Overview
This article provides an overview of High Availability Support offered in the StayLinked Product. Points for discussion will include the prerequisites, the architecture, the protocols and the technology.
Prerequisites for High Availability
Customers who purchase and maintain an annual software maintenance contract for their licensed StayLinked Server are entitled to one HA License Key that can be installed on a designated HA/backup server. This HA license key expires at the end of the software maintenance contract. As the software maintenance contract is renewed, a new HA License Key is issued.
StayLinked’s Unique Thin-Client Architecture is the Enabler
The StayLinked Terminal Session Management software provides a true host-based terminal emulation solution. With StayLinked, the Telnet Client session remains on the host, running within the host-based StayLinked Server process. There is no Telnet Client software running on the device, only the StayLinked Thin-Client software resides on the device. The StayLinked Thin-Client software on the device connects to the StayLinked Server running on the host using StayLinked’s 'Client2Host' UDP-based protocol designed exclusively for wireless environments. Once the device connects to the StayLinked Server, a dedicated Telnet (TCP) connection will be created to the selected Telnet host. StayLinked provides built-in mechanisms to support HA host server implementations, both for the 'Client2Host' connection from the StayLinked Client on the user’s device to the StayLinked Server on the host, and also for the 'Telnet' connection from the StayLinked Server to the Telnet Server.
The Client2Host® Protocol and the Telnet Protocol
The StayLinked Advanced Terminal Session Management solution provides two layers of backup protection for your wireless telnet sessions. In each StayLinked Session, there are two separate IP connections that are being managed, the Client2Host® protocol and the Telnet protocol.
Client2Host® Protocol
This protocol is UDP-based and provides the reliable RF connection between the StayLinked Client, running on the device, and the StayLinked Server, running on the host. This protocol is a critical component of the reliability that StayLinked provides. Instead of running a TCP/IP-based Telnet connection through the unreliable RF network, we use the StayLinked Client2Host® protocol which was designed from the ground-up to provide a reliable, high-performance connection in the RF environment.
Telnet Protocol
This protocol is TCP-based and provides the telnet connection between the StayLinked Server, running on the host, and the Telnet server which should be also running on the same machine. When the StayLinked Server and Telnet server are running on the same machine, the Telnet protocol is transported on the ‘local loop back’ connection which is internal to the machine and is inherently reliable. In some cases, the Telnet server may not be running on the same hardware as the StayLinked Server. Under these conditions, the Telnet protocol is transported over the regular network topology and is naturally more exposed to failure or interruption.
High Availability from the StayLinked Client to the StayLinked Server
With the implementation of a primary and backup StayLinked Server configuration, a customer can select the preferred method for handling a failure of the primary server. One method would involve ‘IP Takeover’ where the backup server assumes the IP identity of the failed primary server. In this case, no changes are made to the StayLinked Client software on the devices as they will be connecting to the same IP, but that IP address will now be assigned to the backup server. Another method would be to configure the StayLinked Client software to be aware of both the primary server IP and the HA/backup server IP. In this case, if the StayLinked Client software cannot make contact with the primary server, it will automatically or manually connect to the HA server. For an automatic failover, the StayLinked client software can be configured to go to the HA server after a pre-configured number of failed attempts to connect to the primary server. If a manual failover method is selected, then the user will be prompted to retry the primary server or to failover to the HA server.
High Availability from the StayLinked Server to the Telnet Server
A standard feature of the StayLinked software is support for up to two backup Telnet servers per Telnet Host Entry. These backup telnet servers are configured as 'Emulation Properties' defined in the 'Telnet Host Entry' using the supplied StayLinked Administrator GUI console application. If the StayLinked Client connects to the StayLinked Server, but the StayLinked Server cannot complete a connection to the selected Telnet server, then the StayLinked Server will attempt to connect to the backup Telnet server(s) if they are specified. If this 'Telnet Failover' event occurs, then the user will be notified on the device screen that they are connecting to a configured backup server and a note will also be recorded in log entries and alert entries so that users and administrators are aware that the failover event occurred and exactly when it happened.
High Availability Setup and Configuration
This section provides information about setup and configuration of the StayLinked product in a High-Availability environment. Topics covered are the installation and configuration of the HA server, replication of the server configuration and configuration of the HA connections on the client and on the server.
Steps to Configure StayLinked on an HA Server
There are only a few steps required to get StayLinked up and running on a High Availability server and ready to take over in the case of a primary server failure.
- Install the StayLinked Server onto the HA backup machine using the same process as with the primary system.
- Connect the StayLinked Administrator to the unique IP address of the HA server.
