Nebulex v1.2.0 Nebulex.Adapters.Replicated View Source
Built-in adapter for replicated cache topology.
The replicated cache excels in its ability to handle data replication, concurrency control and failover in a cluster, all while delivering in-memory data access speeds. A clustered replicated cache is exactly what it says it is: a cache that replicates its data to all cluster nodes.
There are several challenges to building a reliably replicated cache. The
first is how to get it to scale and perform well. Updates to the cache have
to be sent to all cluster nodes, and all cluster nodes have to end up with
the same data, even if multiple updates to the same piece of data occur at
the same time. Also, if a cluster node requests a lock, ideally it should
not have to get all cluster nodes to agree on the lock or at least do it in
a very efficient way (:global
is used for this), otherwise it will scale
extremely poorly; yet in the case of a cluster node failure, all of the data
and lock information must be kept safely.
The best part of a replicated cache is its access speed. Since the data is replicated to each cluster node, it is available for use without any waiting. This is referred to as "zero latency access," and is perfect for situations in which an application requires the highest possible speed in its data access.
However, there are some limitations:
Cost Per Update - Updating a replicated cache requires pushing the new version of the data to all other cluster members, which will limit scalability if there is a high frequency of updates per member.
Cost Per Entry - The data is replicated to every cluster member, so Memory Heap space is used on each member, which will impact performance for large caches.
Based on "Distributed Caching Essential Lessons" by Cameron Purdy.
Options
These options can be set through the config file:
:primary
- The module for the primary storage. The value must be a valid local cache adapter so that the partitioned adapter can store the data in there. For example, you can set theNebulex.Adapters.Local
as value, unless you want to provide another one.
Runtime options
These options apply to all adapter's functions.
:timeout
- The time-out value in milliseconds for the command that will be executed. If the timeout is exceeded, then the current process will exit. This adapter usesTask.await/2
internally, therefore, check the function documentation to learn more about it. For commands likeset_many
andget_many
, if the timeout is exceeded, the task is shutted down but the current process doesn't exit, only the result associated to that task is just skipped in the reduce phase.task_supervisor_opts
- Defines the options passed toTask.Supervisor.start_link/1
when the adapter is initialized.
Example
Nebulex.Cache
is the wrapper around the cache. We can define the local
and replicated cache as follows:
defmodule MyApp.ReplicatedCache do
use Nebulex.Cache,
otp_app: :my_app,
adapter: Nebulex.Adapters.Replicated
defmodule Primary do
use Nebulex.Cache,
otp_app: :my_app,
adapter: Nebulex.Adapters.Local
end
end
Where the configuration for the cache must be in your application environment,
usually defined in your config/config.exs
:
config :my_app, MyApp.ReplicatedCache.Primary,
n_shards: 2,
gc_interval: 3600
config :my_app, MyApp.ReplicatedCache,
primary: MyApp.ReplicatedCache.Primary
For more information about the usage, check out Nebulex.Cache
.
Extended API
This adapter provides some additional functions to the Nebulex.Cache
API.
__primary__
Returns the local cache adapter (the local backend).
__task_sup__
Returns the task supervisor module that manages RPC calls.
__nodes__
Returns the nodes that belongs to the caller Cache.