Return a temp file without creating one on disk

Context of the problem:

Last problem (flattening map of ) was about preparing the data for creating the CSV file. This problem is about creating the CSV file itself. The file is to be created in memory and returned to the caller. The caller is a ReST endpoint that will stream the file to the client. The file is to be deleted as soon as the client has downloaded it.

The size of the file can be quite large, so it's not a good idea to store it on disk, even on the temp folder of the OS. The caller may not download the file and instead re attempt the whole workflow another day. The server will not be restarted often enough to clean up temp folders before eating up disk space.

Solution:

Today's problem is about delaying the file creating until the user requests for it, and then return the file without storing anything in disk. The whole content of the CSV file (headers and rows) will be created and persisted into a DB table. Due to the dynamic nature of the headers, the content will be stored in a CLOB column (which has the capacity to store up to 4 GB of data). So the DB table will have a primary key, a timestamp column and a CLOB column. When the caller requests for the file, the CLOB column will be read, and the file will be created in memory and returned to the caller. This problem is solved in two similar ways: one using org.springframework.core.io.ByteArrayResource and the other using org.springframework.core.io.FileSystemResource.

Solution 1: Using org.springframework.core.io.ByteArrayResource

usecase.tempfilecsv.me.teenageorge.daily.CsvTempFileUsecase.createByteArrayResource()

public ByteArrayResource createByteArrayResource() {
        var csvString = fileRepository.getCsvString();
        return new ByteArrayResource(csvString.getBytes(StandardCharsets.UTF_8));
    }

...end-code-block

Solution 2: Using org.springframework.core.io.FileSystemResource with an in-memory file system

usecase.tempfilecsv.me.teenageorge.daily.CsvTempFileUsecase.createFileSystemResource()

@SneakyThrows(IOException.class)
    public FileSystemResource createFileSystemResource() {
        var csvString = fileRepository.getCsvString();
        Map<String, Object> env = new HashMap<>();
        env.put("create", "true");

        URI uri = URI.create("jar:file:/tmp/temp-fs");

        try (FileSystem inMemoryFs = FileSystems.newFileSystem(uri, env)) {
            String prefix = "tempfile";
            String suffix = ".csv";
            Path tempFile = Files.createTempFile(inMemoryFs.getPath("/"), prefix, suffix);
            Files.writeString(tempFile, csvString);
            return new FileSystemResource(tempFile);
        }
    }

Conclusion:

Solution 1 is simpler and more straightforward. It's also more efficient, as it doesn't require creating a file system in memory.

Source code: daily-problems/tempfilecsv