When working with Azure Cache for Redis, one of the key aspects is understanding and configuring cache and expiration policies. These policies instruct the Redis Cache on how long to keep the data before discarding or replacing it. Configuration and application of these policies can directly affect the performance, stability, and accommodation of your Azure Cache for Redis.
Understanding Cache Expiration
Cache expiration refers to the set period after which cached data is considered stale and discarded. In Azure Cache for Redis, each key-value pair has its TTL (Time to Live), which defines its lifespan in the cache.
Redis offers two methods to manage the expiration:
- Time-to-Live (TTL): This Set’s a key-value pair’s lifespan in seconds.
- Time-to-Idle (TTI): This sets a key-value pair’s lifespan based on the last time it was accessed.
These two methods enable a fine level of control over the datalife in the cache.
Configuring Cache Expiration in Azure Cache for Redis
In Azure Cache for Redis, setting the TTL is done using the EXPIRE command. This command assigns a specified TTL to a key. The command syntax is:
EXPIRE key seconds
For example:
EXPIRE mykey 60
This command sets the TTL of the key, mykey, to 60 seconds.
Unfortunately, Azure Redis Cache doesn’t natively support TTI. The closest approximation would be the EXPIREAT command, which sets an absolute Unix timestamp for expiration, or removing the expiration using the PERSIST command when data is accessed. You can implement more sophisticated TTI-like behavior by using Redis scripts and additional keys to track access times.
Understanding Cache Eviction Policies
When the memory fills up, Redis needs to replace old data to accommodate new data; here, Eviction policies come into the picture. Redis offers six different eviction policies:
- No eviction: This policy does not remove any data. If the memory limit is reached, Redis will return an error on trying to insert more data.
- All keys LRU (Least Recently Used): This policy eliminates less recently used keys first.
- Volatile LRU (Least Recently Used): This policy discards the least recently used keys out of all keys with an “expire” set.
- All keys random: This policy randomly selects keys to remove.
- Volatile random: This policy randomly selects keys from the set of keys with the “expire” field.
- Volatile TTL (Time To Live): This policy removes keys with the nearest expire time (shortest TTL)
Redis’ default eviction policy is no-eviction.
Configuring Cache Eviction Policies in Azure Cache for Redis
The eviction policy applied can be configured using the maxmemory-policy configuration setting. This can be done via Azure portal, Azure CLI or PowerShell.
For Azure Portal, follow the steps:
- Go to your Redis Cache instance.
- Click on
Advanced settingsunder theSettingssection. - From the
Maxmemory policydropdown, select the eviction policy of your choice.
In Azure CLI, you can do so with the following command:
az redis update --name MyCache --resource-group MyResourceGroup --set redisConfiguration.maxmemory-policy=allkeys-lru
This command sets the policy to allkeys-lru, meaning it would remove the least recently used keys first when the memory is full.
Configuring cache and expiration policies for Azure Cache for Redis are integral to managing your cache’s performance and capacity. Adhering to the best practices for your particular application’s needs can make a significant difference in the overall functioning of your application. Therefore, a solid understanding and careful selection of these policies would be an expectation in the AZ-204 Developing Solutions for Microsoft Azure exam.
Practice Test
True or False: Azure Cache for Redis provides an in-memory data store based on open-source Redis.
- True
- False
Answer: True.
Explanation: Azure Cache for Redis indeed provides an in-memory data store based on the open-source Redis.
Which of the following HTTP methods will enable you to configure the Azure Cache for Redis?
- 1) PUT
- 2) GET
- 3) POST
- 4) DELETE
Answer: POST
Explanation: The POST method is used to configure settings in Azure Cache for Redis.
The __________ parameter allows you to customise the time a cache item can remain in the cache if it has not been accessed.
- SlidingExpiration
- AbsoluteExpiration
- ItemPriority
- Timeout
Answer: SlidingExpiration
Explanation: The SlidingExpiration cache parameter allows items to stay in the cache as long as they are being frequently accessed.
True or False: You cannot configure the cache policy to remove items based on priority.
- True
- False
Answer: False
Explanation: Azure Cache for Redis allows you to configure the cache policy to remove items based on priority.
Which one of the following is not a valid expiration policy in Azure Cache for Redis?
- Absolute expiration
- Sliding expiration
- Rolling expiration
- No expiration
Answer: Rolling expiration
Explanation: The rolling expiration is not a valid policy. The valid policies include: absolute, sliding and no expiration.
True or False: Cache priority in Azure Cache for Redis is used primarily for managing memory and does not directly affect latency or throughput.
- True
- False
Answer: True
Explanation: Cache priority is primarily used for memory management. While it indirectly affects latency and throughput, it does not directly affect these attributes.
