Tag: Multi-Version Concurrency Control (MVCC)

Documentation tagged with Multi-Version Concurrency Control (MVCC) in the Geode graph database. MVCC is a fundamental concurrency control mechanism that enables high-performance concurrent access to data while maintaining ACID transaction guarantees. <h3 id="introduction-to-mvcc" class="position-relative d-flex align-items-center group"> Introduction to MVCC <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="introduction-to-mvcc" aria-haspopup="dialog" aria-label="Share link: Introduction to MVCC"> Share link </button> </h3><div id="headingShareModal" class="heading-share-modal" role="dialog" aria-modal="true" aria-labelledby="headingShareTitle" hidden> <div class="hsm-dialog" role="document"> <div class="hsm-header"> <h2 id="headingShareTitle" class="h6 mb-0 fw-bold">Share this section</h2> <button type="button" class="hsm-close" aria-label="Close"> </button> </div> <div class="hsm-body"> <label for="headingShareInput" class="form-label small text-muted mb-1 text-uppercase fw-bold" style="font-size: 0.7rem; letter-spacing: 0.5px;">Permalink</label> <div class="input-group mb-4 hsm-url-group"> <input id="headingShareInput" type="text" class="form-control font-monospace" readonly aria-readonly="true" style="font-size: 0.85rem;" /> <button class="btn btn-primary hsm-copy" type="button" aria-label="Copy" title="Copy"> </button> </div> <div class="small fw-bold mb-2 text-muted text-uppercase" style="font-size: 0.7rem; letter-spacing: 0.5px;">Share via</div> <div class="hsm-share-grid"> <a id="share-twitter" class="btn btn-outline-secondary w-100" target="_blank" rel="noopener noreferrer"> Twitter </a> <a id="share-linkedin" class="btn btn-outline-secondary w-100" target="_blank" rel="noopener noreferrer"> LinkedIn </a> <a id="share-facebook" class="btn btn-outline-secondary w-100" target="_blank" rel="noopener noreferrer"> Facebook </a> </div> </div> </div> </div> <style> .heading-share-modal { position: fixed; inset: 0; display: flex; justify-content: center; align-items: center; background: rgba(0, 0, 0, 0.6); z-index: 1050; padding: 1rem; backdrop-filter: blur(4px); -webkit-backdrop-filter: blur(4px); } .heading-share-modal[hidden] { display: none !important; } .hsm-dialog { max-width: 420px; width: 100%; background: var(--bs-body-bg, #fff); color: var(--bs-body-color, #212529); border: 1px solid var(--bs-border-color, rgba(0,0,0,0.1)); border-radius: 1rem; box-shadow: 0 25px 50px -12px rgba(0, 0, 0, 0.25); overflow: hidden; animation: hsm-fade-in 0.2s ease-out; } @keyframes hsm-fade-in { from { opacity: 0; transform: scale(0.95); } to { opacity: 1; transform: scale(1); } } [data-bs-theme="dark"] .hsm-dialog { background: #1e293b; border-color: rgba(255,255,255,0.1); color: #f8f9fa; } .hsm-header { display: flex; justify-content: space-between; align-items: center; padding: 1rem 1.5rem; border-bottom: 1px solid var(--bs-border-color, rgba(0,0,0,0.1)); background: rgba(0,0,0,0.02); } [data-bs-theme="dark"] .hsm-header { background: rgba(255,255,255,0.02); border-color: rgba(255,255,255,0.1); } .hsm-close { background: transparent; border: none; color: inherit; opacity: 0.5; padding: 0.25rem 0.5rem; border-radius: 0.25rem; font-size: 1.2rem; line-height: 1; transition: opacity 0.2s; } .hsm-close:hover { opacity: 1; } .hsm-body { padding: 1.5rem; } .hsm-url-group { display: flex !important; align-items: stretch; } .hsm-url-group .form-control { flex: 1; min-width: 0; margin: 0; background: var(--bs-secondary-bg, #f8f9fa); border-color: var(--bs-border-color, #dee2e6); border-top-right-radius: 0; border-bottom-right-radius: 0; height: 42px; } .hsm-url-group .btn { flex: 0 0 auto; margin: 0; margin-left: -1px; border-top-left-radius: 0; border-bottom-left-radius: 0; height: 42px; display: flex; align-items: center; justify-content: center; padding: 0 1.25rem; z-index: 2; } [data-bs-theme="dark"] .hsm-url-group .form-control { background: #0f172a; border-color: #334155; color: #e2e8f0; } .hsm-share-grid { display: flex; flex-direction: column; gap: 0.5rem; } .hsm-share-grid .btn { display: flex; align-items: center; justify-content: center; font-size: 0.9rem; padding: 0.6rem; border-color: var(--bs-border-color); width: 100%; } [data-bs-theme="dark"] .hsm-share-grid .btn { color: #e2e8f0; border-color: #475569; } [data-bs-theme="dark"] .hsm-share-grid .btn:hover { background: #334155; border-color: #cbd5e1; } </style> <script> (function(){ const modal = document.getElementById('headingShareModal'); if(!modal) return; const input = modal.querySelector('#headingShareInput'); const copyBtn = modal.querySelector('.hsm-copy'); const twitter = modal.querySelector('#share-twitter'); const linkedin = modal.querySelector('#share-linkedin'); const facebook = modal.querySelector('#share-facebook'); const closeBtn = modal.querySelector('.hsm-close'); let lastFocus=null; let trapBound=false; function buildUrl(id){ return window.location.origin + window.location.pathname + '#' + id; } function isOpen(){ return !modal.hasAttribute('hidden'); } function hydrate(id){ const url=buildUrl(id); input.value=url; const enc=encodeURIComponent(url); const text=encodeURIComponent(document.title); if(twitter) twitter.href=`https://twitter.com/intent/tweet?url=${enc}&text=${text}`; if(linkedin) linkedin.href=`https://www.linkedin.com/sharing/share-offsite/?url=${enc}`; if(facebook) facebook.href=`https://www.facebook.com/sharer/sharer.php?u=${enc}`; } function openModal(id){ lastFocus=document.activeElement; hydrate(id); if(!isOpen()){ modal.removeAttribute('hidden'); } requestAnimationFrame(()=>{ input.focus(); }); trapFocus(); } function closeModal(){ if(!isOpen()) return; modal.setAttribute('hidden',''); if(lastFocus && typeof lastFocus.focus==='function') lastFocus.focus(); } function copyCurrent(){ try{ navigator.clipboard.writeText(input.value).then(()=>feedback(true),()=>fallback()); } catch(e){ fallback(); } } function fallback(){ input.select(); try{ document.execCommand('copy'); feedback(true);}catch(e){ feedback(false);} } function feedback(ok){ if(!copyBtn) return; const icon=copyBtn.querySelector('i'); if(!icon) return; const prev=copyBtn.getAttribute('data-prev')||icon.className; if(!copyBtn.getAttribute('data-prev')) copyBtn.setAttribute('data-prev',prev); icon.className= ok ? 