📓 What Is a Log?
A log is a sequence of records stored on a stable storage (disk) that keeps track of:
Transaction start
Data items modified
Old and new values
Transaction commit or abort
Log-Based Recovery Methods are essential techniques in DBMS for maintaining Atomicity and Durability—two key ACID properties of transactions. These methods use a log file to track all the changes made by transactions, ensuring that the database can recover from crashes or failures.
🧾 Format of Log Records:
🔄 Two Main Log-Based Recovery Methods:
1. Deferred Update (Deferred Write)
✅ Updates are made to the database only after the transaction commits.
❌ If the transaction fails or the system crashes before commit, no change is made to the DB.
🔧 Steps:
On
START, log the event:<START T1>On update, log only:
<T1, X, old, new>On
COMMIT, apply the updates to DB and write<COMMIT T1>
🔁 Recovery:
Redo transactions that have <COMMIT>
Ignore transactions without <COMMIT>
✅ Pros:
Simple recovery — no need to undo
Ideal for systems that commit infrequently
❌ Cons:
Delayed performance due to postponed writes
2. Immediate Update
✅ Updates are made to the database before the transaction commits.
❌ So if a crash occurs, we may need to UNDO the changes.
🔧 Steps:
On
START, log:<START T1>On update, log:
<T1, X, old, new>and apply change to DBOn
COMMIT, log:<COMMIT T1>
🔁 Recovery:
UNDO uncommitted transactions (using old values)
REDO committed transactions (using new values)
✅ Pros:
Faster writes
Suitable for real-time updates
❌ Cons:
Complex recovery — need both UNDO and REDO operations
<!DOCTYPE html>
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<title>Deferred vs Immediate Update — Try It Yourself</title>
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<body>
<div class="infographic-recoverymethods" id="recoverymethods">
<div class="rm-head">
<h2>Log-Based Recovery: Deferred vs Immediate Update</h2>
<p>Same transaction — update X from 10 to 20 — handled two different ways. Try crashing before commit in each and see what recovery actually has to do.</p>
</div>
<div class="rm-tabs">
<button class="rm-tab-btn is-active" data-tab="deferred" type="button">🕓 Deferred Update</button>
<button class="rm-tab-btn" data-tab="immediate" type="button">⚡ Immediate Update</button>
</div>
<!-- TAB 1: Deferred Update -->
<div class="rm-panel is-active" id="panel-deferred">
<p class="rm-panel-title">Deferred Update</p>
<p class="rm-panel-desc">Updates are written to the log immediately, but only applied to the actual database after COMMIT.</p>
<div class="rm-chip-row">
<span class="rm-chip is-good">✅ Simple recovery — no UNDO needed</span>
<span class="rm-chip is-bad">❌ Writes are delayed</span>
</div>
<div class="rm-state-row">
<div class="rm-state-box"><div class="rm-state-label">Database value of X</div><div class="rm-state-value" id="defDbValue">10</div></div>
</div>
<div class="rm-log-scroll">
<div class="rm-log" id="defLog"><span class="rm-log-placeholder">Click START to begin the transaction...</span></div>
</div>
<div class="rm-run-row">
<button class="rm-run-btn" id="defStartBtn" type="button">▶ START T1</button>
<button class="rm-run-btn" id="defUpdateBtn" type="button" disabled>▶ Update X: 10 → 20</button>
<button class="rm-run-btn is-good" id="defCommitBtn" type="button" disabled>✔️ COMMIT T1</button>
<button class="rm-run-btn is-danger" id="defCrashBtn" type="button" disabled>💥 Crash now</button>
<button class="rm-run-btn is-secondary" id="defReset" type="button">↺ Reset</button>
</div>
<p class="rm-msg" id="defMsg"></p>
</div>
<!-- TAB 2: Immediate Update -->
<div class="rm-panel" id="panel-immediate">
<p class="rm-panel-title">Immediate Update</p>
<p class="rm-panel-desc">Updates are applied to the actual database as soon as they happen — before the transaction has committed.</p>
<div class="rm-chip-row">
<span class="rm-chip is-good">✅ Faster writes</span>
<span class="rm-chip is-bad">❌ Recovery needs UNDO and REDO</span>
</div>
<div class="rm-state-row">
<div class="rm-state-box"><div class="rm-state-label">Database value of X</div><div class="rm-state-value" id="immDbValue">10</div></div>
</div>
<div class="rm-log-scroll">
<div class="rm-log" id="immLog"><span class="rm-log-placeholder">Click START to begin the transaction...</span></div>
</div>
<div class="rm-run-row">
<button class="rm-run-btn" id="immStartBtn" type="button">▶ START T1</button>
<button class="rm-run-btn" id="immUpdateBtn" type="button" disabled>▶ Update X: 10 → 20 (apply now)</button>
<button class="rm-run-btn is-good" id="immCommitBtn" type="button" disabled>✔️ COMMIT T1</button>
<button class="rm-run-btn is-danger" id="immCrashBtn" type="button" disabled>💥 Crash now</button>
<button class="rm-run-btn is-secondary" id="immReset" type="button">↺ Reset</button>
</div>
<p class="rm-msg" id="immMsg"></p>
</div>
</div>
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function showMsg(id, text, kind) {
const el = document.getElementById(id);
el.textContent = text;
el.className = 'rm-msg is-visible st-' + kind;
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/* TAB 1: Deferred Update */
(function () {
const log = document.getElementById('defLog');
const dbValueEl = document.getElementById('defDbValue');
const startBtn = document.getElementById('defStartBtn');
const updateBtn = document.getElementById('defUpdateBtn');
