Inter node IPC with bash

Inter-node IPC with bash

Introduction

Gsatellite uses a bash library that provides inter-node IPC when used on a shared file system like NFS. This library provides message boxes (similar to message queues) and send and receive functions.

Usage

Include the library

To enable bash programs to use this feature you have to source the library in the respective programs:

[...]
. $_LIB/ipc/file.bashlib
[...]

In addition you also need a directory shared between the hosts or cluster you want to operate on that will be the base directory for message boxes. This is defined by the variable _MBOXES. Hence also add a definition for this variable:

[...]
readonly _MBOXES="$HOME/mboxes"

. $_LIB/ipc/file.bashlib
[...]

Create message boxes

Each process needs its own message box, so create one per process:

[...]
_self="$$"
_inboxName="$_self.inbox"

#  create inbox
_inbox=$( ipc/file/createMsgBox "$_inboxName" )
[...]

By default this should use the own PID as first part of the name plus a '.' and an arbitrary string afterwards. The example above shows a convenient naming. ipc/file/createMsgBox() will return the full path to the message box. This includes the "$_MBOXES" string as prefix and the fully qualified domain name (FQDN) of the host the process is running on as super directory, e.g. /home/you/mboxes/yourhost.domain/12345.inbox. If you want a special message box name for a specific process - maybe because you don't want to remember the PIDs of your processes, you can create an alias message box in addition:

[...]
#  create alias inbox
_aliasInboxName="MySpecialProcessA"
_aliasInbox=$( ipc/file/createAliasMsgBox "$_inbox" "$_aliasInboxName" )
[...]

A process can then also be reached via its alias message box.

NOTICE: The name for an alias message box has to be unique per host, but not per cluster!

Send messages

After creating a message box you're ready to send and receive messages. This is done with the library function ipc/file/sendMsg(). This function has two arguments:

1. _messageBox: The destination message box
2. _message: The actual message

You're basically free to use arbitrary strings as messages, but to enable a receiver to detect from which sender a specific message came from, the proposal is to use a string formed like the following:

<MESSAGE_TEXT>;<ANSWER_BOX>

Receive message