'fa-duotone fa-clipboard-check':'fa-duotone fa-circle-exclamation'; setTimeout(()=>{ icon.className=prev; },1800); } function handleShareClick(e){ e.preventDefault(); const btn=e.currentTarget; const id=btn.getAttribute('data-share-target'); if(id) openModal(id); } function bindShareButtons(){ document.querySelectorAll('.h-share').forEach(btn=>{ if(!btn.dataset.hShareBound){ btn.addEventListener('click', handleShareClick); btn.dataset.hShareBound='1'; } }); } bindShareButtons(); if(document.readyState==='loading'){ document.addEventListener('DOMContentLoaded', bindShareButtons); } else { requestAnimationFrame(bindShareButtons); } document.addEventListener('click', function(e){ const shareBtn=e.target.closest && e.target.closest('.h-share'); if(shareBtn && !shareBtn.dataset.hShareBound){ handleShareClick.call(shareBtn, e); } }, true); document.addEventListener('click', e=>{ if(e.target===modal) closeModal(); if(e.target.closest && e.target.closest('.hsm-close')){ e.preventDefault(); closeModal(); } if(copyBtn && (e.target===copyBtn || (e.target.closest && e.target.closest('.hsm-copy')))) { e.preventDefault(); copyCurrent(); } }); document.addEventListener('keydown', e=>{ if(e.key==='Escape' && isOpen()) closeModal(); }); function trapFocus(){ if(trapBound) return; trapBound=true; modal.addEventListener('keydown', f=>{ if(f.key==='Tab' && isOpen()){ const focusable=[...modal.querySelectorAll('a[href],button,input,textarea,select,[tabindex]:not([tabindex="-1"])')].filter(el=>!el.hasAttribute('disabled')); if(!focusable.length) return; const first=focusable[0]; const last=focusable[focusable.length-1]; if(f.shiftKey && document.activeElement===first){ f.preventDefault(); last.focus(); } else if(!f.shiftKey && document.activeElement===last){ f.preventDefault(); first.focus(); } } }); } if(closeBtn) closeBtn.addEventListener('click', e=>{ e.preventDefault(); closeModal(); }); })(); </script>Multi-Version Concurrency Control (MVCC) is a sophisticated concurrency control method that allows multiple transactions to access the same data simultaneously without blocking each other. Unlike traditional locking mechanisms that force readers and writers to wait for each other, MVCC creates multiple versions of data items, allowing readers to access consistent snapshots while writers create new versions. MVCC has become the de facto standard for modern database systems because it solves the fundamental trade-off between consistency and concurrency. Traditional two-phase locking provides strong consistency but suffers from contention—readers block writers and writers block readers. MVCC breaks this deadlock by maintaining multiple timestamped versions of each data item. In Geode’s implementation, MVCC enables true snapshot isolation and serializable snapshot isolation (SSI), ensuring that transactions see consistent views of the database while allowing maximum concurrency. This is critical for graph databases where complex traversal queries might read thousands of nodes and relationships—blocking those reads would cripple performance. <h3 id="core-mvcc-concepts" class="position-relative d-flex align-items-center group"> Core MVCC Concepts <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="core-mvcc-concepts" aria-haspopup="dialog" aria-label="Share link: Core MVCC Concepts"> Share link </button> </h3> <h4 id="version-chains" class="position-relative d-flex align-items-center group"> Version Chains <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="version-chains" aria-haspopup="dialog" aria-label="Share link: Version Chains"> Share link </button> </h4>Every data item (node, relationship, or property) in Geode exists as a version chain—a linked list of versions ordered by transaction timestamp. Each version contains: <ul> <li>Transaction ID (TxID): The transaction that created this version</li> <li>Begin timestamp: When this version became visible</li> <li>End timestamp: When this version was superseded (NULL if current)</li> <li>Data payload: The actual node/relationship/property value</li> <li>Visibility metadata: Information for determining version visibility</li> </ul> When a transaction reads data, MVCC walks the version chain to find the appropriate version based on the transaction’s snapshot timestamp. <h4 id="snapshot-isolation" class="position-relative d-flex align-items-center group"> Snapshot Isolation <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="snapshot-isolation" aria-haspopup="dialog" aria-label="Share link: Snapshot Isolation"> Share link </button> </h4>Each transaction in Geode operates on a consistent snapshot of the database taken at transaction start time. This snapshot represents a point-in-time view where: <ul> <li>All committed transactions up to the snapshot time are visible</li> <li>All uncommitted or future transactions are invisible</li> <li>The data remains consistent throughout the transaction</li> </ul> Snapshot isolation eliminates many concurrency anomalies: <ul> <li>Dirty reads: Can’t happen—uncommitted data is invisible</li> <li>Non-repeatable reads: Can’t happen—the snapshot is immutable</li> <li>Phantom reads: Can’t happen—new rows don’t appear in snapshots</li> </ul> <h4 id="visibility-rules" class="position-relative d-flex