How to: When to update the cache
Since you can only store a limited amount of data in cache, you'll need to determine which cache update strategy works best for your use case.

#### Cache-aside
Source: From cache to in-memory data grid
The application is responsible for reading and writing from storage.  The cache does not interact with storage directly.  The application does the following:

* Look for entry in cache, resulting in a cache miss
* Load entry from the database
* Add entry to cache
* Return entry

```python
def get_user(self, user_id):
    user = cache.get("user.{0}", user_id)
    if user is None:
        user = db.query("SELECT * FROM users WHERE user_id = {0}", user_id)
        if user is not None:
            key = "user.{0}".format(user_id)
            cache.set(key, json.dumps(user))
    return user
```

[Memcached](https://memcached.org/) is generally used in this manner.

Subsequent reads of data added to cache are fast.  Cache-aside is also referred to as lazy loading.  Only requested data is cached, which avoids filling up the cache with data that isn't requested.

##### Disadvantage(s): cache-aside

* Each cache miss results in three trips, which can cause a noticeable delay.
* Data can become stale if it is updated in the database.  This issue is mitigated by setting a time-to-live (TTL) which forces an update of the cache entry, or by using write-through.
* When a node fails, it is replaced by a new, empty node, increasing latency.

#### Write-through
Source: Scalability, availability, stability, patterns
The application uses the cache as the main data store, reading and writing data to it, while the cache is responsible for reading and writing to the database:

* Application adds/updates entry in cache
* Cache synchronously writes entry to data store
* Return

Application code:

```python
set_user(12345, {"foo":"bar"})
```

Cache code:

```python
def set_user(user_id, values):
    user = db.query("UPDATE Users WHERE id = {0}", user_id, values)
    cache.set(user_id, user)
```

Write-through is a slow overall operation due to the write operation, but subsequent reads of just written data are fast.  Users are generally more tolerant of latency when updating data than reading data.  Data in the cache is not stale.

##### Disadvantage(s): write through

* When a new node is created due to failure or scaling, the new node will not cache entries until the entry is updated in the database.  Cache-aside in conjunction with write through can mitigate this issue.
* Most data written might never be read, which can be minimized with a TTL.

#### Write-behind (write-back)
Source: Scalability, availability, stability, patterns
In write-behind, the application does the following:

* Add/update entry in cache
* Asynchronously write entry to the data store, improving write performance

##### Disadvantage(s): write-behind

* There could be data loss if the cache goes down prior to its contents hitting the data store.
* It is more complex to implement write-behind than it is to implement cache-aside or write-through.

#### Refresh-ahead
Source: From cache to in-memory data grid
You can configure the cache to automatically refresh any recently accessed cache entry prior to its expiration.

Refresh-ahead can result in reduced latency vs read-through if the cache can accurately predict which items are likely to be needed in the future.

##### Disadvantage(s): refresh-ahead

* Not accurately predicting which items are likely to be needed in the future can result in reduced performance than without refresh-ahead.