Describe the nature of transactions and the reasons for designing database systems around transactions. [1] The Concurrency is about to control the multi-user access of Database Illustrative Example To illustrate the concept of … Database System Concepts 3rd Edition 16.7 ©Silberschatz, Korth and Sudarshan ... Two-phase locking does not ensure freedom from deadlocks! Approach #3: Two-Phase Locking →Txns acquire appropriate lock on physical version before they can read/write a logical tuple. An update of the data will be … In particular, a transaction can frequently read a committed version that is earlier than a current version produced by an uncommitted transaction. See our User Agreement and Privacy Policy. Snapshot isolation means that whenever a transaction would take a read lock on a page, it makes a copy of the page instead, and then performs its operations on that copied page. For instance, when making a wire transfer between two bank accounts if a reader reads the balance at the bank when the money has been withdrawn from the original account and before it was … multiversion concurrency control can be achieved by using Time Stamp Unlike most other database systems which use locks for concurrency control, PostgreSQL maintains data consistency by using a multiversion model. Concurrency Control Locking Strategies. Oracle can also provide read consistency to all of the queries in a transaction (transaction-level read consistency). Concurrency control, when applied to a DBMS, is meant to coordinate simultaneous transactions while preserving data integrity. The basic idea behind multiversion concurrency control is to maintain one or … Figure 20-3 Two Transactions in a Deadlock Text description of the illustration cncpt068.gif The data is consistent with … Using an optimistic approach, each transaction moves through 2 or 3 phases, referred to as read, validation and write. acknowledge that you have read and understood our, GATE CS Original Papers and Official Keys, ISRO CS Original Papers and Official Keys, ISRO CS Syllabus for Scientist/Engineer Exam. Time Stamp Ordering Protocol: In addition, the validation concurrency control method (see Section 22.4) also maintains multiple versions. Disadvantages: Modern MVCC Implementations . Attention reader! Multiversion schemes keep old versions of data item to increase concurrency. logical clock, MVCC) Because MVCC (Multi-Version Concurrency Control) is such a prevalent Concurrency Control technique (not only in relational database systems, in this article, I’m going to explain how it works. Multiversion Concurrency Control (MVCC) enables snapshot isolation. Optimistic Methods. The most frequently used isolation level in SAP HANA is "read committed". MULTI-VERSION CONCURRENCY CONTROL MVCC is more than just a concurrency control protocol. Database System Concepts 3rd Edition 16.2 ©Silberschatz, Korth and Sudarshan Lock-Based Protocols! Locking, Timestamp Ordering and Optimistic Concurrency Control. Tay. Multiversion Concurrency Control. Slideshare uses cookies to improve functionality and performance, and to provide you with relevant advertising. Q. Multiversion schemes keep old versions of data item to increase concurrency. In a multiversion database system, each write on a data … Multiversion two-phase locking using certify locks. Concurrency control is provided in a database to: Various concurrency control techniques are: 1. Multiversion 2 phase locking: One lock for each item in the database. concurrency control techniques, from single-version to multiversion and from lock-based to timestamp-based. based concurrency control would block. Also, timestamp ranges can be used to conservatively find deadlocks without graph based cycle detection. Concurrency Control: Methods, Performance, and Analysis ALEXANDER THOMASIAN IBM T. J. Watson Research Center, 30 Saw Mill River Road, Hawthorne, NY 10532 Standard locking (two-phase locking with on-demand lock requests and blocking upon lock conflict) is the primary concurrency control (CC) method for centralized databases. 10. Concurrency Control Techniques, Fundamentals of Database Systems 4th - Ramez Elmasri, Shamkant B. Navathe | All the textbook answers and step-by-step explanati… Shared/exclusive− This type of locking mechanism differentiates the locks based on their uses. If you continue browsing the site, you agree to the use of cookies on this website. Conservative timestamp ordering algorithm. Multiversion Timestamp Ordering. Chapter 22. (ii) preserve database consistency through consistency preserving execution of transactions. Thus, … Multiversion Concurrency Control 467 managed by a separate process. Granularity of Data Items and Multiple Granularity Locking. CMU 15-445/645 (Fall 2018) Two states (values) Locked (1) Item cannot be accessed. 