align-items-center group"> Visibility Rules <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="visibility-rules" aria-haspopup="dialog" aria-label="Share link: Visibility Rules"> Share link </button> </h4>MVCC determines version visibility using sophisticated rules: <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-fallback" data-lang="fallback">A version V is visible to transaction T if: 1. V.begin_timestamp <= T.snapshot_timestamp 2. V.end_timestamp > T.snapshot_timestamp OR V.end_timestamp IS NULL 3. V.creating_transaction is committed at T.snapshot_timestamp 4. V.deleting_transaction is not committed at T.snapshot_timestamp </code></pre></div>These rules ensure transactions only see committed data that was valid at their snapshot time. <h4 id="write-write-conflicts" class="position-relative d-flex align-items-center group"> Write-Write Conflicts <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="write-write-conflicts" aria-haspopup="dialog" aria-label="Share link: Write-Write Conflicts"> Share link </button> </h4>While MVCC allows readers and writers to proceed concurrently, write-write conflicts still require coordination. When two transactions attempt to modify the same data: <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-gql" data-lang="gql">-- Transaction 1 BEGIN; MATCH (p:Person {id: 123}) SET p.balance = p.balance + 100; -- Transaction 2 attempts same update COMMIT; -- One transaction succeeds, other aborts with conflict error </code></pre></div>Geode detects write-write conflicts and aborts one transaction, forcing retry. This preserves serializability while maximizing concurrency. <h3 id="how-mvcc-works-in-geode" class="position-relative d-flex align-items-center group"> How MVCC Works in Geode <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="how-mvcc-works-in-geode" aria-haspopup="dialog" aria-label="Share link: How MVCC Works in Geode"> Share link </button> </h3> <h4 id="transaction-lifecycle" class="position-relative d-flex align-items-center group"> Transaction Lifecycle <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="transaction-lifecycle" aria-haspopup="dialog" aria-label="Share link: Transaction Lifecycle"> Share link </button> </h4><ol> <li>BEGIN: Allocate transaction ID, capture snapshot timestamp</li> <li>READ: Walk version chains to find visible versions</li> <li>WRITE: Create new versions with current transaction ID</li> <li>COMMIT: Mark all created versions as committed, update timestamps</li> <li>ROLLBACK: Mark all created versions as aborted</li> </ol> <h4 id="version-storage" class="position-relative d-flex align-items-center group"> Version Storage <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="version-storage" aria-haspopup="dialog" aria-label="Share link: Version Storage"> Share link </button> </h4>Geode stores versions using a highly optimized layout: <ul> <li>In-Memory Version Table: Hot versions kept in memory for fast access</li> <li>Versioned B-Tree: Persistent storage with efficient range scans</li> <li>Version Compression: Delta encoding for similar versions</li> <li>Garbage Collection: Background cleanup of obsolete versions</li> </ul> <h4 id="garbage-collection" class="position-relative d-flex align-items-center group"> Garbage Collection <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="garbage-collection" aria-haspopup="dialog" aria-label="Share link: Garbage Collection"> Share link </button> </h4>Over time, old versions accumulate. Geode’s MVCC garbage collector reclaims space: <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-fallback" data-lang="fallback">A version V can be garbage collected if: 1. V is not the current version 2. No active transaction has a snapshot_timestamp that could see V 3. V has been superseded for longer than the retention period </code></pre></div>The GC runs continuously in the background, using a generational approach similar to modern memory managers. <h4 id="integration-with-wal" class="position-relative d-flex align-items-center group"> Integration with WAL <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="integration-with-wal" aria-haspopup="dialog" aria-label="Share link: Integration with WAL"> Share link </button> </h4>MVCC works hand-in-hand with Write-Ahead Logging (WAL): <ul> <li>Before Image: WAL records contain old version data</li> <li>After Image: WAL records contain new version data</li> <li>Recovery: Replay WAL to reconstruct version chains</li> <li>Point-in-Time Recovery: Use version timestamps to recover to specific moments</li> </ul> <h3 id="use-cases-and-benefits" class="position-relative d-flex align-items-center group"> Use Cases and Benefits <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="use-cases-and-benefits" aria-haspopup="dialog" aria-label="Share link: Use Cases and Benefits"> Share link </button> </h3> <h4 id="high-concurrency-read-workloads" class="position-relative d-flex align-items-center group"> High-Concurrency Read Workloads <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="high-concurrency-read-workloads" aria-haspopup="dialog" aria-label="Share link: High-Concurrency Read Workloads"> Share link </button> </h4>MVCC shines in read-heavy workloads: <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-gql" data-lang="gql">-- Thousands of concurrent read transactions MATCH (p:Person)-[:KNOWS*1..3]->(friend) WHERE p.id = $userId RETURN friend.name, friend.interests </code></pre></div>These queries never block each other or block writes, enabling linear scaling of read throughput. <h4 id="long-running-analytics" class="position-relative d-flex align-items-center group"> Long-Running Analytics <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="long-running-analytics" aria-haspopup="dialog" aria-label="Share link: