Mastery read-modify-write is non-atomic — concurrent updates clobber each other (follow-up to #128, finding #9) #247

Closed
#279
Closed
Mastery read-modify-write is non-atomic — concurrent updates clobber each other (follow-up to #128, finding #9)#247
#279
Labels
P2Medium prioritybackendBackend / APIdata-integrityCorrectness of stored/served datatech-debtCleanup / maintainability

Description

@AndresL230
AndresL230
opened on Jun 22, 2026

Follow-up to #128 (finding #9), deferred deliberately in PR #242.

Problem

PR #242 consolidated every mastery write onto a single path (graph_service.apply_mastery_event), but that write is still a non-atomic read-modify-write:

  1. READ mastery_score / mastery_events from graph_nodes
  2. MODIFY in Python (clamp, append capped event)
  3. WRITE the whole value back

Two concurrent writers to the same node (e.g. a quiz submission and an agent mastery tool, or two quick submissions) both read the old value and the second write clobbers the first — a classic lost-update race. mastery_events entries and score deltas can silently disappear.

Location

backend/services/graph_service.py → apply_mastery_event (the inline NOTE (#9, deferred) marks the exact spot).

Why it was deferred

The real fix is a DB-side atomic append (Postgres RPC / SQL UPDATE ... SET mastery_score = ..., mastery_events = mastery_events || ...), which needs the migration-runner plumbing tracked in #195 / #197. Now that all mastery writes share one path, this becomes a single-place change.

Fix options

  • (preferred) Postgres RPC performing the clamp + append atomically server-side.
  • Optimistic concurrency: a version column, reject/retry on stale write.
  • Single-writer path or row-level lock.

Refs: #128 (finding #9), #195, #197, epic #136.

🤖 Filed via Claude Code

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var __re = new RegExp('^' + "github\\.com" + '

      Skip to content
      Mastery read-modify-write is non-atomic — concurrent updates clobber each other (follow-up to #128, finding #9) #247
      Closed
      #279
      Closed
      Mastery read-modify-write is non-atomic — concurrent updates clobber each other (follow-up to #128, finding #9)#247
      #279
      Labels
      P2Medium prioritybackendBackend / APIdata-integrityCorrectness of stored/served datatech-debtCleanup / maintainability
      Description
      @AndresL230
      AndresL230
      opened on Jun 22, 2026
      Follow-up to #128 (finding #9), deferred deliberately in PR #242.
      Problem
      PR #242 consolidated every mastery write onto a single path (graph_service.apply_mastery_event), but that write is still a non-atomic read-modify-write:
      1. READ mastery_score / mastery_events from graph_nodes
      2. MODIFY in Python (clamp, append capped event)
      3. WRITE the whole value back
      Two concurrent writers to the same node (e.g. a quiz submission and an agent mastery tool, or two quick submissions) both read the old value and the second write clobbers the first — a classic lost-update race. mastery_events entries and score deltas can silently disappear.
      Location
      backend/services/graph_service.py → apply_mastery_event (the inline NOTE (#9, deferred) marks the exact spot).
      Why it was deferred
      The real fix is a DB-side atomic append (Postgres RPC / SQL UPDATE ... SET mastery_score = ..., mastery_events = mastery_events || ...), which needs the migration-runner plumbing tracked in #195 / #197. Now that all mastery writes share one path, this becomes a single-place change.
      Fix options
      • (preferred) Postgres RPC performing the clamp + append atomically server-side.
      • Optimistic concurrency: a version column, reject/retry on stale write.
      • Single-writer path or row-level lock.
      Refs: #128 (finding #9), #195, #197, epic #136.
      🤖 Filed via Claude Code
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      Metadata
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        P2Medium prioritybackendBackend / APIdata-integrityCorrectness of stored/served datatech-debtCleanup / maintainability
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          Milestone
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          None yet
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var __re = new RegExp('^' + ".*" + '