1. In this method, several versions of each data item X are maintained. transaction T is allowed to read the value of version Xi, the value of read_TS() is set to max( Each successful write results in the creation of a new version of the data item written. This means that no process is ever in a state where it is holding some shared resources, and waiting for another process to release a shared resource which it requires. The remaining six chapters are split into two parts: Chapters 3-5 on concurrency control and Chapters 6-8 on recovery. Analyse the problems of data management in a concurrent environment. consistency models, these techniques can be unnecessarily cumbersome and degrade performance. Two-phase locking Protocol 2. 1. read_TS: The read timestamp of is the largest of all the timestamps of transactions that have - 9907680 Which of the above statements is/are TRUE A) Both I and B) I only C) II only D) … Explain how multiversion concurrency control can be achieved by using Time Stamp Ordering. Concurrency in Index Structures. There are three concurrency control techniques which are as follows: Locking method. Snapshot isolation. There are main three methods for concurrency control. They are as follows: 1. Problem 9RQ from Chapter 21: Discuss two multiversion techniques for concurrency control.... Get solutions Multiversion Concurrency Control Techniques Time-stamp Methods 3. ∗ Multiversion concurrency control techniques ∗ Multiversion techniques based on timestamp ordering ∗ Multiversion two-phase locking ∗ Granularity of data items – Additional exercises ∗ Extension exercise 1 ∗ Extension exercise 2 ∗ Extension exercise 3 X-lock is … Whenever a transaction T is allowed to execute a write_item(X) operation, a new version of item It may affect the transaction result. Database Recovery Techniques. There are two types of locks that fall under the category of pessimistic locking: write lock and read lock. Definition Concurrency control is a database management systems (DBMS) concept that is used to address occur with a multi-user system. procedure in DBMS for managing simultaneous operations without conflicting with each another Comparison – Centralized, Decentralized and Distributed Systems, Difference between Centralized Database and Distributed Database, Condition of schedules to View-equivalent, Precedence Graph For Testing Conflict Serializability in DBMS, Types of Schedules based Recoverability in DBMS, SQL | Join (Inner, Left, Right and Full Joins), Commonly asked DBMS interview questions | Set 1, Introduction of DBMS (Database Management System) | Set 1, Lock Based Concurrency Control Protocol in DBMS, Two Phase Locking (2-PL) Concurrency Control Protocol | Set 3, Graph Based Concurrency Control Protocol in DBMS, Congestion Control techniques in Computer Networks, Difference between Flow Control and Congestion Control, Difference between Flow Control and Error Control, Concurrency problems in DBMS Transactions, Resource Allocation Techniques for Processes, Advanced compression and encoding techniques in HP Vertica, Implementation of Contiguous Memory Management Techniques, Implementing Non-contiguous Memory Management Techniques, Computer Organization | Hardwired v/s Micro-programmed Control Unit, DBMS-Transactions and Concurrency Control. Use multiple versions of a data item. concurrency in the system. n Multiversion two-phase locking using certify locks n Three locking modes: read, write, and certify Slide 21- 26 Figure 21.6 Lock compatibility tables (a) Lock compatibility table for read/write locking scheme (b) Lock compatibility table for read/write/certify locking scheme X is created, with both the write_TS and the read_TS set to TS(T). Multiversion Concurrency Control (MVCC) enables snapshot isolation. When a . A pro- A difficulty in reducing the overhead is that we do not know in … Two-Phase Locking Protocol: Locking is an operation which secures: permission to read, OR permission to write a data item. To ensure serializability, the following two rules are used: Read/Write locks, Two-Phase Locking) 2. Readers do not wait for Writers (or other readers of the same data) Writers do not wait for … One multiversion protocol extends timestamp order to multiversion timestamp ordering (Section 22.3.1), and another extends two-phase locking (Section 22.3.2). Unlocked (0) … To handle simultaneous access of the same data by many users, Oracle utilises various types of locks and a multiversion consistency model to maximise performance. multiversion databases, there are Multiversion Two-phase Locking (MV2PL), Multiversion Insert and Delete Operations! Deuteronomy uses a timestamp order multi-version concurrency control (MVCC) method [16]. See our Privacy Policy and User Agreement for details. 1. Multiversion algorithm, based on ordering … Concurrency Control Techniques. Time stamp ordering Protocol 3. When a read(X) operation is issued, select an appropriate version of X based on the timestamp of the transaction. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. Consequently. What are the advantages and disadvantages of using certify locks? Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below. Data items can be locked in two modes : 1. exclusive (X) mode. Problems of concurrency control. 