Long-Running Analytics"> Share link </button> </h4>Complex graph analytics can run without locking: <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-gql" data-lang="gql">-- Multi-minute PageRank computation MATCH (n:Page) RETURN n.id, pageRank(n) AS rank ORDER BY rank DESC LIMIT 100 </code></pre></div>The transaction operates on a consistent snapshot, unaffected by concurrent updates. <h4 id="time-travel-queries" class="position-relative d-flex align-items-center group"> Time-Travel Queries <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="time-travel-queries" aria-haspopup="dialog" aria-label="Share link: Time-Travel Queries"> Share link </button> </h4>MVCC enables querying historical data: <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-gql" data-lang="gql">-- See data as it existed at specific time SET TRANSACTION SNAPSHOT AT '2025-01-01T00:00:00Z'; MATCH (account:Account {id: $accountId}) RETURN account.balance -- Balance on Jan 1, 2025 </code></pre></div>This is invaluable for auditing, debugging, and historical analysis. <h4 id="optimistic-concurrency" class="position-relative d-flex align-items-center group"> Optimistic Concurrency <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="optimistic-concurrency" aria-haspopup="dialog" aria-label="Share link: Optimistic Concurrency"> Share link </button> </h4>MVCC supports optimistic locking patterns: <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-gql" data-lang="gql">-- Optimistic update with version check MATCH (product:Product {sku: $sku}) WHERE product.version = $expectedVersion SET product.quantity = product.quantity - $amount, product.version = product.version + 1 RETURN product.version </code></pre></div>If the version changed, the WHERE clause fails, signaling a conflict. <h3 id="best-practices" class="position-relative d-flex align-items-center group"> Best Practices <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="best-practices" aria-haspopup="dialog" aria-label="Share link: Best Practices"> Share link </button> </h3> <h4 id="transaction-management" class="position-relative d-flex align-items-center group"> Transaction Management <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="transaction-management" aria-haspopup="dialog" aria-label="Share link: Transaction Management"> Share link </button> </h4><ol> <li> Keep transactions short: Long transactions hold snapshots, preventing garbage collection <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-gql" data-lang="gql">-- Good: Short transaction BEGIN; MATCH (p:Person {id: $id}) SET p.last_login = now(); COMMIT; -- Bad: Long-running transaction BEGIN; -- Complex processing... -- Hours later... COMMIT; </code></pre></div></li> <li> Use appropriate isolation levels: Choose between snapshot isolation and serializable snapshot isolation based on requirements </li> <li> Handle conflicts gracefully: Implement retry logic for write-write conflicts <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-python" data-lang="python">for attempt in range(max_retries): try: with db.transaction(): # Perform updates break except ConflictError: if attempt == max_retries - 1: raise time.sleep(backoff_delay) </code></pre></div></li> </ol> <h4 id="performance-optimization" class="position-relative d-flex align-items-center group"> Performance Optimization <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="performance-optimization" aria-haspopup="dialog" aria-label="Share link: Performance Optimization"> Share link </button> </h4><ol> <li>Monitor version chain length: Long chains indicate hot spots</li> <li>Tune garbage collection: Balance retention needs with storage efficiency</li> <li>Use bulk operations: Reduce transaction overhead for batch updates</li> <li>Partition hot data: Distribute high-contention data across nodes</li> </ol> <h4 id="debugging-and-monitoring" class="position-relative d-flex align-items-center group"> Debugging and Monitoring <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="debugging-and-monitoring" aria-haspopup="dialog" aria-label="Share link: Debugging and Monitoring"> Share link </button> </h4>Track MVCC metrics: <ul> <li>Active transactions: Number of concurrent transactions</li> <li>Version chain depth: Average and max version count per item</li> <li>GC throughput: Versions reclaimed per second</li> <li>Conflict rate: Write-write conflicts per second</li> <li>Snapshot age: Age of oldest active snapshot</li> </ul> <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-gql" data-lang="gql">-- Query MVCC statistics CALL dbms.monitor.mvcc.stats() YIELD metric, value RETURN metric, value </code></pre></div> <h3 id="troubleshooting" class="position-relative d-flex align-items-center group"> Troubleshooting <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="troubleshooting" aria-haspopup="dialog" aria-label="Share link: Troubleshooting"> Share link </button> </h3> <h4 id="long-version-chains" class="position-relative d-flex align-items-center group"> Long Version Chains <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="long-version-chains" aria-haspopup="dialog" aria-label="Share link: Long Version Chains"> Share link </button> </h4>Symptom: Queries slow down over time Cause: Hot-spot data with frequent updates Solution: Partition data, increase GC frequency, or redesign schema <h4 id="garbage-collection-stalls" class="position-relative d-flex align-items-center group"> Garbage Collection Stalls <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="garbage-collection-stalls" aria-haspopup="dialog" aria-label="Share link: Garbage Collection Stalls"> Share link </button> </h4>Symptom: Storage keeps growing Cause: Long-running transactions preventing GC Solution: Identify and terminate long transactions, tune