shared lock

90.967 -> and exclusive lock

92.02 -> We use shared lock when if the transaction locked the data item

95.62 -> in the shared more then it is allowed to read only

98.56 -> and if in exlusive mode data is locked

102.812 -> then read and write both can be done, it means

105.767 -> Lets say transaction is T1

107.787 -> And Transaction has to perform Read(A)

111 -> A only want perform read

113.06 -> so what we'll do

114.887 -> We'll take shared lock on A

117.607 -> Now in this case there is no benefit of taking exclusive

120.293 -> Why? If only read is to be done

122.513 -> then we can achieve our work with shared mood only

125.932 -> and when is work is over

127.767 -> Then what we'll do

129.532 -> We'll unlock it

130.57 -> So S means shared and U means unlock

133.467 -> A means on which data item you're taking A,B,C you can take anything

138.052 -> Same if we talk about transaction T2

141.207 -> Read(A) and Write (A) both

144.487 -> If transaction has to perform Read(A) and Write (A) both

147.68 -> and in this case if I say that we've to take shared lock

150.573 -> That won't apply

151.972 -> What will apply in this case ?

153.947 -> In this there will be exclusive lock on A

156.812 -> What we can do in exclusive lock case

159.692 -> Read and Write both

161.867 -> and on shared lock case what we'll do

164.667 -> only read

165.72 -> and now if you are only read in exclusive

167.993 -> you can do that also but there is no benefit of that, by default we set the algorithm

172.42 -> or set software in such a way

174.727 -> that if want to read take shared

176.692 -> and if want Read and Write both

178.913 -> Then use exclusive lock

180.972 -> and when its work is over means when read and

183.792 -> write is done then it will perform unlock A

186.612 -> Like if someone else request they can get the lock

190.327 -> because in a transaction there is work done on multiple data items

193.567 -> So it is not unnecessary that only T1 need lock

196.06 -> There is T2, T3, T4

198.292 -> everyone acquires lock

200.28 -> so they will request for lock

202.092 -> so the concurrency control component

204.68 -> that will check

205.755 -> It will check compatibility table

208.212 -> and here the compatibility table is important

210.767 -> Questions may come on this

212.852 -> There is simple thing is compatibility table

215.773 -> On one side we're saying grant

218.413 -> and on other side we're saying request

221.153 -> As we know there are only 2 locks- Shared and exclusive

224.332 -> so how many case can form

225.653 -> 4 case, Shared - Shared

227.547 -> Shared- exclusive

228.607 -> Exclusive-Shared

229.927 -> Exclusive- exclusive

231.16 -> So same we are doing here

232.62 -> Let's say grant means

234.347 -> If on this side grant is there and on other side request

236.773 -> so lets say we already grant shared lock

240.492 -> Means concurrency control component

242.8 -> shared lock on A

244 -> I'm taking about A data item, not separately A, B, C

247.332 -> Lets say we're only taking about A data component

251.107 -> and I've given shared lock (A)

253.247 -> and if someone else also request that I also want shared lock(A)

257.407 -> means on same data item

258.773 -> and will be allowing the request

262.22 -> obviously yes

263.881 -> Reason I've just told you what we do in shared mode

266.852 -> Only Read is done

268.447 -> and there is never a conflict between read and read

271.132 -> No

271.713 -> As I've already told you

273.692 -> Conflict serializibity

276.567 -> all these conflict read, read, read, read, write

278.899 -> what are they that I already made videos about

281.467 -> Link of that I've mentioned in description box, do check it once

285.907 -> As there are many things in this that we had already discussed

288.732 -> and we're using it directly here

291.492 -> So there is no conflict in Read and Read

293.812 -> we're only checking data

295.467 -> So here you can grant

297.292 -> Let's say we already granted shared lock

300.813 -> and request came on exclusive lock

303.34 -> Means A needs exclusive lock

305.498 -> So will that be granted ? No we'll not what we'll do

309.527 -> We can no it, as I've written X here, so here I'm doing yes

314.17 -> and here I'll write no

316.092 -> No means that we won't grant it and reason is

319.473 -> Means if a transaction has taken shared lock let's say it R(A)

324.252 -> Read(A)

325.72 -> and second transaction is saying that I need exclusive lock

328.333 -> means what it want to do Read(A) and Write (A) both

331.852 -> and if we allow

333.607 -> here in future there can be Read(A) and Write(A) conflict

337.652 -> So to prevent that what we'll do is No

340.353 -> So next is

341.833 -> if we had already granted exclusive lock

345.052 -> already we granted exclusive lock (A)

348.147 -> and if someone ask for share lock, So will we give ?

351.053 -> No, same

352.24 -> Write and read, means we're already doing read and write

355.68 -> one transaction is already doing

357.547 -> and second one is saying that I've to read

359.573 -> so same Write, read problem so we won't allow

362.313 -> and on 4th case also we won't allow

365.207 -> Write- write problem, one is already doing Read-Write

368.167 -> and second one is saying that I want to perform same on Read-write on same item

372.667 -> so if we allow that

374.412 -> So again problem on inconsistency can come so to prevent what we'll do is no

379.332 -> So basically our Shared-exclusive locking protocol

383.5 -> It's main purpose is that we are trying to achieve serializabity

387.46 -> because of which we are trying to achieve consistency

391.48 -> if we achieve this so the acid property that we discussed earlier

395.172 -> In that C which is consistency

397.547 -> we are trying to achieve that

399.532 -> So this compatibility table is very important you should know this

Source: https://www.youtube.com/watch?v=94C0V7f2zm4