- Retrieve the Serial Number of the HA Server from the StayLinked Administrator main dialog.
- Submit the Serial Number of the HA Server to StayLinked Sales so that your HA license key can be generated and delivered. (HA licenses are provided as a feature of your StayLinked Software Maintenance Agreement).
- Note: A separate key is required because the serial number of the HA server is not the same as the primary system and StayLinked license keys are generated by serial number.
- Install the HA License Key onto the HA Server using the StayLinked Administrator 'Manage-> Licenses' dialog.
- Set up synchronization of the StayLinked Configuration Files from the Primary Server to the HA Server. Most of these files are found in the 'stay-linked\config' folder on the Primary Server machine.
StayLinked Configuration Files that Require Synchronization
Following are the details of the configuration files that must be synchronized or mirrored from the Primary server to the HA server:
Files for All StayLinked Versions
• stay-linked\config\devices.xml
• stay-linked\config\hosts.xml
• stay-linked\config\keyboards.xml
• stay-linked\config\tapspots.xml
Additional Files and Folders for StayLinked Version 7.0.0 and Greater
• stay-linked\config\deploy.xml
• stay-linked\config\partners.xml
• stay-linked\config\settings.xml
• stay-linked\deploy\*.*
Additional Files and Folders for StayLinked Version 9.0.0 and Greater
• stay-linked\config\manifests.xml
• stay-linked\config\profiles.xml
• stay-linked\manifests\*.*
• stay-linked\devices\*.*
(The ‘devices’ folder may be locked while the server is running. In this case, we would recommend excluding this folder from your synchronization, or synchronizing this folder when the primary server is not running.)
Additional Files for StayLinked Version 10.0.0 and Greater
• stay-linked\config\screenmaps.xml
Additional Folders for StayLinked Version 11.0.0 and Greater
• stay-linked\usage\*.*
(The ‘usage’ folder may be locked while the server is running. In this case, we would recommend excluding this folder from your synchronization, or synchronizing this folder when the primary server is not running.)
Additional Folders for StayLinked Version 12.1.0 and Greater
• stay-linked\images\*.*
Files to be Excluded from Synchronization
• stay-linked\config\espadmin.xml (This file contains the license key information for the unique servers)
Notes for StayLinked Version 15.3 build 214 and Greater
With the release of Server version 15.3 build 214, licenses can be added for a server serial number that does not match the current serial number. Clicking the SN field in the license installation dialogue allows the change of the SN. This means that all files can be replicated from production to backup servers. Details on this can be found in the Administrator User Guide under the Server Administration section.
StayLinked Configuration Files Synchronization Considerations
One of the configuration files, 'hosts.xml', contains the Telnet Host Groups and Telnet Host Entries that point to the telnet servers that are available to the wireless StayLinked clients.
Typically, the telnet server is running on the same server machine where the StayLinked server is running, and so the IP address of the Telnet server will be the local loop-back IP number (127.0.0.1). These telnet host IP addresses should be taken into consideration for HA implementations, in case the Telnet Server IP addresses are different under HA than they are under Primary operations.
Note about shared file systems. Sharing a file system (switchable IASP) across multiple partitions that each run StayLinked could cause issues if multiple StayLinked processes are writing to the same file set. A non-switchable file set would not be subject to this issue, since each StayLinked instance would have an independent copy of each required file. Likewise, if you do not run multiple StayLinked instances that access the same shared file system, the files would be exclusive to that StayLinked instance
StayLinked Backup Server and Backup Hosts Support
The StayLinked Advanced Terminal Session Management solution provides two layers of backup protection for your wireless telnet sessions. In each StayLinked Session, there are two separate IP connections that are being managed, the Client2Host® protocol and the Telnet protocol. In order to provide the most reliable solution for wireless Terminal Emulation, StayLinked provides automated failover capability for both of these IP connections that are managed by the solution.
Failover from Primary StayLinked Server to Backup StayLinked Server
On the device running the StayLinked Client, you can implement a ‘servers.ini’ file that will enable the StayLinked Client to provide access to both a primary and a backup StayLinked Server. The configuration and setup of this ‘servers.ini’ file is described in the StayLinked Client User Guide.
Failover from Primary Telnet Server to Backup Telnet Servers
Using the StayLinked Administrator to manage the Telnet Host entries, you can specify one or two backup telnet servers for your primary telnet server. With backup telnet servers configured, if the StayLinked Server cannot connect a telnet session to the primary telnet server, it will automatically attempt to connect to the first backup server and then to the second backup server. If a backup telnet server is connected, the user will be notified and alerts and log entries will be generated.
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