retention policy <h4 id="high-conflict-rates" class="position-relative d-flex align-items-center group"> High Conflict Rates <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="high-conflict-rates" aria-haspopup="dialog" aria-label="Share link: High Conflict Rates"> Share link </button> </h4>Symptom: Many transaction aborts Cause: Write-write conflicts on shared data Solution: Redesign to reduce contention, use application-level sharding <h3 id="related-topics" class="position-relative d-flex align-items-center group"> Related Topics <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="related-topics" aria-haspopup="dialog" aria-label="Share link: Related Topics"> Share link </button> </h3><ul> <li><a href="/tags/ssi/" >Serializable Snapshot Isolation (SSI)</a> - Advanced isolation level built on MVCC</li> <li><a href="/tags/wal/" >Write-Ahead Logging (WAL)</a> - Durability mechanism integrated with MVCC</li> <li><a href="/tags/acid/" >ACID Transactions</a> - Transaction guarantees enabled by MVCC</li> <li><a href="/tags/concurrency/" >Concurrency Control</a> - Broader concurrency mechanisms</li> <li><a href="/tags/isolation/" >Isolation Levels</a> - Transaction isolation guarantees</li> <li><a href="/tags/performance/" >Performance</a> - MVCC performance characteristics</li> <li><a href="/tags/transactions/" >Transactions</a> - Transaction management</li> </ul> <h3 id="further-reading" class="position-relative d-flex align-items-center group"> Further Reading <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="further-reading" aria-haspopup="dialog" aria-label="Share link: Further Reading"> Share link </button> </h3> <h4 id="documentation" class="position-relative d-flex align-items-center group"> Documentation <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="documentation" aria-haspopup="dialog" aria-label="Share link: Documentation"> Share link </button> </h4><ul> <li><a href="/docs/transactions/" >Transaction Guide</a> - Complete transaction documentation</li> <li><a href="/tags/concurrency/" >Concurrency Control</a> - Detailed concurrency mechanisms</li> <li><a href="/docs/query/performance-tuning/" >Performance Tuning</a> - MVCC optimization techniques</li> </ul> <h4 id="implementation-details" class="position-relative d-flex align-items-center group"> Implementation Details <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="implementation-details" aria-haspopup="dialog" aria-label="Share link: Implementation Details"> Share link </button> </h4><ul> <li><a href="/docs/architecture/" >Architecture Overview</a> - System architecture including MVCC</li> <li><a href="/tags/storage/" >Storage Engine</a> - Version storage and indexing</li> <li><a href="/docs/recovery/" >Recovery and Durability</a> - WAL integration with MVCC</li> </ul> <h4 id="advanced-topics" class="position-relative d-flex align-items-center group"> Advanced Topics <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="advanced-topics" aria-haspopup="dialog" aria-label="Share link: Advanced Topics"> Share link </button> </h4><ul> <li><a href="/tags/distributed/" >Distributed MVCC</a> - MVCC in distributed environments</li> <li><a href="/tags/performance/" >Garbage Collection</a> - Version cleanup strategies</li> <li><a href="/tags/temporal/" >Temporal Queries</a> - Historical query capabilities</li> </ul> <h3 id="mvcc-implementation-deep-dive" class="position-relative d-flex align-items-center group"> MVCC Implementation Deep Dive <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="mvcc-implementation-deep-dive" aria-haspopup="dialog" aria-label="Share link: MVCC Implementation Deep Dive"> Share link </button> </h3> <h4 id="version-storage-architecture" class="position-relative d-flex align-items-center group"> Version Storage Architecture <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="version-storage-architecture" aria-haspopup="dialog" aria-label="Share link: Version Storage Architecture"> Share link </button> </h4>Geode stores versions using an optimized multi-tier approach: <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-fallback" data-lang="fallback">Node Version Chain Example: ┌─────────────────────────────────────────────────────────┐ │ Node ID: 42 (Person:Alice) │ ├─────────────────────────────────────────────────────────┤ │ Current Version (TxID: 1005, committed) │ │ age: 31 │ │ email: "[email protected]" │ │ ↓ │ │ Previous Version (TxID: 1003, committed) │ │ age: 31 │ │ email: "[email protected]" │ │ ↓ │ │ Previous Version (TxID: 1001, committed) │ │ age: 30 │ │ email: "[email protected]" │ │ ↓ │ │ NULL (end of chain) │ └─────────────────────────────────────────────────────────┘ </code></pre></div> <h4 id="version-visibility-algorithm" class="position-relative d-flex align-items-center group"> Version Visibility Algorithm <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="version-visibility-algorithm" aria-haspopup="dialog" aria-label="Share link: Version Visibility Algorithm"> Share link </button> </h4><div class="highlight"><pre tabindex="0" class="chroma"><code class="language-zig" data-lang="zig">// geode/src/mvcc/visibility.zig pub fn isVersionVisible( version: *Version, snapshot: *Snapshot ) bool { // Check version begin timestamp if (version.begin_timestamp > snapshot.timestamp) { return false; // Created after snapshot } // Check version end timestamp if (version.end_timestamp != null and version.end_timestamp.? <= snapshot.timestamp) { return false; // Deleted before snapshot } // Check creating transaction status if (!isCommitted(version.creating_tx, snapshot.timestamp)) { return false; // Creating transaction not committed } // Check deleting transaction status if (version.deleting_tx != null and isCommitted(version.deleting_tx.?, snapshot.timestamp)) { return