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18.7 THE TREE PROTOCOL

18.7 THE TREE PROTOCOL. MALLIKA PEREPA ID: 115. Outline-. Introduction Motivation Rules for Access to Tree-Structured Data Why the Tree Protocol Works. Introduction- In this section, we deal with tree structures that are formed by the link pattern .

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18.7 THE TREE PROTOCOL

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  1. 18.7 THE TREE PROTOCOL MALLIKA PEREPA ID: 115

  2. Outline- • Introduction • Motivation • Rules for Access to Tree-Structured Data • Why the Tree Protocol Works

  3. Introduction- • In this section, we deal with tree structures that are formed by the link pattern. • Database elements are disjoint pieces of data, but the only way to get to a node is through its parent. • B-trees are an important example of this sort of data. • Traversing a particular path to an element gives us freedom to manage locks differently from the two-phase locking approaches we have seen previously.

  4. B-tree Details- • Basic DS: - Keeps records in sorted order, allows searches, sequential access, insertions and deletions in logarithmic time. - It is used in databases and file systems. • Locking structure: - Granularity is at node level. Treating Smaller pieces as elements is not beneficial and entire B-tree as a single element is infeasible!

  5. Motivation for Tree based locking- • If we use a standard set of lock modes (shared, update and exclusive) and two phase locking, then concurrent use of the B-tree is almost impossible. • The reason is that every transaction must begin by locking the root node of the B-tree. • Any transaction that inserts or deletes could wind up rewriting the root of the B-tree. • Thus, only one transaction that is not read-only can access the B-tree at any time • However, in most situations a B-tree node will not be rewritten, even if the transaction inserts or deletes a tuple.

  6. Thus, as soon as a transaction moves to a child of the root and observes the situation that rules out a rewrite of the root, we would like to release the lock on the root. • Releasing the lock on the root early will violate two phase locking, so we cannot be sure that the schedule of several transactions accessing the B-tree will be serializable. • The solution is a specialized protocol for transactions that access tree structured data like B-trees.

  7. Rules for Access to Tree-Structured Data- • The following restrictions on locks form tree protocol. Assumptions: • There is only ONE kind of a lock, represented by the form li(X) • Transactions are consistent, and schedules must be legal but there is no two-phase locking requirement on transactions. Rules: • A transaction’s first lock may be at any node of the tree • Subsequent locks may only be acquired if the transaction currently has a lock on the parent node. • Nodes may be unlocked at any time. • A transaction may not relock a node on which it has released a lock, even if it still holds a lock on the node’s parent

  8. Example… A tree of lockable elements A B C D E F G

  9. 3 transactions following the protocol… T1 T2 T3 l1 (A);r1(A); l1 (B); r1(B); l1(C); l1 (C); w1(A);u1(A); l1(D); r1(D); w1 (B); u1(B); l2(B);r2(B); l3(E);r3(E); w1(D);u1(D); w1(c);u1(C); l2(E) denied l3(F);r3(F); w3(F);u3(F); l3(G);r3(G) w3(E);u3(E); l2(E);r2(E); w3(G);u3(G); w2 (B); u2(B); w1 (E); u1(E);

  10. Why Tree Protocol works- • The tree protocol implies a serial order on the transactions involved in a schedule. • The order of precedence can be defined as- Ti<S Tj ;If in a schedule S, the transactions Ti and Tj lock a node in common, and Ti locks the node first. • If precedence graph drawn from the precedence relations that we defined above has no cycles, then we claim that any topological order of transactions is an equivalent serial schedule. • For Example either ( T1,T2,T3) or (T3,T1,T2) is an equivalent serial schedule the reason for this serial order is that all the nodes are touched in the same order as they are originally scheduled.

  11. If two transactions lock several elements in common, then they are all locked in same order. Precedence graph derived from schedule: 1 2 3

  12. Example:--4 Path of elements locked by two transactions X T locks first P U locks first Z Y U locks first • Now Consider an arbitrary set of transactions T1, T2;.. . . Tn,, that obey the tree protocol and lock some of the nodes of a tree according to schedule S. • First among those that lock, the root. they do also in same order. • If Ti locks the root before Tj, Then Ti locks every node in common with Tj does. That is Ti<sTj, But not Tj>sTi.

  13. THANK YOU

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