Cache invalidation strategies in Express - Step-by-Step Execution

Concept Flow - Cache invalidation strategies

Request comes in

↓

Check cache for data

↓

Serve response

↓

Trigger cache invalidation?

↓

End process

This flow shows how a request checks cache first, serves cached data if found, otherwise fetches fresh data and caches it. Cache invalidation happens after data changes to keep cache fresh.

Execution Sample

Express

app.get('/data', async (req, res) => {
  const cached = cache.get('data');
  if (cached) return res.send(cached);
  const fresh = await fetchData();
  cache.set('data', fresh);
  res.send(fresh);
});

This code checks cache for 'data'. If found, it returns cached data. Otherwise, it fetches fresh data, caches it, then returns it.

Execution Table

Step	Action	Cache State	Response Sent	Cache Invalidation
1	Request received for /data	{}	No	No
2	Check cache for 'data'	{}	No	No
3	Cache miss, fetch fresh data	{}	No	No
4	Store fresh data in cache	{'data': freshData}	No	No
5	Send fresh data in response	{'data': freshData}	Yes	No
6	Data updated elsewhere, invalidate cache	{}	No	Yes
7	Next request checks cache again	{}	No	No
8	Cache miss again, fetch fresh data	{}	No	No
9	Store fresh data in cache	{'data': freshData2}	No	No
10	Send fresh data in response	{'data': freshData2}	Yes	No

💡 Process ends after response sent and cache updated or invalidated as needed.

Variable Tracker

Variable	Start	After Step 4	After Step 6	After Step 9	Final
cache	{}	{'data': freshData}	{}	{'data': freshData2}	{'data': freshData2}
responseSent	No	No	No	No	Yes

Key Moments - 3 Insights

Why do we check the cache before fetching fresh data?

Checking the cache first (see step 2) avoids unnecessary data fetching, making responses faster and reducing load.

When and why do we invalidate the cache?

Cache is invalidated after data changes (step 6) to prevent serving outdated information, ensuring users get fresh data.

What happens if we don't invalidate the cache after data updates?

Without invalidation, the cache keeps old data, so users may see stale responses, causing inconsistency.

Visual Quiz - 3 Questions

Look at the execution table, what is the cache state after step 4?

A {'data': freshData2}

B {}

C {'data': freshData}

D null

💡 Hint

Check the 'Cache State' column at step 4 in the execution_table.

At which step does cache invalidation occur?

A Step 6

B Step 5

C Step 3

D Step 9

💡 Hint

Look at the 'Cache Invalidation' column for the step marked 'Yes'.

If cache invalidation did not happen at step 6, what would be the cache state at step 7?

A {}

B {'data': freshData}

C {'data': freshData2}

D null

💡 Hint

Refer to variable_tracker for cache state changes and consider what happens if invalidation is skipped.

Concept Snapshot

Cache Invalidation Strategies in Express:
- Check cache before fetching data.
- Serve cached data if available.
- Fetch and cache fresh data if cache miss.
- Invalidate cache after data changes to avoid stale data.
- Common invalidation: remove or update cache entries.
- Keeps responses fast and data fresh.

Full Transcript

Cache invalidation strategies in Express involve checking if requested data is already stored in cache. If yes, the cached data is sent immediately, speeding up response. If not, fresh data is fetched from the source, stored in cache, then sent. When data changes elsewhere, the cache must be invalidated by removing or updating the cached entry. This prevents serving outdated information. The process repeats for each request, ensuring fast and fresh responses.