          Skip to content
          Mastery read-modify-write is non-atomic — concurrent updates clobber each other (follow-up to #128, finding #9) #247
          Closed
          #279
          Closed
          Mastery read-modify-write is non-atomic — concurrent updates clobber each other (follow-up to #128, finding #9)#247
          #279
          Labels
          P2Medium prioritybackendBackend / APIdata-integrityCorrectness of stored/served datatech-debtCleanup / maintainability
          Description
          @AndresL230
          AndresL230
          opened on Jun 22, 2026
          Follow-up to #128 (finding #9), deferred deliberately in PR #242.
          Problem
          PR #242 consolidated every mastery write onto a single path (graph_service.apply_mastery_event), but that write is still a non-atomic read-modify-write:
          1. READ mastery_score / mastery_events from graph_nodes
          2. MODIFY in Python (clamp, append capped event)
          3. WRITE the whole value back
          Two concurrent writers to the same node (e.g. a quiz submission and an agent mastery tool, or two quick submissions) both read the old value and the second write clobbers the first — a classic lost-update race. mastery_events entries and score deltas can silently disappear.
          Location
          backend/services/graph_service.py → apply_mastery_event (the inline NOTE (#9, deferred) marks the exact spot).
          Why it was deferred
          The real fix is a DB-side atomic append (Postgres RPC / SQL UPDATE ... SET mastery_score = ..., mastery_events = mastery_events || ...), which needs the migration-runner plumbing tracked in #195 / #197. Now that all mastery writes share one path, this becomes a single-place change.
          Fix options
          • (preferred) Postgres RPC performing the clamp + append atomically server-side.
          • Optimistic concurrency: a version column, reject/retry on stale write.
          • Single-writer path or row-level lock.
          Refs: #128 (finding #9), #195, #197, epic #136.
          🤖 Filed via Claude Code
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          Metadata
          Assignees
          No one assigned
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            P2Medium prioritybackendBackend / APIdata-integrityCorrectness of stored/served datatech-debtCleanup / maintainability
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, 'i');
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})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

              Skip to content
              Mastery read-modify-write is non-atomic — concurrent updates clobber each other (follow-up to #128, finding #9) #247
              Closed
              #279
              Closed
              Mastery read-modify-write is non-atomic — concurrent updates clobber each other (follow-up to #128, finding #9)#247
              #279
              Labels
              P2Medium prioritybackendBackend / APIdata-integrityCorrectness of stored/served datatech-debtCleanup / maintainability
              Description
              @AndresL230
              AndresL230
              opened on Jun 22, 2026
              Follow-up to #128 (finding #9), deferred deliberately in PR #242.
              Problem
              PR #242 consolidated every mastery write onto a single path (graph_service.apply_mastery_event), but that write is still a non-atomic read-modify-write:
              1. READ mastery_score / mastery_events from graph_nodes
              2. MODIFY in Python (clamp, append capped event)
              3. WRITE the whole value back
              Two concurrent writers to the same node (e.g. a quiz submission and an agent mastery tool, or two quick submissions) both read the old value and the second write clobbers the first — a classic lost-update race. mastery_events entries and score deltas can silently disappear.
              Location
              backend/services/graph_service.py → apply_mastery_event (the inline NOTE (#9, deferred) marks the exact spot).
              Why it was deferred
              The real fix is a DB-side atomic append (Postgres RPC / SQL UPDATE ... SET mastery_score = ..., mastery_events = mastery_events || ...), which needs the migration-runner plumbing tracked in #195 / #197. Now that all mastery writes share one path, this becomes a single-place change.
              Fix options
              • (preferred) Postgres RPC performing the clamp + append atomically server-side.
              • Optimistic concurrency: a version column, reject/retry on stale write.
              • Single-writer path or row-level lock.
              Refs: #128 (finding #9), #195, #197, epic #136.
              🤖 Filed via Claude Code
              Metadata
              Metadata
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              No one assigned
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                P2Medium prioritybackendBackend / APIdata-integrityCorrectness of stored/served datatech-debtCleanup / maintainability
                Type
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                Projects
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                  Milestone
                  No milestone
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                  None yet
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                  No branches or pull requests
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, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Strip utm_, fbclid, gclid, etc. from all links on page\n(function() {\n var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',\n 'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',\n 'ref', 'ref_src', 'source', 'medium', 'campaign'];\n \n function cleanUrl(url) {\n try {\n var u = new URL(url, window.location.origin);\n var changed = false;\n trackingParams.forEach(function(p) {\n if (u.searchParams.has(p)) {\n u.searchParams.delete(p);\n changed = true;\n }\n });\n return changed ? u.toString() : url;\n } catch (e) {\n return url;\n }\n }\n \n function cleanLinks() {\n document.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n \n cleanLinks();\n \n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1) {\n if (node.tagName === 'A') cleanLinks();\n node.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Remove Tracking Parameters from Links");
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})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '

                  Skip to content
                  Mastery read-modify-write is non-atomic — concurrent updates clobber each other (follow-up to #128, finding #9) #247
                  Closed
                  #279
                  Closed
                  Mastery read-modify-write is non-atomic — concurrent updates clobber each other (follow-up to #128, finding #9)#247
                  #279
                  Labels
                  P2Medium prioritybackendBackend / APIdata-integrityCorrectness of stored/served datatech-debtCleanup / maintainability
                  Description
                  @AndresL230
                  AndresL230
                  opened on Jun 22, 2026
                  Follow-up to #128 (finding #9), deferred deliberately in PR #242.
                  Problem
                  PR #242 consolidated every mastery write onto a single path (graph_service.apply_mastery_event), but that write is still a non-atomic read-modify-write:
                  1. READ mastery_score / mastery_events from graph_nodes
                  2. MODIFY in Python (clamp, append capped event)
                  3. WRITE the whole value back
                  Two concurrent writers to the same node (e.g. a quiz submission and an agent mastery tool, or two quick submissions) both read the old value and the second write clobbers the first — a classic lost-update race. mastery_events entries and score deltas can silently disappear.
                  Location
                  backend/services/graph_service.py → apply_mastery_event (the inline NOTE (#9, deferred) marks the exact spot).
                  Why it was deferred
                  The real fix is a DB-side atomic append (Postgres RPC / SQL UPDATE ... SET mastery_score = ..., mastery_events = mastery_events || ...), which needs the migration-runner plumbing tracked in #195 / #197. Now that all mastery writes share one path, this becomes a single-place change.
                  Fix options
                  • (preferred) Postgres RPC performing the clamp + append atomically server-side.
                  • Optimistic concurrency: a version column, reject/retry on stale write.
                  • Single-writer path or row-level lock.
                  Refs: #128 (finding #9), #195, #197, epic #136.
                  🤖 Filed via Claude Code
                  Metadata
                  Metadata
                  Assignees
                  No one assigned
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                    P2Medium prioritybackendBackend / APIdata-integrityCorrectness of stored/served datatech-debtCleanup / maintainability
                    Type
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                    Projects
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                      Milestone
                      No milestone
                      Relationships
                      None yet
                      Development
                      No branches or pull requests
                      Issue actions
                      
, 'i');
if (__m === '*' || __re.test(location.href)) {
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})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

