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Pipeline for zipping multiple file chanel into a single output channel
I am in need of downloading some files (from dynamic list of URLs) and archive them into a single zip file. Since these files can be arbitrarily sized (totaling hundreds of MB), I want to avoid downloading each entry individually, creating the entire archive in-memory and then uploading it, but rather make a sort of streamed pipeline for it.
In short, I want to have something going through the list of URLs, streaming each file into a ZipOutputStream (adding a new entry per file), and another thing consuming that stream (ktor multipart request, for example).
I have tried creating a ByteChannel and passing .toOutputStream() to a ZipOutputStream. Then, spawn a background job to grab each URL, create a zip entry and write packets into the stream, and finally return the channel wrapper in a ChannelProvider to pass into FormBuilder#append.
However, sometimes ktor times out, sometimes it doesn't and I manage to upload the archive as a multipart request, but it is corrupt.
Has anyone tried anything similar? If so, how have you tackled it?
Here's the code I have:
ChannelProvider {
val fileChannel = ByteChannel()
val zos = ZipOutputStream(fileChannel.toOutputStream())
coroScope.launch {
files.forEach { f ->
httpClient.get(f.url).reader {
zos.putNextEntry(ZipEntry(f.filename))
while (!channel.isClosedForRead) {
channel.readRemaining(DEFAULT_BUFFER_SIZE.toLong()).let {
zos.writePacket(it)
}
}
zos.closeEntry()
}
}
zos.close()
fileChannel.close()
}
fileChannel
}
1 answer
The following users marked this post as Works for me:
| User | Comment | Date |
|---|---|---|
| tmpod |
Thread: Works for me OP's own solution. |
Sep 12, 2023 at 00:31 |
Well, ain't this fun hehe
Turns out I failed to see .reader launches its own job, which resulted in multiple coroutines contending for the zip stream, each trying to write an entry... 🤦
The solution was just to get a channel and use that instead. Additionally, to mitigate the still blocking nature of Java's ZipOutputStream, I spawned the job in the appropriate Dispatchers.IO.
Here it is:
ChannelProvider {
val fileChannel = ByteChannel()
val zos = ZipOutputStream(fileChannel.toOutputStream())
coroScope.launch(Dispatchers.IO) {
files.forEach { f ->
val body = httpClient.get(f.url).bodyAsChannel()
zos.putNextEntry(ZipEntry(f.filename))
while (!body.isClosedForRead) {
body.readRemaining(DEFAULT_BUFFER_SIZE.toLong()).let {
zos.writePacket(it)
}
}
zos.closeEntry()
}
zos.close()
fileChannel.close()
}
fileChannel
}
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