I'm building a client-server application and I am looking at adding failover to the client so that when a server is down it will try to connect to another available server. Are there any standards or specifications covering server failover? I'd rather adopt an existing standard than implement my own mechanism.
I don't there is, or needs to be any. It's pretty straight forward and all depends on how you can connect to your sever, but basically you need to keep sending pings/keepalives/heartbeats whatever you want to call em, and when a fail occurs (or n fails in a row, if you want) change a switch in your config.
Typically, the above would be running as a separate service on the client machine. Altenativly, you could create a method execution handler which handles thr execution of all server calls you make, and on Communication failure, in your 'catch' block, flick your switch in config
You're question is very general. here are some general answers:
Google for Fault Tolerant Computing
Google for High Availability Solutions
This is usually handled at either the load balancer or the server level. This isn't something you normally do in code at the client.
Typically, you multihome the servers each having their own IP + one that is shared between all of them. Further, they communicate with each other over tcp for the heartbeat to know which is the Active node in an Active / Passive cluster.
I can't tell what type of servers you have, but most of the windows servers can do this natively.
You might consider asking the question at serverfault to see how to properly configure your servers to support this.
Related
I'd like to know if there's a way to communicate directly between two (or more) flask-socketio servers. I want to pass information between servers, and have clients connect a single web socket server, which would have all the combined logic and data from the other servers.
I found this example in JS Socket IO Server to Server where the solution was to use a socket.io-client to connect to another server.
I've looked through the Flask-SocketIO documentation, as well as other resources, however it doesn't appear that Flask-SocketIO has a client component to it.
Any suggestions or ideas?
Flask-SocketIO 2.0 can (maybe) do what you want. This is explained in the Using Multiple Workers section of the documentation.
Basically, the servers are configured to connect to a shared message queue service (redis, for example), and then a load balancer in front of them assigns clients to any of the servers in the pool using sticky sessions. Broadcasting operations are coordinated automatically among the servers by passing messages on the queue.
As an additional feature, if you use this set up, you can have any process connect to the message queue to post messages for clients, so for example, you can emit events to clients from a worker or other auxiliary process that is not a SocketIO server.
From your question it is unclear if you were looking to implement something like this, or if you wanted to have the servers communicate for a different reason. Sending of custom messages on the queue is currently not supported, but your question gave me the idea, this might be useful for some scenarios.
As far as using a SocketIO client as in the question you referenced, that shouud also work. You can use this Python package: https://pypi.python.org/pypi/socketIO-client. If you go this route, you can have a server be a client and receive events or join rooms.
By reading documents on MSDN, I realized that it is recommended to create separate hosts by functionality (Sending hosts, Receiving hosts and Processing hosts). And if there is only one host in this bizTalk server, this host can perform all receiving, sending, and processing messages functionality.
My question is: Is it possible to have multiple hosts that each host can perform its own sending, receiving and processing function , and not affect each other?
This is for multiple developers working on the same project, because our current situation doesn't allow us to have a full set of SQL Server Database and SQL server for each developer or using VM.
Thanks a lot!
Multiple hosts is not a solution for letting multiple developers work on a single server. A single send/receive adapter can only be assigned to one host.
You will also run into other problems, as all the configuration settings are shared in a single database, a change from 1 developer will effect the others.
This same question was asked and answered at MSDN. What you are trying to do is not supported and will not work. There is no way around this.
You must deploy the same application code to each computer in a BizTalk Group.
Sharing a BizTalk computer for development work is not a workable or productive solution and will have a definite negative affect on productivity.
You are correct, the best way to handle DEV is a VM with the entire stack. This is the issue you must address in your environment.
I'm writing a bit of desktop software which has two components. Component B queries component A. Creating a web service seems like an ideal way to do IPC in principle. The data model fits, there are ready-made client and server libraries, a well known way to encode and decode parameters etc.
But setting up an HTTP server on a network socket doesn't seem right for a local application. For example what port do I choose? I don't really want people to be able to scan and talk to the app from outside etc.
So I was thinking that I might be able to do HTTP over a domain socket. Does that make any sense? Is there any precedence for it? Is there an equivalent protocol that I could use for IPC which has the same properties as HTTP (requests for specified resources (URIs), encoded parameters, response)?
Looking for C libraries (and possibly Go and ObjC for bonus points).