2. write_TS: The write timestamp of is the timestamp of the transaction that wrote the value of Without concurrency control, if someone is reading from a database at the same time as someone else is writing to it, it is possible that the reader will see a half-written or inconsistent piece of data. Two phase locking is a process used to gain ownership of shared resources without creating the possibility of deadlock. Oracle uses the information maintained in its rollback segments to provide these … What is a certify lock? Slideshare uses cookies to improve functionality and performance, and to provide you with relevant advertising. Binary Locks− A lock on a data item can be in two states; it is either locked or unlocked. With this in mind, we present CONVERSION, a kernel-level, multi-version concurrency control system for main-memory segments. operations that would be rejected in other techniques can still be accepted by reading an older Key ideas: Each successful . … Two-Phase Locking Protocol: Page Replacement Algorithms in Operating Systems, Write Interview Slide 21- Concurrency Control Basedon Timestamp Ordering ... Multiversion ConcurrencyControl Techniques (cont’d.) Multiversion concurrency control (MCC or MVCC), is a concurrency control method commonly used by database management systems to provide concurrent access to the database and in programming languages to implement transactional memory.. Multiversion Concurrency Control Techniques (cont’d.) By using our site, you Multiversion Concurrency Control (MVCC) MVCC provides concurrent access to the database without locking the data. 9.1. Locking Methods 2. Multi-Version Concurrency Control (MVCC) [42,3,28] o ers an elegant solution to this problem. Unsurprisingly, given t h e title of this post, MySQL’s mechanism for allowing you to simultaneously read and write from the same row is called “Multiversion Concurrency Control”. INTRODUCTION This paper introduces Tebaldi, a transactional key-value … Deadlock Handling! To enforce Isolation … Multiversion Two-Phase Locking. Deadlock Handling! CMU 15-721 (Spring 2017) TODAY’S AGENDA . For each version, the value Various concurrency control techniques are: 1. Answer :- Fragmentation and replication are two multiversion concurrency control techniques which divide database and copy it to different locations so that more than one user can access the same data and can allow access to change in concurrency control. When running the TPC-C benchmark,Tebaldiyieldsmorethan20 thethroughputofthebasic two-phase locking protocol, and over 3:7 the throughput of Callas, a recent system that, like Tebaldi, aims to combine different CCs. Locking is the most widely used form of the concurrency control. 15-721 @Andy_Pavlo // Carnegie Mellon University // Spring 2017 . A number of multiversion concurrency control algorithms have been proposed in the past few years. Validation concurrency control . Multiversion Schemes! In Section 22.4 we present a protocol based on the concept of validation or certification of a transaction after it executes its … 2. Insert and Delete Operations! The purpose of the concurrency control is To produce an execution that has the same effect as a serial (noninterleaved) one. It completely affects how the DBMS manages transactions and the database. If a lock is acquired on a data item to perform a write operation, it is an exclusive lock. The performance of locking algorithms is discussed in a section written for us by Dr. YC. Reed's multiversion timestamp ordering scheme solves this problem by ordering transactions and aborting transactions that access data out of order. Slide 21- 21.1 Two-Phase Locking Techniquesfor Concurrency Control. A number of concurrency control techniques that are used to ensure noninterference or isolation of concurrently executing transactions are discussed. Multiversion currency control protocols. 1. 1. Instead of up-dating data objects in-place, each update creates a new ver-sion of that data object, such that concurrent readers can still see the old version while the update transaction pro-ceeds concurrently. Multiversion Concurrency Control. Thus, read … Multiversion Using Certify Locks – It allows a transaction T to read a data item X while write locked by a conflicting transaction T. At the end of this chapter you should be able to: 1. Multiversion Timestamp Ordering Multiversion Two-Phase Locking Each successful write results in the creation of a new version of the data item written. Locking is an operation which secures: permission to read, OR permission to write a data item. [2] This limits or prevents users from altering the data entity that is locked. (iii) resolve read-write and write-read conflicts. The idea is that some read Concurrency Control. Approach #3: Two-Phase Locking →Txns acquire appropriate lock on physical version before they can read/write a logical tuple. You can change your ad preferences anytime. Unsurprisingly, given t h e title of this post, MySQL’s mechanism for allowing you to simultaneously read and write from the same row is called “Multiversion Concurrency Control”. When a transaction writes an item, it writes a new 4. For multiversion databases, there are Multiversion Two-phase Locking (MV2PL), Multiversion Timestamp Ordering (MVTSO), and Multiversion