false; // Deleting transaction committed } return true; } </code></pre></div> <h4 id="snapshot-isolation-example" class="position-relative d-flex align-items-center group"> Snapshot Isolation Example <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="snapshot-isolation-example" aria-haspopup="dialog" aria-label="Share link: Snapshot Isolation Example"> Share link </button> </h4><div class="highlight"><pre tabindex="0" class="chroma"><code class="language-python" data-lang="python">from geode_client import Client import asyncio async def demonstrate_snapshot_isolation(): client1 = Client("localhost", 3141) client2 = Client("localhost", 3141) # Setup: Create test data await tx1.execute(""" CREATE (account:Account {id: 'acc_123', balance: 1000}) """) # Transaction 1: Long-running read async with client1.connection() as tx1: await tx1.begin() # Take snapshot at t=1 result1 = await tx1.execute(""" MATCH (account:Account {id: 'acc_123'}) RETURN account.balance AS balance """) balance_t1 = (result1.rows[0] if result1.rows else None)['balance'] print(f"T1 reads balance: {balance_t1}") # 1000 # Transaction 2: Concurrent update async with client2.connection() as tx2: await tx2.begin() await tx2.execute(""" MATCH (account:Account {id: 'acc_123'}) SET account.balance = account.balance - 100 """) # T2 commits here # Sleep to ensure T2 commits await asyncio.sleep(0.1) # T1 reads again from same snapshot result2 = await tx1.execute(""" MATCH (account:Account {id: 'acc_123'}) RETURN account.balance AS balance """) balance_t2 = (result2.rows[0] if result2.rows else None)['balance'] print(f"T1 reads balance again: {balance_t2}") # Still 1000! # T1 sees consistent snapshot despite T2's committed change # After T1 commits, new snapshot sees T2's change result3 = await tx1.execute(""" MATCH (account:Account {id: 'acc_123'}) RETURN account.balance AS balance """) balance_final = (result3.rows[0] if result3.rows else None)['balance'] print(f"New snapshot sees: {balance_final}") # 900 asyncio.run(demonstrate_snapshot_isolation()) </code></pre></div> <h3 id="advanced-mvcc-patterns" class="position-relative d-flex align-items-center group"> Advanced MVCC Patterns <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="advanced-mvcc-patterns" aria-haspopup="dialog" aria-label="Share link: Advanced MVCC Patterns"> Share link </button> </h3> <h4 id="optimistic-concurrency-control" class="position-relative d-flex align-items-center group"> Optimistic Concurrency Control <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="optimistic-concurrency-control" aria-haspopup="dialog" aria-label="Share link: Optimistic Concurrency Control"> Share link </button> </h4><div class="highlight"><pre tabindex="0" class="chroma"><code class="language-python" data-lang="python">class OptimisticLock: """Implement optimistic locking using MVCC.""" def __init__(self, client): self.client = client async def read_with_version(self, entity_id): """Read entity and its current version.""" result, _ = await self.client.query(""" MATCH (e:Entity {id: $id}) RETURN e, e.version AS version """, {'id': entity_id}) row = result.rows[0] if result.rows else None return row['e'], row['version'] async def update_with_version_check(self, entity_id, expected_version, updates): """Update entity only if version matches.""" result, _ = await self.client.query(""" MATCH (e:Entity {id: $id}) WHERE e.version = $expected_version SET e += $updates, e.version = e.version + 1 RETURN e.version AS new_version """, { 'id': entity_id, 'expected_version': expected_version, 'updates': updates }) row = result.rows[0] if result.rows else None if not row: raise OptimisticLockException("Version mismatch - entity was modified") return row['new_version'] # Usage async def safe_concurrent_update(): lock = OptimisticLock(client) # Read current state entity, version = await lock.read_with_version(entity_id="entity_1") # Perform business logic new_quantity = entity['quantity'] - 5 # Attempt update with version check try: new_version = await lock.update_with_version_check( entity_id="entity_1", expected_version=version, updates={'quantity': new_quantity} ) print(f"Updated to version {new_version}") except OptimisticLockException: print("Concurrent modification detected - retry") </code></pre></div> <h4 id="time-travel-queries-1" class="position-relative d-flex align-items-center group"> Time-Travel Queries <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="time-travel-queries-1" aria-haspopup="dialog" aria-label="Share link: Time-Travel Queries"> Share link </button> </h4>MVCC enables querying historical database states: <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-gql" data-lang="gql">-- Query database as it existed at specific time BEGIN TRANSACTION AS OF SYSTEM TIME '2025-01-01T00:00:00Z'; MATCH (account:Account {id: 'acc_123'}) RETURN account.balance AS balance_at_jan1; COMMIT; -- Query multiple historical points MATCH (account:Account {id: 'acc_123'}) WITH account.id AS account_id UNWIND [ '2025-01-01T00:00:00Z', '2025-02-01T00:00:00Z', '2025-03-01T00:00:00Z' ] AS timestamp WITH account_id, timestamp CALL { BEGIN TRANSACTION AS OF SYSTEM TIME timestamp; MATCH (account:Account {id: account_id}) RETURN account.balance AS balance; COMMIT; } RETURN timestamp, balance ORDER BY timestamp; </code></pre></div><div class="highlight"><pre tabindex="0" class="chroma"><code class="language-python" data-lang="python">async def historical_balance_report(account_id, start_date, end_date): """Generate historical balance report using time-travel.""" # Generate monthly timestamps timestamps = generate_monthly_timestamps(start_date, end_date) balances = [] for ts in timestamps: result, _ = await client.query(""" BEGIN TRANSACTION AS OF SYSTEM TIME $timestamp; MATCH (account:Account {id: $account_id}) RETURN account.balance AS balance; COMMIT; """, {'account_id': account_id, 'timestamp': ts}) row = result.rows[0] if result.rows else None balances.append({ 'timestamp': ts, 'balance': row['balance'] if row else None }) return balances </code></pre></div> <h3 id="garbage-collection-strategies" class="position-relative d-flex align-items-center group"> Garbage Collection Strategies <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="garbage-collection-strategies" aria-haspopup="dialog" aria-label="Share link: Garbage Collection Strategies"> Share link </button> </h3> <h4 id="configurable-gc-policies" class="position-relative d-flex align-items-center group"> Configurable GC Policies <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="configurable-gc-policies" aria-haspopup="dialog" aria-label="Share link: Configurable GC Policies"> Share link </button> </h4><div class="highlight"><pre tabindex="0" class="chroma"><code class="language-python" data-lang="python">class MVCCGarbageCollector: """Configure and manage MVCC garbage collection.""" def __init__(self, client): self.client = client async def set_retention_policy(self, retention_hours): """Set how long to retain old versions.""" await self.client.execute(""" CALL dbms.mvcc.setRetentionPolicy($hours) """, {'hours': retention_hours}) async def run_gc_manually(self): """Trigger manual garbage collection.""" result, _ = await self.client.query(""" CALL dbms.mvcc.garbageCollect() YIELD versionsReclaimed, bytesFreed RETURN versionsReclaimed, bytesFreed """) stats = result.rows[0] if result.rows else None return stats async def get_gc_stats(self): """Get garbage collection statistics.""" result, _ = await self.client.query(""" CALL dbms.mvcc.stats() YIELD metric, value RETURN metric, value """) return {row['metric']: row['value'] for row in result.rows} # Usage async def manage_storage(): gc = MVCCGarbageCollector(client) # Set 24-hour retention await gc.set_retention_policy(retention_hours=24) # Run GC and get stats stats = await gc.run_gc_manually() print(f"Reclaimed {stats['versionsReclaimed']} versions") print(f"Freed {stats['bytesFreed'] / 1024 / 1024:.2f} MB") # Monitor GC health gc_stats = await gc.get_gc_stats() print(f"Average version chain length: {gc_stats['avg_chain_length']}") print(f"Oldest active snapshot age: {gc_stats['oldest_snapshot_age_seconds']}s") </code></pre></div> <h4 id="aggressive-vs-conservative-gc" class="position-relative d-flex align-items-center group"> Aggressive vs. Conservative GC <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="aggressive-vs-conservative-gc" aria-haspopup="dialog" aria-label="Share link: Aggressive vs. Conservative GC"> Share link </button> </h4><div class="highlight"><pre tabindex="0" class="chroma"><code class="language-python" data-lang="python"># Conservative: Keep more history (better for time-travel) await client.execute(""" CALL dbms.mvcc.configure({ retention_hours: 168, // 1 week gc_frequency_minutes: 60, max_chain_length: 100 }) """) # Aggressive: Minimize storage (faster queries) await client.execute(""" CALL dbms.mvcc.configure({ retention_hours: 1, gc_frequency_minutes: 5, max_chain_length: 10 }) """) </code></pre></div> <h3 id="performance-monitoring" class="position-relative d-flex align-items-center group"> Performance Monitoring <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="performance-monitoring" aria-haspopup="dialog" aria-label="Share link: Performance Monitoring"> Share link </button> </h3> <h4 id="mvcc-performance-metrics" class="position-relative d-flex align-items-center group"> MVCC Performance Metrics <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="mvcc-performance-metrics" aria-haspopup="dialog" aria-label="Share link: MVCC Performance Metrics"> Share link </button> </h4><div class="highlight"><pre tabindex="0" class="chroma"><code class="language-python" data-lang="python">from prometheus_client import Gauge, Histogram # Define metrics version_chain_length = Histogram( 'geode_mvcc_version_chain_length', 'Length of version chains', buckets=[1, 5, 10, 25, 50, 100, 250] ) active_snapshots = Gauge( 'geode_mvcc_active_snapshots', 'Number of active snapshots' ) gc_reclaimed_versions = Counter( 'geode_mvcc_gc_reclaimed_versions_total', 'Total versions reclaimed by GC' ) async def collect_mvcc_metrics(): """Collect and export MVCC metrics.""" stats, _ = await client.query(""" CALL dbms.mvcc.detailedStats() YIELD metric, value RETURN metric, value """) for row in stats.rows: metric = row['metric'] value = row['value'] if metric == 'avg_version_chain_length': version_chain_length.observe(value) elif metric == 'active_snapshots': active_snapshots.set(value) elif metric == 'gc_reclaimed_versions': gc_reclaimed_versions.inc(value) </code></pre></div> <h4 id="identifying-performance-bottlenecks" class="position-relative d-flex align-items-center group"> Identifying Performance Bottlenecks <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="identifying-performance-bottlenecks" aria-haspopup="dialog" aria-label="Share link: Identifying Performance Bottlenecks"> Share link </button> </h4><div class="highlight"><pre tabindex="0" class="chroma"><code class="language-python" data-lang="python">async def diagnose_mvcc_issues(): """Diagnose MVCC-related performance issues.""" # Check for long version chains result, _ = await client.query(""" CALL dbms.mvcc.versionChainStats() YIELD nodeId, chainLength WHERE chainLength > 50 RETURN nodeId, chainLength ORDER BY chainLength DESC LIMIT 10 """) long_chains = [row for row in result.rows] if long_chains: print("WARNING: Long version chains detected") for chain in long_chains: print(f" Node {chain['nodeId']}: {chain['chainLength']} versions") print("Recommendation: