The messages that pass through a flow are plain JavaScript objects that can have properties set on them.

They usually have a payload property - this is the default property that most nodes will work with.

When creating flows, the choice of properties used on a message will largely be determined by what the nodes in the flow require.

• payload as the main content
• properties at top level, vs everything in payload
• Take care of node side-effects (msg.topic for example)
• Use properties to identify the origin of a message
• Preserve message properties
• Cloning messages