Write-through and write-behind caching: when to update Redis on writes

Keeping cache consistent on writes

Cache-aside handles reads. For writes you need a strategy to keep Redis and your database consistent.

Write-through

Write to Redis and the database in the same operation, synchronously. The cache is always up to date.

async function updateUser(userId, data) {
  // Write to DB first
  await db.query('UPDATE users SET name=$1 WHERE id=$2',
    [data.name, userId]);

  // Immediately update cache
  const cacheKey = `user:${userId}`;
  await client.setEx(cacheKey, 300, JSON.stringify(data));
}

Write-through increases write latency (two round trips) but eliminates stale cache entries for that key.

Write-behind (write-back)

Write to Redis immediately and return success. A background worker asynchronously flushes changes to the database.

async function updateUserAsync(userId, data) {
  await client.setEx(`user:${userId}`, 300, JSON.stringify(data));
  await client.rPush('db:write:queue', JSON.stringify({ userId, data }));
  // Worker drains queue and writes to DB
}

Write-behind is faster but risks data loss if Redis crashes before the flush. Use it only when you can tolerate some data loss or have Redis persistence enabled.