Access modes and Compatibility:

We say that two lock requests for the similar node by two different transactions are compatible if they can be granted concurrently. The mode of the demand determines its compatibility with requests made by other transactions. The three modes X, S, and I are unsuited with one another but distinct S requests may be granted together and distinct I requests may be granted together.

The compatibilities between modes derive from their semantics. Share mode permits reading but not modification of the corresponding resource by the requestor and by other transactions. The semantics of elite mode is that the grantee may perhaps read and modify the resource however no other transaction may read or modify the resource while the exclusive lock is set. The reason for dichotomizing share furthermore exclusive access is that several share requests can be granted concurrently (are compatible) whereas an exclusive request is not compatible with any other request. Intention mode was commenced to be incompatible with share and exclusive mode (to prevent share and exclusive locks). Nevertheless intention mode is compatible with itself since two transactions having intention access to a node will explicitly lock descendants of the node in X, S or I mode as well as thereby will either be compatible with one another or will be scheduled on the basis of their requests at the finer level. For illustration two transactions can simultaneously be granted the database and some area and some file in intention mode. In these circumstances their explicit locks on particular records in the file will resolve any conflicts among them.

The notion of intention mode is refined to intention share mode (IS) as well as intention exclusive (IX) for two reasons intention share mode only requests share or intention share locks at the lower nodes of the tree (that is never requests an exclusive lock below the intention share node.) Therefore IS mode is compatible with S mode. Since read only is a common outline of access it will be profitable to distinguish this for greater concurrency. Secondly if a transaction has an purpose share lock on a node it can convert this to a share lock at a later time but one cannot convert an intention exclusive lock to a share lock on a node Rather to get the combined rights of share node and intention exclusive mode one must obtain an X or SIX mode lock.

We distinguish one further refinement of modes namely share and intention exclusive mode (SIX). Suppose one transaction needs to read an entire sub-tree and to update particular nodes of that sub-tree. Using the modes provided therefore far it would have the options of- (a) requesting exclusive access to the root of the sub-tree as well as doing no further locking or (b) requesting intention exclusive access to the root of the sub-tree as well as explicitly locking the lover nodes in intention share or exclusive mode.

On the other hand (a) has low concurrency. If merely a small fraction of the read nodes are updated then alternative (b) has nigh locking overhead The correct access mode would be share access to the sub-tree thereby allowing the transaction to read all nodes of the sub-tree without further locking as well as intention exclusive access to the sub-tree thereby allowing the transaction co set exclusive locks on those nodes in the sub-tree that are to be updated as well as IX or SIX locks on the intervening nodes. SIX modes are introduced because this is a common case. It is compatible with IS mode as other transactions requesting IS mode will explicitly lock lower nodes in IS or S mode thereby avoiding any updates (IX or X mode) produced by the SIX mode transaction. However SIX modes aren’t compatible with IX, S, SIX or X mode requests.

The table below provides the compatibility of the request modes, where null mode (NL) represents the absence of a request.

 
Table: Compatibilities between access modes.

To summarize we distinguish six modes of access to a resource:

NL: Gives no access to a node that is represents the absence of a request of a resource.

IS: Gives intention share access to the requested node as well as allows the requestor to lock descendant nodes in S or IS mode It doesn’t implicit locking.

IX: Gives intention exclusive access to the requested node as well as allows the requestor to explicitly lock descendants in X, S, SIX, IX or IS mode It doesn’t implicit locking.

S: Gives shared access to the requested node as well as to all descendants of the requested node without setting further locks. It implicitly sets S locks on every descendants of the requested node.

SIX: Gives share as well as intention exclusive access to the requested node. Particularly it implicitly locks all descendants of the node in share mode and allows the requestor to explicitly lock descendant nodes in X, SIX or IX mode.)

X: Gives exclusive access to the requested node as well as to all descendants of the requested node without setting further locks. (It absolutely sets X Locks on all descendants. Locking lover nodes in S or else IS mode would give no increased access.)

IS mode is the feeblest non-null form of access to a resource. It carries zeal privileges than IX or S modes. IX mode permits IS, IX, S, SIX and X mode locks to be set on descendant nodes while S mode permits read-only access to all descendants of the node without further locking. SIX mode carries the privileges of S as well as of IX mode (hence the name SIX). X mode is the most privileged form of access as well as allows reading and writing of all descendants of a node without further locking. Therefore the modes can be ranked in the partial order of privileges shown the figure below. Note that it isn’t a total order since IX and S are incomparable.

Figure: The biased ordering of modes by their privileges.

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