{"id":117,"date":"2026-03-06T09:38:01","date_gmt":"2026-03-06T09:38:01","guid":{"rendered":"https:\/\/gigz.pk\/sql\/?post_type=lesson&#038;p=117"},"modified":"2026-03-16T18:55:53","modified_gmt":"2026-03-16T18:55:53","slug":"locks-deadlocks","status":"publish","type":"lesson","link":"https:\/\/gigz.pk\/sql\/lesson\/locks-deadlocks\/","title":{"rendered":"Locks &amp; Deadlocks"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">n database systems and multi-threaded applications, <strong>locks<\/strong> and <strong>deadlocks<\/strong> are critical concepts that ensure data integrity and prevent conflicts. Understanding these concepts helps in designing efficient and safe applications.<\/p>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">1. What is a Lock?<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">A <strong>lock<\/strong> is a mechanism that prevents multiple users or processes from accessing the same resource simultaneously in a way that could cause inconsistencies. Locks are used to maintain <strong>data consistency<\/strong> and <strong>isolation<\/strong>.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Types of Locks:<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Shared Lock (Read Lock):<\/strong> Multiple transactions can read a resource simultaneously but cannot modify it.<\/li>\n\n\n\n<li><strong>Exclusive Lock (Write Lock):<\/strong> Only one transaction can read or modify the resource at a time.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Locking Modes:<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Pessimistic Locking:<\/strong> Locks the resource before performing operations to prevent conflicts.<\/li>\n\n\n\n<li><strong>Optimistic Locking:<\/strong> Assumes conflicts are rare and checks for conflicts only before committing changes.<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">2. What is a Deadlock?<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">A <strong>deadlock<\/strong> occurs when two or more processes are waiting for each other to release resources, creating a cycle of dependency where none of the processes can proceed.<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Example Scenario:<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Process A holds Resource 1 and waits for Resource 2.<\/li>\n\n\n\n<li>Process B holds Resource 2 and waits for Resource 1.<br>Neither process can continue, resulting in a deadlock.<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">3. Causes of Deadlocks<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Circular wait: Processes form a circular chain where each process waits for a resource held by the next.<\/li>\n\n\n\n<li>Mutual exclusion: Some resources can only be used by one process at a time.<\/li>\n\n\n\n<li>Hold and wait: A process holding one resource waits for another resource.<\/li>\n\n\n\n<li>No preemption: Resources cannot be forcibly taken from a process.<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">4. Deadlock Detection and Prevention<\/h2>\n\n\n\n<h3 class=\"wp-block-heading\">Prevention Techniques:<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Resource Ordering:<\/strong> Assign a global order to resources and acquire them in that order.<\/li>\n\n\n\n<li><strong>Request All Resources at Once:<\/strong> Avoid partial acquisition of resources.<\/li>\n\n\n\n<li><strong>Timeouts:<\/strong> Abort or rollback a transaction if it takes too long to acquire a resource.<\/li>\n<\/ul>\n\n\n\n<h3 class=\"wp-block-heading\">Detection Techniques:<\/h3>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Use a <strong>wait-for graph<\/strong> to identify cycles in resource allocation.<\/li>\n\n\n\n<li>Periodically check for processes that have been waiting longer than a threshold.<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">5. Handling Deadlocks<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Transaction Rollback:<\/strong> Abort one of the deadlocked processes to break the cycle.<\/li>\n\n\n\n<li><strong>Deadlock Recovery:<\/strong> Identify the victim process based on priority, cost, or age and restart it.<\/li>\n\n\n\n<li><strong>Avoidance Algorithms:<\/strong> Apply algorithms like Banker&#8217;s Algorithm to ensure safe allocation of resources.<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<h2 class=\"wp-block-heading\">6. Best Practices for Locks &amp; Deadlocks<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li>Keep transactions short to reduce the time resources are held.<\/li>\n\n\n\n<li>Avoid nested locks whenever possible.<\/li>\n\n\n\n<li>Use consistent locking order across all processes.<\/li>\n\n\n\n<li>Monitor the system for long waits or repeated deadlocks.<\/li>\n<\/ul>\n\n\n\n<hr class=\"wp-block-separator has-alpha-channel-opacity\"\/>\n\n\n\n<p class=\"wp-block-paragraph\">.<\/p>\n\n\n<div class=\"yoast-breadcrumbs\"><span><span><a href=\"https:\/\/gigz.pk\/sql\/\">Home<\/a><\/span> \u00bb <span class=\"breadcrumb_last\" aria-current=\"page\">Learn Advanced SQL &#038; Database Engineering (SQL-301) > Transactions &#038; Concurrency > Locks &#038; Deadlocks<\/span><\/span><\/div>\n\n\n<div class=\"schema-faq wp-block-yoast-faq-block\"><div class=\"schema-faq-section\" id=\"faq-question-1773601690992\"><strong class=\"schema-faq-question\"><\/strong> <p class=\"schema-faq-answer\"><\/p> <\/div> <\/div>\n","protected":false},"menu_order":62,"template":"","class_list":["post-117","lesson","type-lesson","status-publish","hentry"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.6 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Locks &amp; 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