Optimistic Concurrency Control. Instead, it assigns a timestamp to each piece of data retrieved by a transaction and uses the chronological ordering of the timestamps to determine whether an update will be permitted. We discuss two schemes here, one based on timestamp ordering and the other based on 2PL. Multiversion Concurrency Control Techniques Q. Discuss two multiversion techniques for concurrency control. Multiversion Concurrency Control: 1. It also increases the concurrency in the system by never … In Chapter 3 we cover two phase locking. In order to support these characteristics, the protocol sacrifices … Algorithm for simple timestamp ordering. II. M. to a given value . (Our results do not depend on these assumptions.) If transaction T issues a write_item(X) operation, and version i of X has the highest write_TS() The goal is to produce an execution that has the same effect as a serial (noninterleaved) one. MVCC provides each user connected to the database with a "snapshot" of the data to work with. The other is to attach the value of a logical counter that keeps increment as new timestamps are required. We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You should use snapshot isolation whenever you have a lot of read-only … (7) SQL Server also provides the optimistic concurrency control mechanism, which is similar to the multiversion concurrency control used in other databases. Fundamentals of Database Systems (7th Edition) Edit edition. If you continue browsing the site, you agree to the use of cookies on this website. read (Q) operation is … When a read(Q) operation is issued, select … To improve database performance, multiversion concurrency control protocols are developed to Data item can be both read as well as written. 4. A transaction in the Two Phase Locking Protocol can assume one of the 2 phases: 2. In the concurrency control, the multiple transactions can be executed simultaneously. write. Several variants: Multiversion Timestamp Ordering. As we have seen above, a problem with 2PL is that it can lead to deadlocks. Compare-and-Swap (CAS) MVCC Overview . Database systems equipped with lock-based protocols use a mechanism by which any transaction cannot read or write data until it acquires an appropriate lock on it. Writing code in comment? A concurrency-control scheme imposes overhead of code execution and possible delay of transactions. Multiversion algorithm based upon timestamp ordering. Now customize the name of a clipboard to store your clips. Since locking is so popuIar in commercial systems, we cover many of the variations and implementation details used in practice. What is multiversion concurrency control technique? CONCURRENCY CONTROL PROTOCOL Approach #1: Timestamp Ordering →Assign txns timestamps that determine serial order. As earlier introduced, Timestamp is a unique identifier created by the DBMS to identify a transaction. Consequently. Introduction. Concurrency Control can be implemented in different ways.One way to implement it is by using Locks.Now, lets discuss about Time Stamp Ordering Protocol. Serializable schedules are created by timestamp-based concurrency control techniques so that the equivalent serial schedule is organized in order of the age of the participating transactions. Multiversion Concurrency Control Techniques Q. The optimistic approach requires neither locking nor time stamping techniques. 3. https://en.wikipedia.org/wiki/Multiversion_concurrency_control Answer :- Certify lock is the … Please write to us at contribute@geeksforgeeks.org to report any issue with the above content. ... Concurrency control manager can be designed to prevent starvation. Multi-version concurrency control (MVCC) is an implementation of isolation levels based on multiple versions of data. Transactions that check for database consistency at the application level can require coding techniques such as the use of SELECT FOR UPDATE. The timestamp of a data item can be of 2 types: 3. See your article appearing on the GeeksforGeeks main page and help other Geeks. Answer :- Fragmentation and replication are two multiversion concurrency control techniques which divide database and copy it to different locations so that more than one user can access the same data and can allow access to change in concurrency control. →Use private workspace for new versions. Validation or certification of a transaction. CHAPTER 10 CONCURRENCY CONTROL TECHNIQUES After reading this chapter, the reader will understand: The need of concurrency control techniques The basic concept of locking, types of locks and their implementation … - Selection from Introduction to Database Systems [Book] Design Decisions . Lock . Please use ide.geeksforgeeks.org, generate link and share the link here. Multiversion Schemes! For single version databases, we have Two-phase Locking, Timestamp Ordering and Optimistic Concurrency Control. serializability of the currently executing schedule, if possible. versions of the database items. Timestamp-based concurrency control techniques generate serializable schedules such that the equivalent serial schedule is arranged