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How to Create a Cache Module

In this document, you will learn how to build your own Medusa cache module.

Overview​

Medusa provides ready-made modules for cache, including in-memory and Redis modules. If you prefer another technology used for caching in your commerce application, you can build a module locally and use it in your Medusa backend. You can also publish to NPM and reuse it across multiple Medusa backend instances.

In this document, you will learn how to build your own Medusa cache module based on Memcached as an example. This gives you a real-life example of creating the Cache Module. You can follow the general steps with any other caching system or service.

Prerequisites​

If you want to create the Memcached cache module as explained in this guide, you must have Memcached installed and running.

(Optional) Step 0: Prepare Module Directory​

Before you start implementing your module, it's recommended to prepare the directory or project holding your custom implementation.

You can refer to the Project Preparation step in the Create Module documentation to learn how to do that.

Step 1: Create the Service​

Create the file src/services/memcached-cache.ts which will hold your cache service. Note that the name of the file is recommended to be in the format <service_name>-cache. So, if you’re not integrating memcached, you should replace the name with what’s relevant for your module.

Add the following content to the file:

src/services/memcached-cache.ts
import { ICacheService } from "@medusajs/types"

class MemcachedCacheService implements ICacheService {
  async get<T>(key: string): Promise<T> {
    throw new Error("Method not implemented.")
  }
  async set(
    key: string, 
    data: unknown, 
    ttl?: number
  ): Promise<void> {
    throw new Error("Method not implemented.")
  }
  async invalidate(key: string): Promise<void> {
    throw new Error("Method not implemented.")
  }
}

export default MemcachedCacheService

This creates the class MemcachedCacheService that implements the ICacheService interface imported from @medusajs/types. Feel free to rename the class to what’s relevant for your cache service.

In the class, you must implement three methods: get, set, and invalidate. You’ll learn what each of these methods do in the upcoming section.

Step 2: Implement the Methods​

constructor​

The constructor method of a service allows you to prepare any third-party client or service necessary to be used in other methods. It also allows you to get access to the module’s options which are typically defined in medusa-config.js, and to other services and resources in the Medusa backend using dependency injection.

Here’s an example of how you can use the constructor to create a memcached instance and save the module’s options:

src/services/memcached-cache.ts
import { ICacheService } from "@medusajs/types"
import Memcached from "memcached"

const DEFAULT_CACHE_TIME = 30

export type MemcachedCacheModuleOptions = {
  /**
   * Time to keep data in the cache (in seconds)
   */
  ttl?: number

  /**
    * Allow passing the configuration for Memcached client
    */
  location: Memcached.Location
  options?: Memcached.options
}

class MemcachedCacheService implements ICacheService {
  protected readonly memcached: Memcached
  protected readonly TTL: number

  constructor(
    {
      // inject services through dependency injection
      // for example you can access the logger
      logger,
    },
    options: MemcachedCacheModuleOptions
  ) {
    this.memcached = new Memcached(
      options.location, 
      options.options
    )
    this.TTL = options.ttl || DEFAULT_CACHE_TIME
  }
  // ...
}

get​

The get method allows you to retrieve the value of a cached item based on its key. The method accepts a string as a first parameter, which is the key in the cache. It either returns the cached item or null if it doesn’t exist.

Here’s an example implementation of this method for a Memcached service:

src/services/memcached-cache.ts
class MemcachedCacheService implements ICacheService {
  // ...
  async get<T>(cacheKey: string): Promise<T | null> {
    return new Promise((res, rej) => {
      this.memcached.get(cacheKey, (err, data) => {
        if (err) {
          res(null)
        } else {
          if (data) {
            res(JSON.parse(data))
          } else {
            res(null)
          }
        }
      })
    })
  }
}

set​

The set method is used to set an item in the cache. It accepts three parameters:

  1. The first parameter key is a string that represents the key of the data being added to the cache. This key can be used later to get or invalidate the cached item.
  2. The second parameter data is the data to be added to the cache. There’s no defined type for this data.
  3. The third parameter ttl is optional. It’s a number indicating how long (in seconds) the data should be kept in the cache.

Here’s an example of an implementation of this method for a Memcached service:

src/services/memcached-cache.ts
class MemcachedCacheService implements ICacheService {
  // ...
  async set(
    key: string,
    data: Record<string, unknown>,
    ttl: number = this.TTL
  ): Promise<void> {
    return new Promise((res, rej) =>
      this.memcached.set(
        key, JSON.stringify(data), ttl, (err) => {
        if (err) {
          rej(err)
        } else {
          res()
        }
      })
    )
  }
}

invalidate​

The invalidate method removes an item from the cache using its key. By default, the item is removed from the cache when the time-to-live (ttl) expires. The invalidate method can be used to remove the item beforehand.

The method accepts a string as a first parameter, which is the key of the item to invalidate and remove from the cache.

Here’s an example of an implementation of this method for a Memcached service:

src/services/memcached-cache.ts
class MemcachedCacheService implements ICacheService {
  // ...
  async invalidate(key: string): Promise<void> {
    return new Promise((res, rej) => {
      this.memcached.del(key, (err) => {
        if (err) {
          rej(err)
        } else {
          res()
        }
      })
    })
  }
}

Step 3: Export the Service​

After implementing the cache service, you must export it so that the Medusa backend can use it.

Create the file src/index.ts with the following content:

src/index.ts
import { ModuleExports } from "@medusajs/modules-sdk"

import { 
  MemcachedCacheService,
} from "./services/memcached-cache"

const service = MemcachedCacheService

const moduleDefinition: ModuleExports = {
  service,
}

export default moduleDefinition

This exports a module definition, which requires at least a service. If you named your service something other than MemcachedCacheService, make sure to replace it with that.

You can learn more about what other properties you can export in your module definition in the Create a Module documentation.

Step 4: Test your Module​

You can test your module in the Medusa backend by referencing it in the configurations.

To do that, add the module to the exported configuration in medusa-config.js as follows:

medusa-config.js
module.exports = {
  // ...
  modules: { 
    // ...
    cacheService: {
        resolve: "path/to/custom-module/src/index.ts", 
        options: {
          // any necessary options
          ttl: 30, 
          location: "localhost:55000", 
        },
      },
    },
}

Make sure to replace the path/to/custom-module with a relative path from your Medusa backend to your module. You can learn more about module reference in the Create Module documentation.

You can also add any necessary options to the module. The options added in the example above are relevant to the Memcached Module and you can replace them with your own options.

Then, to test the module, run the Medusa backend which also runs your module:

npx medusa develop

(Optional) Step 5: Publish your Module​

You can publish your cache module to NPM. This can be useful if you want to reuse your module across Medusa backend instances, or want to allow other developers to use it.

You can refer to the Publish Module documentation to learn how to publish your module.

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