Middleware pour Step Functions : stocker et charger automatiquement les charges utiles depuis Amazon S3

J'aimerais vous présenter middy-store, une nouvelle bibliothèque que j'ai construite au cours des derniers mois. Je réfléchis à cette idée depuis un moment, revenant sur cette demande de fonctionnalité que j'ai ouverte il y a plus d'un an. middy-store est un middleware pour Middy qui stocke et charge automatiquement les charges utiles depuis et vers un magasin comme Amazon S3 ou potentiellement d'autres services.

Motivation

Les services AWS ont certaines limites dont il faut être conscient. Par exemple, AWS Lambda a une limite de charge utile de 6 Mo pour les appels synchrones et de 256 Ko pour les appels asynchrones. AWS Step Functions autorise une taille d'entrée ou de sortie maximale de 256 Ko de données sous forme de chaîne codée en UTF-8. Si vous dépassez cette limite lors du renvoi des données, vous rencontrerez la tristement célèbre exception States.DataLimitExceeded.

La solution habituelle pour contourner cette limitation consiste à vérifier la taille de votre charge utile et à la sauvegarder temporairement dans un stockage persistant tel qu'Amazon S3. Ensuite, vous renvoyez l’URL ou l’ARN de l’objet pour S3. Le Lambda suivant vérifie s'il y a une URL ou un ARN dans l'entrée et charge la charge utile depuis S3. Comme on peut l'imaginer, cela entraîne beaucoup de code passe-partout pour stocker et charger la charge utile depuis et vers Amazon S3, qui doit être répété dans chaque Lambda.

Cela devient encore plus fastidieux lorsque vous souhaitez uniquement enregistrer une partie de la charge utile sur S3 et laisser le reste tel quel. Par exemple, lorsque vous travaillez avec Step Functions, la charge utile peut contenir des données de flux de contrôle pour des états comme Choice ou Map, auxquelles il faut accéder directement. Cela signifie que le premier Lambda enregistre une charge utile partielle sur S3, et que le Lambda suivant doit charger la charge utile partielle de S3 et la fusionner avec le reste de la charge utile. Cela nécessite de s'assurer que les types sont cohérents dans plusieurs fonctions, ce qui est bien sûr très sujet aux erreurs.

Comment ça marche

middy-store est un middleware pour Middy. Il est attaché à une fonction Lambda et est appelé deux fois lors d'une invocation Lambda : avant et après le gestionnaire Lambda() s'exécute. Il reçoit l'entrée avant l'exécution du gestionnaire et reçoit la sortie du gestionnaire une fois l'exécution terminée.

Commençons par la fin avec la sortie après une invocation réussie pour faciliter le suivi : middy-store reçoit la sortie (la charge utile) de la fonction handler() et vérifie la taille. Pour calculer la taille, il chaîne la charge utile, s'il s'agit d'un objet, et utilise Buffer.byteLength() pour calculer la taille de la chaîne codée en UTF-8. Si la taille est supérieure à un certain seuil configurable, la charge utile est stockée dans un Store comme Amazon S3. La référence à la charge utile stockée (par exemple, une URL ou un ARN S3) est ensuite renvoyée en tant que sortie au lieu de la sortie d'origine.

Regardons maintenant la prochaine fonction Lambda (par exemple dans une machine à états), qui recevra cette sortie comme entrée. Cette fois, nous examinons l'entrée avant que le handler() soit invoqué : middy-store reçoit l'entrée du gestionnaire et recherche une référence à une charge utile stockée. S'il en trouve un, la charge utile est chargée depuis le Store et renvoyée en entrée au gestionnaire. Le gestionnaire utilise la charge utile comme si elle lui avait été transmise directement.