const commitBtn = document.getElementById('defCommitBtn');
const crashBtn = document.getElementById('defCrashBtn');
startBtn.addEventListener('click', () => {
startBtn.disabled = true;
appendLog(log, '<START T1>');
updateBtn.disabled = false;
crashBtn.disabled = false;
});
updateBtn.addEventListener('click', () => {
updateBtn.disabled = true;
appendLog(log, '<T1, X, 10, 20> <span style="color:#9a968c;">— logged only, DB untouched</span>');
commitBtn.disabled = false;
showMsg('defMsg', 'The change exists only in the log right now — X in the actual database is still 10.', 'warn');
});
commitBtn.addEventListener('click', () => {
commitBtn.disabled = true;
crashBtn.disabled = true;
appendLog(log, '<span class="rm-log-commit"><COMMIT T1></span>');
dbValueEl.textContent = '20';
dbValueEl.classList.add('is-changing');
setTimeout(() => dbValueEl.classList.remove('is-changing'), 600);
showMsg('defMsg', '✔️ Only now, at COMMIT, does the database actually get updated to 20.', 'good');
});
crashBtn.addEventListener('click', () => {
crashBtn.disabled = true;
updateBtn.disabled = true;
commitBtn.disabled = true;
appendLog(log, '<span class="rm-log-crash">💥 CRASH</span>');
showMsg('defMsg', '🔁 Recovery scans the log: no <COMMIT T1> found, so T1 is ignored entirely. Since the database was never touched, there is nothing to undo — X is still safely 10.', 'bad');
});
document.getElementById('defReset').addEventListener('click', () => {
log.innerHTML = '<span class="rm-log-placeholder">Click START to begin the transaction...</span>';
dbValueEl.textContent = '10';
startBtn.disabled = false;
updateBtn.disabled = true;
commitBtn.disabled = true;
crashBtn.disabled = true;
document.getElementById('defMsg').classList.remove('is-visible');
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/* TAB 2: Immediate Update */
(function () {
const log = document.getElementById('immLog');
const dbValueEl = document.getElementById('immDbValue');
const startBtn = document.getElementById('immStartBtn');
const updateBtn = document.getElementById('immUpdateBtn');
const commitBtn = document.getElementById('immCommitBtn');
const crashBtn = document.getElementById('immCrashBtn');
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startBtn.disabled = true;
appendLog(log, '<START T1>');
updateBtn.disabled = false;
crashBtn.disabled = false;
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dbValueEl.textContent = '20';
dbValueEl.classList.add('is-changing');
setTimeout(() => dbValueEl.classList.remove('is-changing'), 600);
commitBtn.disabled = false;
showMsg('immMsg', '⚠️ The database is already showing 20 — but T1 hasn\u2019t committed yet. If something goes wrong now, this change has to be undone.', 'warn');
});
commitBtn.addEventListener('click', () => {
commitBtn.disabled = true;
crashBtn.disabled = true;
appendLog(log, '<span class="rm-log-commit"><COMMIT T1></span>');
showMsg('immMsg', '✔️ Now that T1 has committed, 20 is permanent. A crash from here on would just REDO using the logged new value — no undo needed.', 'good');
});
crashBtn.addEventListener('click', () => {
crashBtn.disabled = true;
updateBtn.disabled = true;
commitBtn.disabled = true;
appendLog(log, '<span class="rm-log-crash">💥 CRASH</span>');
const wasUpdated = dbValueEl.textContent === '20';
if (wasUpdated) {
showMsg('immMsg', '🔁 Recovery scans the log: no <COMMIT T1> found, but the DB was already changed to 20. It must UNDO using the old value (10) from the log entry.', 'bad');
setTimeout(() => {
dbValueEl.textContent = '10';
dbValueEl.classList.add('is-changing');
setTimeout(() => dbValueEl.classList.remove('is-changing'), 600);
}, 900);
} else {
showMsg('immMsg', '🔁 Recovery scans the log: no <COMMIT T1> found, and the update hadn\u2019t even been applied yet — nothing to undo.', 'bad');
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});
document.getElementById('immReset').addEventListener('click', () => {
log.innerHTML = '<span class="rm-log-placeholder">Click START to begin the transaction...</span>';
dbValueEl.textContent = '10';
startBtn.disabled = false;
updateBtn.disabled = true;
commitBtn.disabled = true;
crashBtn.disabled = true;
document.getElementById('immMsg').classList.remove('is-visible');
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</body>
</html>.demo {
font-family: system-ui, sans-serif;
padding: 16px;
}
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margin: 0 0 8px;
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.demo p {
color: #4b5563;
margin: 0;
}document.querySelector('.demo h1')?.addEventListener('click', () => {
alert('Hello from live preview!');
});🧠 Summary Table:
Feature | Deferred Update | Immediate Update |
|---|---|---|
When DB is updated | After COMMIT | Before COMMIT |
Log Required | REDO only | UNDO + REDO |
Recovery Complexity | Simple | Complex |
Speed | Slower | Faster |
Failure Before Commit | No effect (nothing written) | Must UNDO |
🧰 Types of Recovery Operations
Operation | Purpose |
|---|---|
UNDO | Reverse changes made by uncommitted transactions |
REDO | Reapply changes made by committed transactions |
💡 Bonus Tip: Write-Ahead Logging (WAL)
Before applying any changes to the database, the log must be written to disk first. This ensures that:
If a failure occurs, the log is there to recover from.
Maintains data integrity.