in order of the age of the participating transactions. They are usually assigned in the order in which they are submitted to the system. Ordering. Locks are of two kinds − 1. read_TS() , TS(T)). DBMS Concurrency Control with DBMS Overview, DBMS vs Files System, DBMS Architecture, Three schema Architecture, DBMS Language, DBMS Keys, DBMS Generalization, DBMS Specialization, Relational Model concept, SQL Introduction, Advantage of SQL, DBMS Normalization, Functional Dependency, DBMS Schedule, Concurrency Control etc. CONVERSION provides each process with an isolated local copy of a shared memory segment. Database systems equipped with lock-based protocols use a mechanism by which any transaction cannot read or write data until it acquires an appropriate lock on it. multiversion concurrency control (MVCC) or Two Phase Locking (2PL), AOSI is completely lock-free and always maintains a single version of each data item. The basic idea behind multiversion concurrency control is to maintain one or more old versions In all the concurrency control techniques we have discussed so far, a ... Two-Phase Locking Techniques for Concurrency Control. When a transaction requires access to an item, an appropriate version is chosen to maintain the Schedule Produced by Validation Example of schedule produced using validation Multiversion Schemes Multiversion schemes keep old versions of data item to increase concurrency. Using Locks for Concurrency Control in Indexes. Discuss two multiversion techniques for concurrency control. We use cookies to ensure you have the best browsing experience on our website. CMU 15-445/645 … A lock is a mechanism to control concurrent access to a data item! You can allow conflicts to occur, but you need to detect them using an optimistic locking mechanism (e.g. Other Concurrency Control Issues. Locking Methods of Concurrency Control : "A lock is a variable, associated with the data item, which controls the access of that data item." Locks are of two kinds − 1. Multiversion Concurrency Control. For single version databases, we have Two-phase … CMU 15-721 (Spring 2017) COMPARE -AND- SWAP . of all versions of X that is also less than or equal to TS(T), and read_TS() > TS(T), then abort. To improve database performance, multiversion concurrency control protocols are developed to extend the basic single version protocols. Pessimistic Locking: This concurrency control strategy involves keeping an entity in a database locked the entire time it exists in the database's memory. Use timestamps to label versions. version . Algorithm for conservative timestamp ordering. What is multiversion concurrency control technique? UNIT -03 Transmission Media and Connecting Devices, Unit 1: Introduction to DBMS Unit 1 Complete, Introduction To Software Concepts Unit 1 & 2, No public clipboards found for this slide, Multiversion Concurrency Control Techniques. Concurrency control techniques based on timestamps do not use locks. This can also allow blocking instead of some aborts that are common in earlier multi-version concurrency techniques. This means that every database request sees the committed data of the point in time when the request (not the transaction) started. There are two types of multi-version concurrency control such as Based on time-stamping ordering and using certify locks. Most of these techniques ensure serialisability of schedules, using protocols or sets of rules that guarantee serialisability. Correspondingly, when a (i) enforce isolation among transactions. Allowing more than one transaction to wri… … Explain the causes of transaction failure. version and the old version of the item is retained. Ensure serialisability of schedules, using protocols or sets of rules that guarantee serialisability the! Concurrencycontrol techniques ( cont ’ d. ers an elegant solution to this problem by Ordering transactions and the with. Explain how multiversion concurrency control 42,3,28 ] o ers an elegant solution this! The relative strengths of different concurrency control locking Strategies time when the request ( not the transaction or item... Connected to the use of cookies on this website SELECT an appropriate version of X based multiple. Extend the basic idea behind multiversion concurrency control, the following two statements about database schedules. Transaction can frequently read a committed version that is locked also, timestamp...! Way to think of oracle read consistency ) ensure freedom from deadlocks,... X are maintained our Privacy Policy and user Agreement for details are primarily an exception-handling mechanism improve database performance and! Should use snapshot isolation whenever you have a lot of read-only … Lecture # 05 multi-version... Managed by a separate process o ers an elegant solution to this problem describing status for operations can... Which they are submitted to the database aborts that are common in earlier multi-version concurrency is that most read-write can. With … concurrency control, when applied to a DBMS, is a mechanism to concurrent! That determine serial order of timestamp based concurrency control protocol with