Voici un exemple pour illustrer le fonctionnement du magasin intermédiaire :

/* ./src/functions/handler1.ts */
export const handler1 = middy()
  .use(
    middyStore({
      stores: [new S3Store({ /* S3 options */ })],
    })
  )
  .handler(async (input) => {
    // Return 1MB of random data as a base64 encoded string as output 
    return randomBytes(1024 * 1024).toString('base64');
  });

/* ./src/functions/handler2.ts */
export const handler2 = middy()
  .use(
    middyStore({
      stores: [new S3Store({ /* S3 options */ })],
    })
  )
  .handler(async (input) => {
    // Print the size of the input
    return console.log(`Size: ${Buffer.from(input, "base64").byteLength / 1024 / 1024} MB`);
  });

/* ./src/workflow.ts */
// First Lambda returns a large output
// It automatically uploads the data to S3 
const output1 = await handler1({});

// Output is a reference to the S3 object: { "@middy-store": "s3://bucket/key"}
console.log(output1); 

// Second Lambda receives the output as input
// It automatically downloads the data from S3
const output2 = await handler2(output1);

Qu'est-ce qu'un magasin ?

En général, un Store est tout service qui vous permet de stocker et de charger des charges utiles arbitraires, comme Amazon S3 ou d'autres systèmes de stockage persistants. Les bases de données comme DynamoDB peuvent également faire office de Store. Le Store reçoit une charge utile du gestionnaire Lambda, la sérialise (s'il s'agit d'un objet) et la stocke dans un stockage persistant. Lorsque le prochain gestionnaire Lambda a besoin de la charge utile, le Store charge la charge utile depuis le stockage, la désérialise et la renvoie.

middy-store interagit avec un Store via une interface StoreInterface, que chaque Store doit implémenter. L'interface définit les fonctions canStore() et store() pour stocker les charges utiles, et canLoad() et load() pour charger les charges utiles.

interface StoreInterface<TPayload = unknown, TReference = unknown> {
  name: string;
  canLoad: (args: LoadArgs<unknown>) => boolean;
  load: (args: LoadArgs<TReference | unknown>) => Promise<TPayload>;
  canStore: (args: StoreArgs<TPayload>) => boolean;
  store: (args: StoreArgs<TPayload>) => Promise<TReference>;
}

• canStore() sert de garde pour vérifier si le Store peut stocker une charge utile donnée. Il reçoit la charge utile et sa taille en octets et vérifie si la charge utile correspond aux limites de taille maximale du magasin. Par exemple, un magasin soutenu par DynamoDB a une taille d'élément maximale de 400 Ko, tandis qu'un magasin S3 n'a en réalité aucune limite sur la taille de la charge utile qu'il peut stocker.

• store() receives a payload and stores it in its underlying storage system. It returns a reference to the payload, which is a unique identifier to identify the stored payload within the underlying service. For example, the Amazon S3 Store uses an S3 URI in the format s3:/// as a reference, while other Amazon services might use ARNs.

• canLoad() acts like a filter to check if the Store can load a certain reference. It receives the reference to a stored payload and checks if it's a valid identifier for the underlying storage system. For example, the Amazon S3 Store checks if the reference is a valid S3 URI, while a DynamoDB Store would check if it's a valid ARN.

• load() receives the reference to a stored payload and loads the payload from storage. Depending on the Store, the payload will be deserialized into its original type according to the metadata that was stored alongside it. For example, a payload of type application/json will get parsed back into a JSON object, while a plain string of type text/plain will remain unaltered.

Single and Multiple Stores

Most of the time, you will only need one Store, like Amazon S3, which can effectively store any payload. However, middy-store lets you work with multiple Stores at the same time. This can be useful if you want to store different types of payloads in different Stores. For example, you might want to store large payloads in S3 and small payloads in DynamoDB.

middy-store accepts an Array in the options to provide one or more Stores. When middy-store runs before the handler and finds a reference in the input, it will iterate over the Stores and call canLoad() with the reference for each Store. The first Store that returns true will be used to load the payload with load().

On the other hand, when middy-store runs after the handler and the output is larger than the maximum allowed size, it will iterate over the Stores and call canStore() for each Store. The first Store that returns true will be used to store the payload with store().

Therefore, it is important to note that the order of the Stores in the array is important.

References

When a payload is stored in a Store, middy-store will return a reference to the stored payload. The reference is a unique identifier to find the stored payload in the Store. The value of the identifier depends on the Store and its configuration. For example, the Amazon S3 Store will use an S3 URI by default. However, it can also be configured to return other formats like an ARN arn:aws:s3:::/, an HTTP endpoint https://.s3.us-west-1.amazonaws.com/, or a structured object with the bucket and key.

