The FBP Language/ Links & Data

Links & Data

Links & Data

A link connects one node's output port to another node's input port:

A out -> in B

Packets written to A's out port travel the link and arrive on B's in port. Links can be chained on a single line by continuing the right-hand side:

A out -> in B out -> in C out -> in D

This is the same as three separate links, and chains up to four hops per line parse the same way.

Initial information packets (IIPs)

An IIP is a literal value fed directly into a port — no source node:

'Hello World!' -> msg Log(console/log)

The left-hand side can be a quoted string or a JSON literal:

{"title": "test"} -> in MyProc
["a", "b", "c"] -> in MyProc

An IIP is not a link: it appears in the flow model as initial data, delivered when the flow starts.

Context feeds

Prepending @ to the target port turns the data into context — config stored on the node, not a wired link:

'Hello World!' -> @msg Log(console/log)

Now the value travels inside the flow definition itself (it shows up under nodes[].context in the compiled model), and the node's input is fulfilled by context, so the node starts without any link. This is how a flow distributes configuration that should live with the definition rather than with the wiring.

$ fbpx convert hello.fbp --yaml
nodes:
  - id: Log
    ns: console
    name: log
    context:
      msg: Hello World!

A few modifiers attach to the arrow or the ports to change how data travels.

^ — persist

A ^ before a target port keeps the port's last value, so the node can re-read it on every run without new data:

Test out -> ^IN SomeNode

The port holds its last packet and read() returns a fresh clone of it after it has been drained.

[index] — extraction

A [N] (or [name]) after a source port pulls one element out of the packet before it travels:

GetCommands out [0] -> in BuildPath
Message event [srcElement] -> msg Log

[0] extracts element zero from an array packet; [srcElement] extracts the named property from an object packet. This is data extraction on the wire — the downstream node receives the single element, not the whole structure.

Masks

A -(/path)> arrow extracts an object path from each packet:

{"title": "my title"} -> in MyProc out -(/title)> in Proc2

Each packet that crosses the link is masked down to the value at /title. Masks take either an object mapping names to paths or a single path.

=> and >= — streams

Two arrows change how a whole batch of packets behaves rather than a single packet. They are documented in full on the Streams page:

["a", "b", "c"] => in Echo(stream/echo)   # one delivery per element
Echo out >= in Collect(console/log)        # one array when the batch closes

Export declarations

A -> or <- with nothing on the left is not an IIP or a link — it declares an external port on the flow itself:

-> in:ExtName Process
<- out:ExtName Process
  • -> in:ExtName Process declares an external input port ExtName, wired to Process's in port.
  • <- out:ExtName Process declares an external output port ExtName, fed by Process's out port.

The colon renames the port for the outside world (in:command exposes command). The rename is optional:

-> in Process     # external port named "in"
<- out Click      # external port named "out"

Exports land in the flow's input/output ports, which is exactly what lets a flow be used as a node elsewhere — the subgraph's exports are its ports.

A chain can pass through an export:

-> in d(utils/dummy) out -> in Click

declares d's in as an external input, then links d.out onward to Click.

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