Database Locks

Locking is a mechanism to control concurrent access to data in order to maintain consistency. Lock requests are made to concurrency-control manager. Transaction can proceed only after the request for lock is approved. Data can be locked in eXclusive mode or Shared mode. When a transaction sets an exclusive mode, it can both read and alter it. Whereas a shared mode will only let the transaction to read the data.

A transaction may be granted a lock on an item if the requested lock is compatible (see the compatibility matrix) with locks already held on the item by other transactions. For example if a request for an exclusive lock is made on data that is already locked, the request will not be approved until the existing lock (shared or exclusive) is removed.

SX
STRUEFALSE
XFALSEFALSE

Table: Compatibility matrix

The concurrency control manager records all the locks granted in a system table called the Lock table.

Two transactions T1 and T2 enter a deadlock state when a transaction T1 that has locked data X waits for access to some other data Y which is locked by the transaction T2, which is waiting for access to X--a catch-22 situation! A deadlock can be removed by rolling back one of the transactions. The rollback can be partial or complete. The idea is to remove the locks held by one transaction to let the other transaction complete. If a transaction reaches starvation state if it is being rolled back too many times.

Database locks in Oracle can be of the following granularity:
  1. Field
  2. Row
  3. Table
  4. Tablespace
  5. Database
If a particular lock is acquired at a granularity then the same lock is applied for all the lower granularity level. Example: If a lock is set to a table, it applied to rows and fields in the table.

Comments

Post a Comment

Popular posts from this blog

OS/DB Migration - CMD. STR, TOC, EXT, R3load, DDLDBS.TPL and more

Image
With increasing need to improve performance or reduce costs or vanishing support for operating systems/databases it becomes inevitable to migrate to a better OS or DB. We feel take a quick look at the primary ingredients of the tools for OS/DB migration handled by R3SETUP. Tools - R3load, R3ldctl and R3szchk R3ldctl creates STRucture files, TemPLate files by reading the technical settings for table/indexes in SAP data dictionary. R3szchk calculates the database size object sizes (EXTent files) for the target system based on the STR files created by R3ldctl. The EXT and STR files can be later split into smaller groups. The most crucial tool R3load is responsible for unloading and loading data from the database. R3load uses the files created by R3ldctl and R3szchk and dumps the data into compressed binary format as .001, .002 .. files. It uses cmd files to achieve the task. If the cmd files are not present, it creates on its own. Files Command files*.cmd  are creat...
Read more

Fixing Inconsistent Table - Table activation fails due to inconsistency between DD and DB

If you find a table to be available on the database, but not active in ABAP dictionary, try calling SE14 as DDIC user, enter the table name and choose Table > Reconstruct, Execute. If this method fails follow these steps: Call transaction code SE37 and execute the function module  DD_TABL_ACTM with single test On the next screen, do not change any other field except the following and set them as shown below: MODE: 12 NTAB_PUTSTATE: N TABNAME: <enter the correct table name> Click on Execute The export parameter ACT_RESULT should show 0. To verify if the inconsistency is fixed, call transaction SE14 > Extras > Database Object > Check and  SE14 > Extras > Runtime object > Check.
Read more

301 Redirect Using SAP Web Dispatcher

301 redirects are typically used to redirect accesses that include www to non-www or vice-versa. Example, when you call http://blogger.com, the call is redirected to www.blogger.com. An end-user may prefer to call a URL in any manner. You must ensure that all these calls are redirected to relevant URLs with content. SAP web dispatcher can be configured to send a redirect (301) to the client. To enable redirection add icm/HTTP/redirect_<xx> to the web dispatcher's profile file. The parameter uses the following syntax: icm/HTTP/redirect_<xx> = PREFIX=<URL prefix>[, FROM=<pattern for URL>, FROMPROT=<incoming protocol>, FOR=<pattern for host name:port>,TO=<new URL prefix>, PROT=<protocol>, HOST=<host>, PORT=<port number/name>] Let's say you want to redirect all the HTTP requests on mysource.com on 80 port to mytarget.com on HTTPS 443 port icm/HTTP/redirect_0 = PREFIX=/, FROM=*, FROMPROT=http, FOR=mysource.com:80,...
Read more