The output from the handler after middy-store will contain the reference to the stored payload:

/* Output with reference */
{
  "@middy-store": "s3://bucket/key"
}

middy-store embeds the reference from the Store in the output as an object with a key "@middy-store". This allows middy-store to quickly find all references when the next Lambda function is called and load the payloads from the Store before the handler runs. In case you are wondering, middy-store recursively iterates through the input object and searches for the "@middy-store" key. That means the input can contain multiple references, even from different Stores, and middy-store will find and load them.

Selecting a Payload

By default, middy-store will store the entire output of the handler as a payload in the Store. However, you can also select only a part of the output to be stored. This is useful for workflows like AWS Step Functions, where you might need some of the data for control flow, e.g., a Choice state.

middy-store accepts a selector in its storingOptions config. The selector is a string path to the relevant value in the output that should be stored.

Here's an example:

const output = {
  a: {
    b: ['foo', 'bar', 'baz'],
  },
};

export const handler = middy()
  .use(
    middyStore({
      stores: [new S3Store({ /* S3 options */ })],
      storingOptions: {
        selector: '',          /* select the entire output as payload */
        // selector: 'a';      /* selects the payload at the path 'a' */
        // selector: 'a.b';    /* selects the payload at the path 'a.b' */
        // selector: 'a.b[0]'; /* selects the payload at the path 'a.b[0]' */
        // selector: 'a.b[*]'; /* selects the payloads at the paths 'a.b[0]', 'a.b[1]', 'a.b[2]', etc. */
      }
    })
  )
  .handler(async () => output);

await handler({});

The default selector is an empty string (or undefined), which selects the entire output as a payload. In this case, middy-store will return an object with only one property, which is the reference to the stored payload.

/* selector: '' */
{
  "@middy-store": "s3://bucket/key"
}

The selectors a, a.b, or a.b[0] select the value at the path and store only this part in the Store. The reference to the stored payload will be inserted at the path in the output, thereby replacing the original value.

/* selector: 'a' */
{
  a: {
    "@middy-store": "s3://bucket/key"
  }
}
/* selector: 'a.b' */
{
  a: {
    b: {
      "@middy-store": "s3://bucket/key"
    }
  }
}
/* selector: 'a.b[0]' */
{
  a: {
    b: [
      { "@middy-store": "s3://bucket/key" }, 
      'bar', 
      'baz'
    ]
  }
}

A selector ending with [*] like a.b[*] acts like an iterator. It will select the array at a.b and store each element in the array in the Store separately. Each element will be replaced with the reference to the stored payload.

/* selector: 'a.b[*]' */
{
  a: {
    b: [
      { "@middy-store": "s3://bucket/key" }, 
      { "@middy-store": "s3://bucket/key" }, 
      { "@middy-store": "s3://bucket/key" }
    ]
  }
}

Size Limit

middy-store will calculate the size of the entire output returned from the handler. The size is calculated by stringifying the output, if it's not already a string, and calculating the UTF-8 encoded size of the string in bytes. It will then compare this size to the configured minSize in the storingOptions config. If the output size is equal to or greater than the minSize, it will store the output or a part of it in the Store.

export const handler = middy()
  .use(
    middyStore({
      stores: [new S3Store({ /* S3 options */ })],
      storingOptions: {
        minSize: Sizes.STEP_FUNCTIONS,  /* 256KB */
        // minSize: Sizes.LAMBDA_SYNC,  /* 6MB */
        // minSize: Sizes.LAMBDA_ASYNC, /* 256KB */
        // minSize: 1024 * 1024,        /* 1MB */
        // minSize: Sizes.ZERO,         /* 0 */
        // minSize: Sizes.INFINITY,     /* Infinity */
        // minSize: Sizes.kb(512),      /* 512KB */
        // minSize: Sizes.mb(1),        /* 1MB */
      }
    })
  )
  .handler(async () => output);

await handler({});

middy-store provides a Sizes helper with some predefined limits for Lambda and Step Functions. If minSize is not specified, it will use Sizes.STEP_FUNCTIONS with 256KB as the default minimum size. The Sizes.ZERO (equal to the number 0) means that middy-store will always store the payload in a Store, ignoring the actual output size. On the other hand, Sizes.INFINITY (equal to Math.POSITIVE_INFINITY) means that it will never store the payload in a Store.

