Goal

Provide a very easy way to run one program with different settings on a bunch of computers in parallel and collect the results. Simple configuration and wide applicability is the aim. Fault tolerance in respect to network and adminstration errors.

Project Page

The project is registered on Sourceforge.net and is called simparex. It is really used productively by several people.

Why not use something like MPI?

  • Well, first of all the program I want to run are not witten in C.
  • I could write a MPI client that executes my program with the parameters, but then I can just use a shell script.
  • I need a more flexibel way to grasp the results. For example some output goes to a file. A shell or perl script seams to be more convinient to do a flexible conversion.

Terminology:

  • Master: is the computer where the server runs
  • Slave: is one of many computers that do the work
  • Server: program that coordinates the process
  • Client: program that runs on slaves
  • Worker: program that does the computation (any)
  • Task: a set of parameters/ settings for the Worker
  • Result: a file with the results of the computation
  • SessionID: unique number for client-server communication (should be unique over multiple runs)
  • Ticket: Computation identification. Unique within one run.

Specification

Cluster

  • One Master
  • Many Slaves
  • primary Linux machines, however the design is platform independent
  • any TCP/IP network connection (can be non permanent, but should not :-))
  • SSH (public key authentication) on Slave
  • SCP sequrity copy (public key authentication) on Slave
  • HTTP (any port open on Master, prefered #80)

Features

  • one master computer with server program (acts as HTTP server). SSH and SCP is needed to get the client program to the slaves and start it.
  • list of slave computers (host names or IPs).
  • platform dependent workers possible
  • command specification: commandline pattern with place holders for variables and input file generation
  • result specification: standart output and/or files
  • validation of the results on the master
  • list of tasks characterised through parameters.
  • timeouts and multiply task assigments if necessary (timeout, free resources and so on)
  • collecting rules: a) plain concat b) blockwise with parameters
  • simple online (via Web) and offline statistics: which slave did what and which parameter sets failed.
  • test mode with different bias to check configuration.
  • NFS aware

Error detection/ dealing

  • error while connection/authentication (ssh, scp)
  • slave dead / client killed (don't care there are other slaves :-) )
  • server breaks or get stopped (all clients should terminate at some near point)
  • worker terminates without success
  • worker doesn't return within timeout

Server

  • Format of communication is specified in Protocol section
  • initialisation: for every slave: try to start client (ssh). If it fails: check ssh connection with dummy ssh command. Success → copy client to slave using scp and try to start it.
  • on http request for configuration: reply client configuration (i.e. worker name, MD5 checksum and what to do on exit)
  • on http request for worker executeable: reply with the binary for the right platform
  • on http request for new task: reply with the next command to execute and all parameters.
  • on http request for statistics (normal website): reply statistics webpage
  • on post: validate result, mark task as completed and collect results
  • no more task to process: exit and display statistics.

Client

  • Format of communication is specified in Protocol section
  • gets via command line: Session ID, server URL and port
  • register at the server and fetch configuration
  • check for the worker: if not already at local filesystem or the MD5 checksum is wrong: fetch it (for own platform) from the server
  • fetch a task
  • run worker
  • check return code: if failed → Post failture otherwise take the results and post them.
  • fetch next task
  • die if there is no more task or the server is not responding.
  • different settings for termination: delete executeable (if fetched), delete the client program, delete results?

Protocol

Configuration

  • Request: GET http://master/config?sessionid=SESSIONID&platform=PLATFORM
  • Fail (due to unsupported platform): 415 Unsupported Media Type
  • Fail (due to wrong session id): 403 (Forbidden)
  • Successful Reply: List of Key = Value pairs.
Worker=name of the executeable
MD5=md5 checksum of the executeable
DeleteWorker=Yes/No
DeleteClient=Yes/No
DeleteResults=Yes/No
Ping=#
  • PLATFORM: one of “Linux, Unix, BSD, WinNT, Win95” (TODO: need a better way then $^0)

Ping (HTTP)

  • Ping interval is given in seconds. 0 for no ping.
  • Purpose of the ping is that the client realises if the server is stopped or finished or even dead.
  • Request: GET http://master/ping?sessionid=SESSIONID&ticket=TICKET
  • Fail due to wrong session id: 403 (Forbidden)
  • Successful, but ticket expired (task already done): 205 (Reset Content)
  • Successful (keep on it!): 204 (No Content)

Worker

  • Request: GET http://master/worker?sessionid=SESSIONID
  • Fail (due to wrong session id): 403 (Forbidden)
  • Fail (due to file not found): 403 (Forbidden)
  • Success: binary file

