ZeroMQ

Symbolic Links

While using a staging area is a nice, simple API, it also creates costs for senders. If I already have a 2 GB video file on a camera and I want to send it via FileMQ, the current implementation asks that I copy it to a staging area before it will be sent to subscribers.

One option is to mount the whole content directory (e.g., /home/me/Movies), but this is fragile since it means the application can’t decide to send individual files. It’s everything or nothing.

A simple answer is to implement portable symbolic links. As Wikipedia explains:

A symbolic link contains a text string that is automatically interpreted and followed by the operating system as a path to another file or directory. This other file or directory is called the “target”. The symbolic link is a second file that exists independently of its target. If a symbolic link is deleted, its target remains unaffected.

This doesn’t affect the protocol in any way; it’s an optimization in the server implementation. Let’s make a simple portable implementation:

• A symbolic link consists of a filename with the extension .ln.

• The filename without .ln is the published filename.

• The link file contains one line, which is the real path to the file.

Since we’ve collected all operations on files in a single class (fmq_file), it’s a clean change. When we create a new file object we check if it’s a symbolic link; if so, all read-only actions (get file size, read file) operate on the target file, not the link.