Stores

Currently, there is only one Store implementation for Amazon S3, but I'm planning to implement a Store backed by DynamoDB and DAX. DynamoDB, with its Time-To-Live (TTL) feature, provides a great option for short-term payloads that only need to exist during the execution of a workflow like Step Functions.

Amazon S3

The middy-store-s3 package provides a store implementation for Amazon S3. It uses the official @aws-sdk/client-s3 package to interact with S3.

import { middyStore } from 'middy-store';
import { S3Store } from 'middy-store-s3';

const handler = middy()
  .use(
    middyStore({
      stores: [
        new S3Store({
          config: { region: "us-east-1" },
          bucket: "bucket",
          key: () => randomUUID(),
          format: "arn",
        }),
      ],
    }),
  )
  .handler(async (input) => {
    return { /* ... */ };
  });

The S3Store only requires a bucket where the payloads are being stored. The key is optional and defaults to randomUUID(). The format configures the style of the reference that is returned after a payload is stored. The supported formats include arn, object, or one of the URL formats from the amazon-s3-url package. It's important to note that S3Store can load any of these formats; the format config only concerns the returned reference. The config is the S3 client configuration and is optional. If not set, the S3 client will resolve the config (credentials, region, etc.) from the environment or file system.

Custom Store

A new Store can be implemented as a class or a plain object, as long as it provides the required functions from the StoreInterface interface.

Here's an example of a Store to store and load payloads as base64 encoded data URLs:

import { StoreInterface, middyStore } from 'middy-store';

const base64Store: StoreInterface<string, string> = {
  name: "base64",
  /* Reference must be a string starting with "data:text/plain;base64," */
  canLoad: ({ reference }) => {
    return (
      typeof reference === "string" &&
      reference.startsWith("data:text/plain;base64,")
    );
  },
  /* Decode base64 string and parse into object */
  load: async ({ reference }) => {
    const base64 = reference.replace("data:text/plain;base64,", "");
    return Buffer.from(base64, "base64").toString();
  },
  /* Payload must be a string or an object */
  canStore: ({ payload }) => {
    return typeof payload === "string" || typeof payload === "object";
  },
  /* Stringify object and encode as base64 string */
  store: async ({ payload }) => {
    const base64 = Buffer.from(JSON.stringify(payload)).toString("base64");
    return `data:text/plain;base64,${base64}`;
  },
};

const handler = middy()
  .use(
    middyStore({
      stores: [base64Store],
      storingOptions: {
        minSize: Sizes.ZERO, /* Always store the data */ 
      }
    }),
  )
  .handler(async (input) => {
    /* Random text with 100 words */ 
    return `Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.`;
  });

const output = await handler(null, context);

/* Prints: { '@middy-store': 'data:text/plain;base64,IkxvcmVtIGlwc3VtIGRvbG9yIHNpdC...' } */
console.log(output);

This example is the perfect way to try middy-store, because it doesn't rely on external resources like an S3 bucket. You will find it in the repository at examples/custom-store and should be able to run it locally.

Contributions and Feedback

I've been tinkering with the API design for a while, and it's definitely not stable yet. I would love to get feedback on the current state as well as suggestions for changes or improvements. If you are eager to contribute to this project, please go ahead and submit feature requests or pull requests.

zirkelc / middy-store

Middleware for Step Functions: Automatically Store and Load Payloads

Middleware middy-store

middy-store is a middleware for Lambda that automatically stores and loads payloads from and to a Store like Amazon S3 or potentially other services.

Installation

You will need @middy/core >= v5 to use middy-store Please be aware that the API is not stable yet and might change in the future. To avoid accidental breaking changes, please pin the version of middy-store and its sub-packages in your package.json to an exact version.

npm install --save-exact @middy/core middy-store middy-store-s3 

Motivation

AWS services have certain limits that one must be aware of. For example, AWS Lambda has a payload limit of 6MB for synchronous invocations and 256KB for asynchronous invocations. AWS Step Functions allows for a maximum input or output size of 256KB of data as a UTF-8 encoded string. If you exceed this limit when returning data, you will encounter the infamous States.DataLimitExceeded exception.

The usual workaround for this…

View on GitHub

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