Increase GC frequency or reduce update rate") # Check for old snapshots blocking GC result, _ = await client.query(""" CALL dbms.transactions.list() YIELD transactionId, startTime WHERE duration.between(startTime, datetime()) > duration('PT5M') RETURN transactionId, startTime """) old_txs = [row for row in result.rows] if old_txs: print("WARNING: Long-running transactions blocking GC") for tx in old_txs: print(f" Transaction {tx['transactionId']}") print(f" Started: {tx['startTime']}") print("Recommendation: Terminate or shorten these transactions") </code></pre></div> <h3 id="distributed-mvcc" class="position-relative d-flex align-items-center group"> Distributed MVCC <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="distributed-mvcc" aria-haspopup="dialog" aria-label="Share link: Distributed MVCC"> Share link </button> </h3> <h4 id="multi-node-version-coordination" class="position-relative d-flex align-items-center group"> Multi-Node Version Coordination <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="multi-node-version-coordination" aria-haspopup="dialog" aria-label="Share link: Multi-Node Version Coordination"> Share link </button> </h4><div class="highlight"><pre tabindex="0" class="chroma"><code class="language-python" data-lang="python">class DistributedMVCC: """Coordinate MVCC across multiple nodes.""" def __init__(self, nodes): self.nodes = nodes self.clock = HybridLogicalClock() async def begin_distributed_transaction(self): """Start transaction across all nodes with synchronized snapshot.""" # Get synchronized timestamp from all nodes timestamps = [] for node in self.nodes: ts = await node.get_current_timestamp() timestamps.append(ts) # Use maximum timestamp to ensure consistent snapshot snapshot_timestamp = max(timestamps) # Begin transaction on all nodes with same snapshot transactions = [] for node in self.nodes: tx = await node.begin(snapshot_timestamp) transactions.append(tx) return DistributedTransaction(transactions, snapshot_timestamp) class HybridLogicalClock: """Hybrid logical clock for distributed timestamp coordination.""" def __init__(self): self.logical_time = 0 self.physical_time = 0 def now(self): """Get current HLC timestamp.""" current_physical = time.time_ns() if current_physical > self.physical_time: self.physical_time = current_physical self.logical_time = 0 else: self.logical_time += 1 return (self.physical_time, self.logical_time) def update(self, remote_timestamp): """Update clock based on received timestamp.""" remote_physical, remote_logical = remote_timestamp current_physical = time.time_ns() self.physical_time = max(current_physical, remote_physical, self.physical_time) if self.physical_time == remote_physical: self.logical_time = max(self.logical_time, remote_logical) + 1 elif self.physical_time == current_physical: self.logical_time += 1 else: self.logical_time = 0 return (self.physical_time, self.logical_time) </code></pre></div> <h3 id="troubleshooting-mvcc-issues" class="position-relative d-flex align-items-center group"> Troubleshooting MVCC Issues <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="troubleshooting-mvcc-issues" aria-haspopup="dialog" aria-label="Share link: Troubleshooting MVCC Issues"> Share link </button> </h3> <h4 id="issue-version-chain-bloat" class="position-relative d-flex align-items-center group"> Issue: Version Chain Bloat <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="issue-version-chain-bloat" aria-haspopup="dialog" aria-label="Share link: Issue: Version Chain Bloat"> Share link </button> </h4>Symptom: Queries slow down over time on frequently updated entities Diagnosis: <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-python" data-lang="python">async def find_bloated_chains(): result, _ = await client.query(""" CALL dbms.mvcc.analyzeChains() YIELD entityId, entityType, chainLength, storageBytes WHERE chainLength > 100 RETURN entityId, entityType, chainLength, storageBytes ORDER BY chainLength DESC """) return [row for row in result.rows] </code></pre></div>Solutions: <ol> <li>Increase GC frequency</li> <li>Reduce retention period</li> <li>Redesign to batch updates</li> <li>Consider partitioning hot entities</li> </ol> <h4 id="issue-gc-starvation" class="position-relative d-flex align-items-center group"> Issue: GC Starvation <button type="button" class="h-share btn btn-link p-0 text-decoration-none link-secondary opacity-50 hover-opacity-100 transition-all ms-1" data-share-target="issue-gc-starvation" aria-haspopup="dialog" aria-label="Share link: Issue: GC Starvation"> Share link </button> </h4>Symptom: Storage grows continuously, GC not reclaiming space Diagnosis: <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-python" data-lang="python">async def check_gc_health(): result, _ = await client.query(""" CALL dbms.mvcc.gcHealth() YIELD lastRunTime, versionsEligible, versionsReclaimed RETURN lastRunTime, versionsEligible, versionsReclaimed """) health = result.rows[0] if result.rows else None if health['versionsEligible'] > 0 and health['versionsReclaimed'] == 0: # GC unable to run - find why blocked_by, _ = await client.query(""" CALL dbms.transactions.blocking() YIELD transactionId, blockedBy RETURN transactionId, blockedBy """) return blocked_by.rows </code></pre></div>Solutions: <ol> <li>Identify and terminate long-running transactions</li> <li>Ensure no snapshots held indefinitely</li> <li>Check for GC configuration issues</li> </ol> Geode’s MVCC implementation provides the foundation for high-concurrency, consistent graph queries. By maintaining multiple versions and using snapshot isolation, Geode achieves both strong consistency guarantees and excellent concurrent performance—essential for production graph workloads.

Popular

Related Articles

Transactions and Data Integrity