Binding to the loopback interface only (127.0.0.1) solves your "external visibility" problem, only processes on the local machine will be able to connect.
It does not solve your port allocation problem though, the port number you choose might be taken by the time your app starts. Then your server can't bind and your client connects to the other process bound to your port.
Old, less hip, but CORBA implementations tend to have the problems you have not thought of yet figured out already.
I'm not sure if I'm asking the question properly. I'm referring to locally installed software, often called an "Agent" that keeps in regular communication with some host via HTTP. e.g. When you install LogMeIn, the Agent keeps in communication with the logmein.com server so that when you visit logmein.com with your web browser and connect to the agent, the server is able to initiate communication. The Agent, however, isn't a webserver, nor are any ports forwarded to the Agent. So, is the Agent constantly polling the server asking like a broken record, "Can I help you? Can I help you? Can I help you?" Or is the http connection from Agent to server somehow kept open? I know you can keep an http connection open, but A) how, and B) for how long? Does the Agent need to act like a less annoying broken record asking, "Can I help you? Yet? Yet? Yet?" with much more time in between each question? Or can the Agent ask once and wait indefinitely, asking again only once it learns that the connection has been dropped?
Bottom line is, I'd like to create a small little sample program for trying my hand at writing a client/server application that communicates via the Internet using HTTP. Either side needs to be able to initiate commands / requests. The Agent would likely communicate with the Server using some sort of API, perhaps RESTful. When I start the experiment, I'll be using Perl. It'd be fun to create a Hello World project that would have samples in many languages for many platforms how to write the agent and how to communicate with the server. The agent code would do client side things (e.g. determine public IP address) and send the data to the server. The server would act on the data (e.g. store IP address in a database). The server might also initiate a command to the Agent (e.g. Hey, Agent! What's your CPU type?) Proper authentication / authorization between Agent and Server is of course a necessity.
Are there any existing projects to model off of? Any existing documents? Perhaps I'm just missing terminology and if I just knew that everything I was asking can be summarized by the term foo, then the doors would be opened wide for what I could find in searches!
I looked into the code of Ubuntu's Landscape. It uses Python's Twister -- a web server for HTML5 Websockets. So I'd say what I was looking for in an answer is Websockets (bi-directional communication). That now has opened up a wealth of options, node.js, twister, mojolicious, and many many more as web servers. Turns out using Ajax to poll every few seconds is a very bad idea -- an overwhelming slam on web servers. Keep the connection open.
I would like to write an application to manage files, directories and processes on hundreds of remote PCs. There are measurement programs running on these machines, which are currently managed manually using TightVNC / RealVNC. Since the number of machines is large (and increasing) there is a need for automatic management. The plan is that our operators would get a scriptable client application, from which they could send queries and commands to server applications running on each remote PC.
For the communication, I would like to use a TCP-based custom protocol, but it is administratively complicated and would take very long to open pinholes in every firewall in the way. Fortunately, there is a program with a built-in TinyWeb-based custom web server running on every remote PC, and port 80 is opened in every firewall. These web servers serve requests coming from a central server, by starting a CGI program, which loads and sends back parts of the log files of measurement programs.
So the plan is to write a CGI program, and communicate with it from the clients through HTTP (using GET and POST). Although (most of) the remote PCs are inside the corporate intranet, they are scattered all over the country, I would like to secure the communication. It would not be wise to send commands, which manipulate files and processes, in plain text. Unfortunately the program which contains the web server cannot be touched, so I cannot simply prepare it for HTTPS. I can only implement the security layer in the client and in the CGI program. What should I do?
I have read all similar questions in SO, but I am still not sure what to do in this specific situation. Thank you for your help.
There are several webshells but as far as I can see ( http://www-personal.umich.edu/~mressl/webshell/features.html ) they run on the top of an existing SSL/TLS layer.
There is also S-HTTP.
There are several ways of authenticating to an server (username/passwort) in a protected way, without SSL. http://www.switchonthecode.com/tutorials/secure-authentication-without-ssl-using-javascript . But these solutions are focused only on sending a username/password to the server.
Would it be possible to implement something like message-level security in SOAP/WS-Security? I realise this might be a bit heavy duty and complicated to implement, but at least it is
standardised
definitely secure
possibly supported by some libraries or frameworks you could use
suitable for HTTP