Tick the boxes of valid eviction policies support by Azure Cache for Redis:
- LRU: Least Recently Used
- LFU: Least Frequently Used
- MRU: Most Recently Used
- MFU: Most Frequently Used
Answer: LRU: Least Recently Used and LFU: Least Frequently Used
Explanation: Azure Cache for Redis supports two eviction policies: least recently used and least frequently used.
True or False: Azure Cache for Redis pricing is based on the quantity of cache used.
- True
- False
Answer: True
Explanation: Azure Cache for Redis is priced based on the capacity of memory used and the features chosen.
You can set cache expiration in Azure Cache for Redis to _______.
- Never Expire
- Expire after a set duration
- Expire after not being accessed for a certain time frame
- All of the above
Answer: All of the above
Explanation: All three options are valid configurations for cache expiration in Azure Cache for Redis.
True or False: Azure Cache for Redis offers data persistence feature.
- True
- False
Answer: True
Explanation: Azure Cache for Redis provides data persistence capabilities that help to recover cache data in the case of a planned reboot or an unplanned issue.
The _________ setting will remove the cache item from the cache once its shelf life duration has been met.
- SlidingExpiration
- AbsoluteExpiration
- ItemPriority
- Timeout
Answer: AbsoluteExpiration
Explanation: The AbsoluteExpiration setting allows you to specify a fixed duration after which the cache item must be expired.
True or False: Azure Cache for Redis supports eviction policy configuration.
- True
- False
Answer: True
Explanation: Azure Cache for Redis supports configuration of eviction policy including Least Recently Used and Least Frequently Used policies.
True or False: Cache item priority can determine the order for cache eviction.
- True
- False
Answer: True
Explanation: Cache item priority can determine the order for cache eviction during memory pressure.
Which Azure CLI command can be used to update properties of Azure Cache for Redis?
Answer: az redis update
Explanation: The command ‘az redis update’ can be used to update properties of Azure Cache for Redis.
True or False: You cannot use Redis commands to access cache features from any Azure Cache for Redis instance.
- True
- False
Answer: False
Explanation: You can indeed use Redis commands to access cache features from any Azure Cache for Redis instance.
Interview Questions
What is Azure Cache for Redis?
Azure Cache for Redis provides an in-memory data store based on the Redis software. It allows developers to leverage this highly performant cache in their applications to improve performance and scalability.
What are some benefits of using Azure Cache for Redis?
Azure Cache for Redis provides benefits such as high throughput, low latency data access to power fast, scalable Azure applications. It also offers advanced features, such as reliable pub/sub and server-side scripting.
What does configuring cache and expiration policies for Azure Cache for Redis mean?
Configuring cache and expiration policies for Azure Cache for Redis involves defining how long data should be stored in cache and when it should be removed to free up memory resources.
What command is used to set an expiration time on a key in Azure Redis Cache?
The command used to set an expiration time on a key in Azure Redis Cache is “EXPIRE”.
How can you remove an expiration from a key in Azure Cache for Redis?
You can remove an expiration from a key in Azure Cache for Redis by using the PERSIST command.
If you forget to set an expiration for Azure Cache for Redis and the memory fills up, What happens?
If memory fills up, Azure Cache for Redis will start removing keys using the eviction policy set on your cache.
What is eviction policy in Azure Cache for Redis?
An eviction policy decides which keys to remove when the cache is filled up.
What are some eviction policies available in Azure Cache for Redis?
Some eviction policies available are noeviction, allkeys-lru, volatile-lru, allkeys-random, volatile-random, and volatile-ttl.
What is the volatile-lru eviction policy in Azure Cache for Redis?
With the volatile-lru eviction policy, when the cache reaches its maximum limit, Azure Cache for Redis removes less recently used keys first, but only among keys that have an expiration set.
How can you change the eviction policy for Azure Cache for Redis?
You can change the eviction policy in the Azure portal by navigating to the Redis Cache, going to the Advanced settings, and then choosing your desired eviction policy from the drop-down.
What’s the role of the “Time To Live” setting in Azure Cache for Redis?
The “Time To Live” setting helps you determine the length of time an item lives in the cache before it expires.
How can you view cache metrics in Azure Cache for Redis?
You can view cache metrics in Azure Cache for Redis from the Azure portal. Navigate to your Redis Cache, then go to the Monitoring section.
Can you set different eviction policies for different data in Azure Cache for Redis?
No, you can only set one eviction policy per Azure Cache for Redis instance.
Can you change the cache size and tier of Azure Cache for Redis on the fly?
No, to change the cache size or tier, you must create a new Azure Cache for Redis instance with the desired size or tier, and then migrate your application to use the new cache.
How can you reset the data in the cache in Azure Cache for Redis?
You can reset the data in the cache in Azure Cache for Redis by using the FLUSHDB or FLUSHALL command.
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