                      Skip to content
                      Mastery read-modify-write is non-atomic — concurrent updates clobber each other (follow-up to #128, finding #9) #247
                      Closed
                      #279
                      Closed
                      Mastery read-modify-write is non-atomic — concurrent updates clobber each other (follow-up to #128, finding #9)#247
                      #279
                      Labels
                      P2Medium prioritybackendBackend / APIdata-integrityCorrectness of stored/served datatech-debtCleanup / maintainability
                      Description
                      @AndresL230
                      AndresL230
                      opened on Jun 22, 2026
                      Follow-up to #128 (finding #9), deferred deliberately in PR #242.
                      Problem
                      PR #242 consolidated every mastery write onto a single path (graph_service.apply_mastery_event), but that write is still a non-atomic read-modify-write:
                      1. READ mastery_score / mastery_events from graph_nodes
                      2. MODIFY in Python (clamp, append capped event)
                      3. WRITE the whole value back
                      Two concurrent writers to the same node (e.g. a quiz submission and an agent mastery tool, or two quick submissions) both read the old value and the second write clobbers the first — a classic lost-update race. mastery_events entries and score deltas can silently disappear.
                      Location
                      backend/services/graph_service.py → apply_mastery_event (the inline NOTE (#9, deferred) marks the exact spot).
                      Why it was deferred
                      The real fix is a DB-side atomic append (Postgres RPC / SQL UPDATE ... SET mastery_score = ..., mastery_events = mastery_events || ...), which needs the migration-runner plumbing tracked in #195 / #197. Now that all mastery writes share one path, this becomes a single-place change.
                      Fix options
                      • (preferred) Postgres RPC performing the clamp + append atomically server-side.
                      • Optimistic concurrency: a version column, reject/retry on stale write.
                      • Single-writer path or row-level lock.
                      Refs: #128 (finding #9), #195, #197, epic #136.
                      🤖 Filed via Claude Code
                      Metadata
                      Metadata
                      Assignees
                      No one assigned
                        Labels
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                        Projects
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                          Milestone
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                          Relationships
                          None yet
                          Development
                          No branches or pull requests
                          Issue actions
                          
, 'i');
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})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

                          Skip to content
                          Mastery read-modify-write is non-atomic — concurrent updates clobber each other (follow-up to #128, finding #9) #247
                          Closed
                          #279
                          Closed
                          Mastery read-modify-write is non-atomic — concurrent updates clobber each other (follow-up to #128, finding #9)#247
                          #279
                          Labels
                          P2Medium prioritybackendBackend / APIdata-integrityCorrectness of stored/served datatech-debtCleanup / maintainability
                          Description
                          @AndresL230
                          AndresL230
                          opened on Jun 22, 2026
                          Follow-up to #128 (finding #9), deferred deliberately in PR #242.
                          Problem
                          PR #242 consolidated every mastery write onto a single path (graph_service.apply_mastery_event), but that write is still a non-atomic read-modify-write:
                          1. READ mastery_score / mastery_events from graph_nodes
                          2. MODIFY in Python (clamp, append capped event)
                          3. WRITE the whole value back
                          Two concurrent writers to the same node (e.g. a quiz submission and an agent mastery tool, or two quick submissions) both read the old value and the second write clobbers the first — a classic lost-update race. mastery_events entries and score deltas can silently disappear.
                          Location
                          backend/services/graph_service.py → apply_mastery_event (the inline NOTE (#9, deferred) marks the exact spot).
                          Why it was deferred
                          The real fix is a DB-side atomic append (Postgres RPC / SQL UPDATE ... SET mastery_score = ..., mastery_events = mastery_events || ...), which needs the migration-runner plumbing tracked in #195 / #197. Now that all mastery writes share one path, this becomes a single-place change.
                          Fix options
                          • (preferred) Postgres RPC performing the clamp + append atomically server-side.
                          • Optimistic concurrency: a version column, reject/retry on stale write.
                          • Single-writer path or row-level lock.
                          Refs: #128 (finding #9), #195, #197, epic #136.
                          🤖 Filed via Claude Code
                          Metadata
                          Metadata
                          Assignees
                          No one assigned
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                            P2Medium prioritybackendBackend / APIdata-integrityCorrectness of stored/served datatech-debtCleanup / maintainability
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                            Projects
                            No projects
                              Milestone
                              No milestone
                              Relationships
                              None yet
                              Development
                              No branches or pull requests
                              Issue actions
                              
, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Universal Dark Mode - works on any site\n(function() {\n var enabled = true;\n \n function applyDarkMode() {\n if (!enabled) return;\n \n // Create style element if it doesn't exist\n var style = document.getElementById('universal-dark-mode-style');\n if (!style) {\n style = document.createElement('style');\n style.id = 'universal-dark-mode-style';\n document.head.appendChild(style);\n }\n \n // Dark mode CSS - inverts colors but preserves images/video\n style.textContent = '\n /* Invert everything except media */\n html {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #1a1a2e !important;\n }\n \n /* Restore images, videos, iframes, canvas */\n img, video, iframe, canvas, svg, picture, [style*=\"background-image\"] {\n filter: invert(1) hue-rotate(180deg) !important;\n }\n \n /* Preserve specific elements that should not be inverted */\n .no-dark-mode, .no-dark-mode *,\n [data-theme=\"light\"], [data-theme=\"light\"],\n .ace_editor, .ace_editor *,\n .CodeMirror, .CodeMirror *,\n .monaco-editor, .monaco-editor *,\n .markdown-body pre, .markdown-body pre *,\n .highlight, .highlight *,\n pre code, pre code * {\n filter: none !important;\n }\n \n /* Fix common UI elements */\n .modal, .popup, .dropdown-menu, .tooltip, .popover {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #2d2d44 !important;\n border-color: #444 !important;\n }\n \n /* Scrollbars */\n ::-webkit-scrollbar { background: #1a1a2e !important; }\n ::-webkit-scrollbar-thumb { background: #444 !important; }\n ::-webkit-scrollbar-thumb:hover { background: #555 !important; }\n \n /* Selection */\n ::selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ';\n }\n \n function removeDarkMode() {\n var style = document.getElementById('universal-dark-mode-style');\n if (style) style.remove();\n }\n \n // Toggle with Alt+Shift+D\n document.addEventListener('keydown', function(e) {\n if (e.altKey && e.shiftKey && e.key === 'D') {\n e.preventDefault();\n enabled = !enabled;\n if (enabled) {\n applyDarkMode();\n console.log('[Universal Dark Mode] Enabled');\n } else {\n removeDarkMode();\n console.log('[Universal Dark Mode] Disabled');\n }\n }\n });\n \n // Apply on load\n applyDarkMode();\n \n // Re-apply on dynamic content\n var observer = new MutationObserver(function(mutations) {\n if (enabled && !document.getElementById('universal-dark-mode-style')) {\n applyDarkMode();\n }\n });\n observer.observe(document.head, { childList: true });\n \n console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');\n})();", "Universal Dark Mode");
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                              Mastery read-modify-write is non-atomic — concurrent updates clobber each other (follow-up to #128, finding #9) #247
                              Closed
                              #279
                              Closed
                              Mastery read-modify-write is non-atomic — concurrent updates clobber each other (follow-up to #128, finding #9)#247
                              #279
                              Labels
                              P2Medium prioritybackendBackend / APIdata-integrityCorrectness of stored/served datatech-debtCleanup / maintainability
                              Description
                              @AndresL230
                              AndresL230
                              opened on Jun 22, 2026
                              Follow-up to #128 (finding #9), deferred deliberately in PR #242.
                              Problem
                              PR #242 consolidated every mastery write onto a single path (graph_service.apply_mastery_event), but that write is still a non-atomic read-modify-write:
                              1. READ mastery_score / mastery_events from graph_nodes
                              2. MODIFY in Python (clamp, append capped event)
                              3. WRITE the whole value back
                              Two concurrent writers to the same node (e.g. a quiz submission and an agent mastery tool, or two quick submissions) both read the old value and the second write clobbers the first — a classic lost-update race. mastery_events entries and score deltas can silently disappear.
                              Location
                              backend/services/graph_service.py → apply_mastery_event (the inline NOTE (#9, deferred) marks the exact spot).
                              Why it was deferred
                              The real fix is a DB-side atomic append (Postgres RPC / SQL UPDATE ... SET mastery_score = ..., mastery_events = mastery_events || ...), which needs the migration-runner plumbing tracked in #195 / #197. Now that all mastery writes share one path, this becomes a single-place change.
                              Fix options
                              • (preferred) Postgres RPC performing the clamp + append atomically server-side.
                              • Optimistic concurrency: a version column, reject/retry on stale write.
                              • Single-writer path or row-level lock.
                              Refs: #128 (finding #9), #195, #197, epic #136.
                              🤖 Filed via Claude Code
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