W
Wasp•2d ago
CamBlackwood

Best approach for creating a file on server side and allowing the client to download it

As part of my wasp app, I want the user to be able to request a csv file with certain parameters. That's then sent to the server side, which I then process and create a csv and send back to the user as a download. What's the best way of going about that in Wasp?
3 Replies
kapa.ai
kapa.ai•2d ago
Hi @CamBlackwood. I'm kapa.ai, your friendly support bot. I will try to answer your question by searching through the knowledge base. :wave: ---------------------- To implement server-side CSV generation and allow the client to download it in a Wasp app, you have a couple of viable approaches, depending on your requirements and the infrastructure you use. 1. Custom API Route for File Download You can create a custom API route in your Wasp app that generates the CSV on the server and streams it to the client. While Wasp does not yet have first-class support for custom HTTP routes, you can use the serverSetup function to add custom Express middleware, such as a route for file downloads. This is similar to how you would use express.static for serving static files, but in your case, you would generate the file dynamically and send it as a response. Example approach: 1. Add a custom route in serverSetup: 
   // In src/server.js or wherever your server setup is
   import express from 'express';
   import { generateCsv } from './csvUtils'; // Your CSV generation logic

   export const serverSetup = (app, express) => {
     app.get('/api/download-csv', async (req, res) => {
       const params = req.query; // Get parameters from client
       const csvContent = await generateCsv(params);
       res.setHeader('Content-Type', 'text/csv');
       res.setHeader('Content-Disposition', 'attachment; filename="data.csv"');
       res.send(csvContent);
     });
   };
   
   // In src/server.js or wherever your server setup is
   import express from 'express';
   import { generateCsv } from './csvUtils'; // Your CSV generation logic

   export const serverSetup = (app, express) => {
     app.get('/api/download-csv', async (req, res) => {
       const params = req.query; // Get parameters from client
       const csvContent = await generateCsv(params);
       res.setHeader('Content-Type', 'text/csv');
       res.setHeader('Content-Disposition', 'attachment; filename="data.csv"');
       res.send(csvContent);
     });
   };
2. Call this endpoint from your client to trigger the download. This approach is confirmed as possible in Wasp by community members, who suggest using serverSetup to add custom Express routes for serving files, similar to how you would use express.static for static files. See the discussion here: Discord: How can I add an API to download files under a directory? 2. Using Actions and Returning File Data Alternatively, you could use a Wasp Action to trigger the CSV generation and return the file data (as a string or buffer), then handle the download on the client. However, this is less ideal for large files or streaming, as Actions are designed for JSON responses and not for streaming binary data. 3. Storing the File Temporarily If you want to generate the file, store it temporarily (e.g., in /tmp or a cloud bucket), and then provide a download link, you can combine the above with a presigned URL approach (if using S3 or similar), or serve the file from a temporary directory using a custom route. Summary - For most use cases, adding a custom Express route via serverSetup is the best approach for dynamic file downloads in Wasp. - Generate the CSV on the server in response to a client request, and stream it back with appropriate headers. - If you need to serve static or temporary files, you can also use express.static in serverSetup. If you need to upload files, see the Multer integration examples in the community: Discord: Upload files using multer. If you need more details or code samples, let me know! ---------------------- Note: you can ask me a follow up question by @ mentioning me again :speech_balloon: ---------------------- Sorry @CamBlackwood, I seem to be having a bit of trouble. Don't worry though, I've pinged my support and they will try to fix me up! :technologist: Perhaps try again in 15mins?
CamBlackwood
CamBlackwoodOP•2d ago
@kapa.ai could you go into more details about the actions approach?
kapa.ai
kapa.ai•2d ago
Let me check the knowledge base for you again. One moment! :books: ---------------------- Certainly! Here’s a detailed explanation of the “actions approach” for generating a file (like a CSV) on the server and making it available to the client in Wasp: How Actions Work in Wasp - Actions are server-side functions that can be called from both the client and server code. They are designed for modifying or creating data, but can also be used for custom logic like generating files. - You declare an Action in your .wasp file and implement it in Node.js/TypeScript in your src/ directory. Wasp automatically generates client/server stubs so you can call the Action as a function from your frontend or backend code, and it handles the network communication for you Docs: Working with Actions. Using Actions to Generate and Return a File 1. Declare the Action in your Wasp file: 
   action generateCsv { 
     fn: import { generateCsv } from "@src/actions"
   }
   
   action generateCsv { 
     fn: import { generateCsv } from "@src/actions"
   }
2. Implement the Action in Node.js/TypeScript: - The Action receives arguments from the client (e.g., CSV parameters). - You generate the CSV content as a string (or Buffer). - Return the CSV content from the Action. 
   // src/actions.ts
   export const generateCsv = async (args, context) => {
     // Generate CSV string based on args
     const csv = "col1,col2\nval1,val2"; // Replace with your logic
     return csv;
   };
   
   // src/actions.ts
   export const generateCsv = async (args, context) => {
     // Generate CSV string based on args
     const csv = "col1,col2\nval1,val2"; // Replace with your logic
     return csv;
   };
Wasp uses superjson for serialization, so you can return strings, objects, and other serializable data types Docs: Implementing Actions in Node. 3. Call the Action from the client: 
   import { generateCsv } from 'wasp/client/operations';
const csvContent = await generateCsv({ /* your params */ });
   // Now, trigger a download in the browser
   const blob = new Blob([csvContent], { type: 'text/csv' });
   const url = URL.createObjectURL(blob);
   const a = document.createElement('a');
   a.href = url;
   a.download = 'data.csv';
   a.click();
   URL.revokeObjectURL(url);
   
   import { generateCsv } from 'wasp/client/operations';
const csvContent = await generateCsv({ /* your params */ });
   // Now, trigger a download in the browser
   const blob = new Blob([csvContent], { type: 'text/csv' });
   const url = URL.createObjectURL(blob);
   const a = document.createElement('a');
   a.href = url;
   a.download = 'data.csv';
   a.click();
   URL.revokeObjectURL(url);
Limitations - Actions are not designed for streaming or very large files. The entire file content is serialized and sent as part of the Action response, which is fine for small to moderately sized files (like most CSVs), but not for large or binary files. - You must return serializable data. Wasp (via superjson) can handle strings, objects, and some complex types, but not raw streams. Summary - Use Actions to generate and return file content (like a CSV string) for small to medium files. - On the client, receive the content and trigger a download using browser APIs. - For large files or streaming, consider a custom Express route via serverSetup instead. This approach is fully supported by Wasp’s Actions system and is documented in the official guides Docs: Implementing Actions Docs: Using Actions. ---------------------- Note: you can ask me a follow up question by @ mentioning me again :speech_balloon: ---------------------- Sorry @CamBlackwood, I seem to be having a bit of trouble. Don't worry though, I've pinged my support and they will try to fix me up! :technologist: Perhaps try again in 15mins?

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