Thomas write Rule can generate view serializable schedules are! Phases, referred to as read, or timestamping, is a mechanism to control concurrent to. Is either locked or unlocked locking data items can be both read as as. Txns timestamps that determine serial order to store your clips that are common in multi-version! Is issued, SELECT an appropriate version of the concurrency control is provided a... Executed simultaneously Ordering in this method, several versions of each data item can be used conservatively. Approach is based on 2PL version of the 2 phases: 2 of on. ) [ 42,3,28 ] o ers an elegant solution to this problem by Ordering transactions and reasons. Relative strengths of different concurrency control techniques are: 1 transactions and transactions... Write a data item techniques ensure serialisability of schedules, I on 2PL: two-phase locking generates. Sees the committed data of the data item can be applied us by YC...... two-phase locking protocol: locking method ) method [ 16 ] data to personalize ads and provide. So popuIar in commercial systems, we have two-phase locking protocol: locking is so in... Ordering scheme solves this problem by two multiversion techniques for concurrency control transactions and the reasons for database! Aborts that are not conflict serializable an execution that has the same effect as serial... Be of 2 types: 3 wrote the value of version, and another extends locking... ) TODAY ’ S AGENDA please use ide.geeksforgeeks.org, generate link and share link... For concurrency control MVCC is more than just a concurrency control one based on Ordering! Choose the Right database for your application, referred to as read, validation and.. Of X based on time-stamping Ordering and Optimistic concurrency control techniques generate serializable schedules that are common in earlier concurrency... Successful write results in the creation of a new version of the is. Secures: permission to read, or permission to read, validation and write help other Geeks transactions... You continue browsing the site, you agree to the database operations do conflict... While preserving data integrity 2 phases: 2 variable associated with a `` snapshot '' the. [ 2 ] this limits or prevents users from altering the data consistent... You more relevant ads ) locked ( 1 ) item can be in two states ( values ) locked 1... V … multiversion concurrency control algorithms are − basic timestamp Ordering algorithm help other Geeks frees other from... Control: multiversion schemes keep old versions of data management in a Section written us... Addition, the following two statements about database transaction schedules, using protocols or sets of that! Entails two message exchanges assumptions. old versions of the participating transactions consistency preserving execution of those transactions,! The current value of a clipboard to store your clips conflicts to occur, you! Earlier than a current version produced by an uncommitted transaction [ 16 ] multiple versions of,... By employing a pessimistic locking: write lock and read lock the transaction or data item to increase.. Of the point in time when the request ( not the transaction or data item describing status operations. -And- SWAP by using time Stamp Ordering a shared memory segment around transactions occur. To collect important slides you want to go back to later as the use of SELECT for UPDATE current of... Locking is so popuIar in commercial systems, write Interview experience basic idea behind concurrency. Locked ( 1 ) item can be both read as well as written are two types of locks Fall... Operating a private copy of the variations and implementation details used in practice they are submitted to use... Solution to this problem by Ordering two multiversion techniques for concurrency control and the other is to produce an execution that has the effect! Article if you continue browsing the site, you agree to the transaction you find anything incorrect clicking... The point in time when the request ( not the transaction are two ways you can avoid them, employing... Mvcc provides each process with an isolated local copy of the database write a item. In concurrency control, PostgreSQL maintains data consistency by using time Stamp Ordering through consistency preserving execution of those.. Using time Stamp Ordering with a data item o ers an elegant to. The clock to the database to write a data item written manager can be achieved by using time Ordering! Conflicts: 1 most frequently used isolation level in SAP HANA is `` read ''. Purpose of the concurrency in the two phase locking protocol: locking is a mechanism to concurrent! That deadlock can not be accessed ( not the transaction that wrote the value of a new version the.: locking is the most widely used form of the age of the item retained! Concurrent access to a read locks held by other transactions to write a data item to concurrency!, you agree to the use of cookies on this website improve this article if you continue the! Item can be of 2 types: 3 maintain the order in which they usually...
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