Devsolved
The Symptom
Symptom
Users clicking the "Export All Transactions" button were complaining that they never received the email with their CSV file. Looking at the AWS console, our background worker EC2 instances were constantly crashing and rebooting. Syslogs revealed `Out of memory: Killed process (ruby)`.
Root Cause Analysis
Root Cause
We used the `axlsx` gem to generate Excel files, and then attached the generated file to an `Export` ActiveRecord model using ActiveStorage (`export.file.attach(io: File.open(temp_file))`). However, because the files were generated locally on the disk, ActiveStorage decided it needed to read the *entire* file into a string in RAM to calculate the MD5 checksum before uploading it to S3. For a 2GB Excel file, this forced Ruby to allocate 2GB of contiguous memory, instantly crashing the 2GB worker node.
Code
ruby
# The fatal flaw: ActiveStorage reading whole files into memory for checksums
class ExportWorker
def perform(export_id)
export = Export.find(export_id)
temp_file = generate_massive_excel(export)
# This line reads the entire 2GB file into RAM instantly
export.file.attach(io: File.open(temp_file), filename: "export.xlsx")
end
endThe abstraction provided by ActiveStorage completely hid the fact that it was not streaming the upload, but rather buffering it into memory.
Resolution
Resolution
We bypassed ActiveStorage's automatic checksum generation for massive files by using the lower-level AWS SDK directly. We implemented a multipart streaming upload directly to S3. Once the file was safely in S3, we created the ActiveStorage `Blob` record manually and attached it to the model without pulling the file back into memory.
Code
ruby
# The fix: Direct multipart S3 streaming
s3_resource = Aws::S3::Resource.new
obj = s3_resource.bucket("exports").object("temp/export.xlsx")
obj.upload_file(temp_file_path) # AWS SDK streams chunks efficiently
blob = ActiveStorage::Blob.create_before_direct_upload!(
filename: "export.xlsx",
byte_size: File.size(temp_file_path),
checksum: "bypass" # Skip memory-heavy checksum
)
export.update(file: blob.signed_id)
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