EMSG Format

EMSG (Email Message Blob) is a compact binary format for storing email content in S3, designed for efficient storage and retrieval.

Overview

EMSG provides a lightweight alternative to JSON for storing email bodies, with:

Binary encoding for smaller size
Forward-compatible versioning
Efficient parsing without full deserialization

Binary Layout

Field	Size	Value
Magic number	4 bytes	`"EMSG"`
Version	1 byte	`0x01`
Flags	4 bytes	Bitmask of the sections that follow
Section 1..N	4 bytes + data each	`[Length 4B][Data...]`, one per flag bit set

Flags

Bit	Flag	Description
0	`FlagPlainText`	Plain-text email body present
1	`FlagHTMLBody`	HTML email body present

S3 Storage

Key Pattern

emails/{YYYY}/{MM}/{DD}/{taskID}.emsg

Example: emails/2026/01/29/550e8400-e29b-41d4-a716-446655440000.emsg

Lifecycle Policies

Configure S3 lifecycle rules to automatically expire old messages:

{
  "Rules": [
    {
      "ID": "expire-old-emsg",
      "Prefix": "emails/",
      "Status": "Enabled",
      "Expiration": {
        "Days": 30
      }
    }
  ]
}

Usage

Encoding

import "github.com/warmbly/warmbly/internal/pkg/emsg"

blob := emsg.New()
blob.SetPlainText("Hello, world!")
blob.SetHTMLBody("<html><body>Hello, world!</body></html>")

data, err := blob.Encode()
// Upload data to S3

Decoding

import "github.com/warmbly/warmbly/internal/pkg/emsg"

// Download data from S3
blob, err := emsg.Decode(data)
if err != nil {
    return err
}

if blob.HasPlainText() {
    text := blob.PlainText()
}

if blob.HasHTMLBody() {
    html := blob.HTMLBody()
}

Code References

Implementation: internal/pkg/emsg/emsg.go

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