Diagnosing Apollo Client Cache Inconsistency After Mutations

Recognizable condition

After a mutation is sent from the UI, the screen still shows the old data or a perpetual loading spinner, even though the network request appears to have succeeded.

Cause / diagnostic table

Symptom	Likely cause
Mutation returns 200 but UI unchanged	Cache not updated after mutation
UI shows loading indicator that never resolves	fetchPolicy set to 'cache-only' or error swallowed
Old data flashes then disappears	Missing or incorrect optimisticResponse
Related fields stay stale	Type policy key fields mis‑configured
Network error but UI shows success	errorPolicy ignores errors

Ordered checks

1. Verify network payload
   • Open browser DevTools → Network tab.
   • Find the mutation request (POST to GraphQL endpoint).
   • Check that status is 200 and the response JSON contains the expected data fields (e.g., { updateItem { id name } }).
   • If the request failed or returned an error, note the error message.
2. Inspect the Apollo Client cache
   • Ensure Apollo Client DevTools extension is installed.
   • In the DevTools panel, select the Cache tab.
   • Search for the typename and id of the object you expect to change (e.g., Item:"123").
   • Confirm that the fields returned by the mutation are present and have correct __typename and id.
   • If the cache still shows the old values, the mutation did not update the cache.
3. Check optimistic response handling
   • Locate the mutation call in your code.
   • Verify that an optimisticResponse object is supplied (if you rely on optimistic UI).
   • If omitted, the UI will not reflect changes until the server response arrives.
4. Review fetchPolicy and errorPolicy
   • Find the useMutation hook or mutate call.
   • Ensure fetchPolicy is not set to 'cache-only' (which would prevent the mutation result from hitting the network).
   • Check errorPolicy: if set to 'ignore', errors are swallowed and the UI may appear stuck.
   • Typical safe values: fetchPolicy: 'network-only' or 'cache-and-network'; errorPolicy: 'all' or 'none'.
5. Examine type policies and key fields
   • When initializing InMemoryCache, verify that each type has a proper key fields definition (usually id or custom keyFields).
   • Incorrect key fields cause Apollo to treat updated objects as new entities, leaving the old cache entry untouched.
   • Example snippet:

     const cache = new InMemoryCache({
       typePolicies: {
         Item: { keyFields: ['id'] },
         // add other types as needed
       }
     });

Fixes tied to findings

• Missing cache update
  • Add an update function to the mutation:

  • const [updateItem] = useMutation(UPDATE_ITEM, {
      update: (cache, { data: { updateItem } }) => {
        cache.modify({
          id: cache.identify(updateItem),
          fields: {
            name: () => updateItem.name,
          },
        });
      },
    });

  • Alternatively, use refetchQueries to refetch a query that reads the updated field.
• Optimistic UI missing
  • Provide an optimisticResponse that matches the shape of the mutation result:

  • const [updateItem] = useMutation(UPDATE_ITEM, {
      optimisticResponse: {
        updateItem: {
          __typename: 'Item',
          id: variables.id,
          name: variables.newName,
        },
      },
    });

• fetchPolicy blocking network
  • Change the fetchPolicy in the mutation options:

  • const [updateItem] = useMutation(UPDATE_ITEM, {
      fetchPolicy: 'network-only',
    });

• errorPolicy swallowing errors
  • Set errorPolicy to 'all' to surface errors, or handle them in the mutation’s onError callback.

  • const [updateItem] = useMutation(UPDATE_ITEM, {
      errorPolicy: 'all',
      onError: (err) => console.error('Mutation failed', err),
    });

• Type policy mis‑configuration
  • Define correct key fields for each type, as shown in the cache initialization example above.
  • If you use composite keys, ensure they uniquely identify the object across the cache.

Escalation criteria

• Stale data persists after applying the relevant fix above.
• Multiple unrelated mutations exhibit the same cache inconsistency, suggesting broader cache corruption.
• You need custom logic that cannot be expressed with standard update or refetchQueries (e.g., conditional updates based on other cache entries).
• Consider enabling cacheRedirects in InMemoryCache to map fields to existing cache entries.
• Or use apollo-link-state to manage local state alongside remote data.
• If the issue remains unresolved, prepare a minimal reproduction (e.g., using MockedProvider) and open a GitHub issue on the Apollo Client repository, including:
  • Apollo Client version (e.g., 3.9.0).
  • Relevant code snippets (mutation call, cache config).
  • Network payload and cache snapshot screenshots.

Verification

1. After applying a fix, re‑run the mutation and confirm the network response contains the expected data (status 200).
2. Open Apollo Client DevTools → Cache tab and verify that the updated fields now reflect the new values.
3. In a unit test, render the component with MockedProvider, mock the mutation to return the new data, and assert that the UI displays the updated information after the mutation resolves.