Realm.LFS.Functions 1.0.0

Realm LFS Functions

Realm LFS (large file storage) is an extension of the Realm.NET SDK that exposes an abstraction for interacting with binary files that are transparently uploaded to a 3rd party service (e.g. S3/Azure Blob Storage) and their URL is subsequently updated in the Realm object for other clients to consume.

This package supplies a RemoteFileManager implementation for the Realm.LFS that uses an Atlas Function to obtain a pre-signed url, which it then uploads the files to.

Usage

For the most part, just replace byte[] properties with FileData ones:

public class Recipe : RealmObject
{
    public string Name { get; set; }

    public string Summary { get; set; }

    public IList<Ingredient> Ingredients { get; set; }

    // Replace this
    public byte[] Photo { get; set; }

    // with this
    public FileData Photo { get; set; }
}

To initialize the SDK, the minimum configuration you need to do is to configure the remote manager factory:

FileManager.Initialize(new FileManagerOptions
{
    RemoteManagerFactory = (config) => new FunctionsFileManager(config, "MyDataFunction")
});

The FileData class can be constructed from a Stream - if you already have a byte[], that can be used to create a MemoryStream.

When displaying an image from a FileData, the code should look something like:

public void PopulateImage(Recipe recipe)
{
    switch (recipe.Photo.Status)
    {
        case DataStatus.Local:
            var imagePath = recipe.Photo.LocalUrl;
            if (File.Exists(imagePath))
            {
                // we are the device that created the image - display it from disk
                MyImage.ImageSource = new FileImageSource(imagePath);
            }
            else
            {
                // this image was created on another device, but it hasn't uploaded it yet
                // to Blob Storage. Display a placeholder until the status changes to Remote
                MyImage.ImageSource = placeHolderImage;
            }
            break;
        case DataStatus.Remote:
            MyImage.ImageSource = new ImageSource(recipe.Photo.Url);
            break;
    }
}

Atlas Function

This package calls an Atlas Function to obtain a pre-signed url which it then uploads the data to.

Function Signature

You can provide your own implementation for the function itself, but it has to have the following signature:

Payload

{
    Operation: "Upload" | "Download" | "Delete",
    FileId: "string"
}

The Operation field indicates the type of the operation requested - upload, download, or delete the file with id FileId.

Response

The shape of the response depends on the requested operation.

• Operation: Upload:

  {
      Success: true | false,  // Whether the operation completed successfully
      PresignedUrl: "string", // The url to upload the file to
      CanonicalUrl: "string", // The url that can be used to fetch the data from
      Error: "string"         // An error message if the operation failed. Should only be set if Success == false
  }
  

• Operation: Download:

  {
      Success: true | false,  // Whether the operation completed successfully
      Url: "string",          // The url that contains the file. It can be a presigned url or a normal public url
      Error: "string"         // An error message if the operation failed. Should only be set if Success == false
  }
  

• Operation: Delete:

  {
      Success: true | false,  // Whether the operation completed successfully
      Error: "string"         // An error message if the operation failed. Should only be set if Success == false
  }
  

Reference implementation

The following is a reference implementation that uses the S3 SDK to generate pre-signed urls:

import { S3Client, PutObjectCommand, DeleteObjectCommand, GetObjectCommand, HeadObjectCommand, NotFound, S3 } from "@aws-sdk/client-s3";
import { getSignedUrl } from "@aws-sdk/s3-request-presigner";

const publicFiles = true;
const validity = 3600;

async function main(payload) {
    const bucket = context.values.get("S3Bucket");
    const accessKeyId = context.values.get("S3AccessKeyIdValue");
    const secretAccessKey = context.values.get("S3SecretAccessKeyValue");
    const region = context.values.get("S3Region");
    const userId = context.user.id;

    const client = new S3Client({
        region,
        credentials: {
            accessKeyId,
            secretAccessKey
        }
    });

    const getCanonicalUrl = (id) => {
        return `https://${bucket}.s3.${region}.amazonaws.com/${id}`;
    }

    const getMetadata = async (id) => {
        try {
            const headCommand = new HeadObjectCommand({ Bucket: bucket, Key: id });
            const response = await client.send(headCommand);
            return response.Metadata || {};
        } catch (err) {
            // There's a bug with the S3 SDK on Atlas Functions - the type hierarchy is messed up and
            // it doesn't have a name property and instanceof NotFound returns false.
            if (`${err}`.indexOf("NotFound") !== -1) {
                return undefined;
            }

            console.log(JSON.stringify(err));
            throw err;
        }
    }

    try {
        switch (payload.Operation) {
            case "Upload":
                const metadata = await getMetadata(payload.FileId);
                if (metadata !== undefined) {
                    return {
                        Success: false,
                        Error: "Object already exists"
                    };
                }

                const uploadCommand = new PutObjectCommand({
                    Bucket: bucket,
                    Key: payload.FileId,
                    ACL: publicFiles ? "public-read" : "private",
                    Metadata: {
                        userid: userId,
                    },
                });

                const uploadUrl = await getSignedUrl(client, uploadCommand, {
                    expiresIn: validity,
                });

                return {
                    Success: true,
                    PresignedUrl: uploadUrl,
                    CanonicalUrl: getCanonicalUrl(payload.FileId),
                };

            case "Download":
                // If files are publicly accessible, there's no need to generate a signed url
                // for the download path.
                if (publicFiles) {
                    return {
                        Success: true,
                        Url: getCanonicalUrl(payload.FileId),
                    };
                }

                const downloadCommand = new GetObjectCommand({ Bucket: bucket, Key: payload.FileId });
                const downloadUrl = await getSignedUrl(client, downloadCommand, { expiresIn: validity });

                return {
                    Success: true,
                    Url: downloadUrl
                };

            case "Delete":
                const deleteMetadata = await getMetadata(payload.FileId);
                if (deleteMetadata === undefined) {
                    return {
                        Success: false,
                        Error: "Object not found"
                    };
                } else if (deleteMetadata.userid !== userId) {
                    return {
                        Success: false,
                        Error: "User issuing the delete needs to match the user that created the request"
                    };
                }

                const deleteCommand = new DeleteObjectCommand({ Bucket: bucket, Key: payload.FileId });
                const response = await client.send(deleteCommand);
                return {
                    Success: response.$metadata.httpStatusCode === 204
                }
            default:
                return {
                    Success: false,
                    Error: `Unknown operation: ${payload.Operation}`
                };
        }
    } catch (err) {
        console.log(`An error occurred executing the function: ${err}`);

        return {
            Success: false,
            Error: "Internal Error. See logs for more details"
        };
    }
}

exports = main;

Dependencies

This function has the following package dependencies that need to be added via the app services UI:

{
    "@aws-sdk/client-s3": "^3.354.0",
    "@aws-sdk/s3-request-presigner": "^3.354.0"
}

Secrets

This function uses the following values:

1. S3Bucket: the bucket where your data will be uploaded - e.g. realm-data-files.
2. S3Region: the region where the bucket is located - e.g. us-east-1.
3. S3AccessKeyIdValue: a value linking to the S3 Access Key Id stored as a secret.
4. S3SecretAccessKeyValue: a value linking to the S3 Secret Access Key stored as a secret.