Task

  • Request: GET http://master/task?sessionid=SESSIONID
  • Fail (due to wrong session id): 403 (Forbidden)
  • Fail (because no task left): 503 Service Unavailable
  • Success:
[[Task]]
Ticket=#  (unique number within session)
CommandLine=commandline
[[Input filename]]*
Content=single-line file content 
or
Content= <<EOT
multi-line file content here (ASCII)
EOT
[[Result name]]+ 
File=filename
  • the * behinds the section means there can be _zero_ or more sections
  • the + behinds the section means there can be _one_ or more sections

Task completed (successful or with failture)

  • Successful: POST http://master/completed?sessionid=SESSIONID&ticket=TICKET
[[Result name]]+
Content= single-line file content
or
Content= <<EOT
multi-line file content here (ASCII)
EOT
  • Failed: GET http://master/failed?sessionid=SESSIONID&ticket=TICKET
  • Reply Fail due to wrong session id: 403 (Forbidden)
  • Reply Last Task (no more task): 204 (No Content)
  • Reply Otherwise: 202 (Accepted)
  • binary content is not supported

Client Starting Status

  • This is used for communication between threads in the server.
  • GET http://master/clientstatus?slaveid=SLAVEID&status=STATUS
  • STATUS: -1: no ssh; 0: error while starting or copying; 1: started
  • Reply Fail due to wrong slave id: 403 (Forbidden)
  • Reply Otherwise: 204 (No Content)

Client Died Notification

  • The client notifies that it is about to die
  • GET http://master/died?session=SESSIONID&normal=yes/no&reason=REASON
  • REASON: String that describes why
  • Reply Fail due to wrong session id: 403 (Forbidden)
  • Reply Otherwise: 204 (No Content)

Implementation Details

Configuation and Files

  • Server config: See server.sample.conf in the side bar.
  • Task file: tasks.csv : csv file, cells seperated with |, parameter names in the headline and every following line contains one parameter set. All lines have to have the same amount of cells like the headline!

Before the headline can occure comments, that start with # until the end of the line.

#comment
string|counter
"eins"|1
"zwei"|2
  • Worker config: See worker.sample.conf in the side bar.
  • Input specification: The input consists of the command line and one ore more files. In the example above the parameter “counter” is passed as an commandline argument and the parameter “string” is written in the file input.file. This file is used as standart input for the worker. One can specify other files as well, in case the worker reads them.
  • Result specification: A result has a name and filename where to get the result values from. In the above example one result is calles “Result” and it comes from the file result.file, which is the standart output of the worker. The second result is calles “Output” and is read from output.file. If the worker doesn't write to this file the result will be empty.
  • Validation: standart implementations are provided and a custom implementation can be provided by the user as a perl function. A validation function gets the result of the worker and returns success or failture. (See Validate.pm)
  • Collection: standart implementations are provided and a custom implementation can be provided by the user as a perl function. A collection function gets task description (number, parameters set) and the result of the worker and can do whatever it wants with it (usually writes in a file). (See Collect.pm)

Server

  • The server is implemented in perl.
  • Perl has no reasonable way to use shared memory in multiply threads. Since the program is written using perl objects and objects can't be shared I decided to make a serial implementation first.
  • That means just one request can be responded at the time
  • However I managed to to implement the swarming in parallel, that means that the starting is reasonably fast.
  • Consequences: less suitable for very small tasks (short computation time) with large input/response data

NFS awareness

The problem is, that if some slaves share files via NFS or another network filesystem it could happen that different clients overwrite their files. Basically there are three points where it occurs:

  1. the client is copied
  2. the client fetches the worker
  3. the worker writes its data to a file.

Solutions:

  1. a) start and copy clients in serial (very slow) b) start the first client an copy the client if necessary. After that start the remaining clients in parallel (quite fast, current implementation :-))
  2. before fetching the worker the client creates a .lock file. The other clients check the existance and wait for the worker.
  3. every worker is started in a separate directory, given by the session id and the ticket number

Error detection

  • No client could be started
  • Swarm: Remote shell command (ssh) termination code:
  • 0 ⇒ Success: The executed command has been executed with success!
  • otherswise ⇒ Failure: Can have the following reasons: connection failed, program not found or terminated without success.

To check a connection and the authentication:

ssh host echo
  • return 0 (success): Connection is OK and machine has a shell. (TODO: check for Windows and Mac machines)
  • otherwise error

projects/simparex.txt · Last modified: 23.06.2009 09:32 by georg