The OceanStore Project OceanStore HelloWorld HOWTO by Hakim Weatherspoon, Chiu Wah Kelvin So, and Jason Lee

Step 2: Locating Three Servers

Goal

A client sends a message to more than one unique server. Then servers print out the messages, "Hello! How are you" and send a response "Great! The weather is wonderful today!" The client prints out the response. After all the responses have been received, client exits.

Directions

• Protocol:
  • All 10 HelloWorldServers publish DDPublishReq
  • The HelloWorldClient locates and sends msg to 3 different HelloWorldServer by sending a HelloWorldLocateMsg.
  • The 3 HelloWorldServers which have received a HelloWorldLocateMsg prints out the message and send back a response to HelloWorldClient using HelloWorldRouteMsg.
  • The HelloWorldClient listens for HelloWorldRouteMsg and prints out the message. After receving HelloWorldRouteMsg from 3 different HelloWorldServer, HelloWorldClient exits.

• In order to locate 3 different HelloWorldServer , you will need to understand how do DDLocateMsg, DDQuery, and DDQueryState work.
  • Here is a simple tutorial in how they work.
  • DDLocateMsg is sent to root of GUID. On the way to the root, there can be intermediate nodes that DDLocateMsg goes through. At each of the intermediate nodes, Router will run the DDLocateMsg's query on all the pointer(included GUID and the tag) if GUID of the pointers matches the DDLocateMsg's GUID. If DDQueryResultMatch is returned by the query and DDLocateMsg's forward is true, then the DDLocateMsg will get forward to the GUID object. If DDQueryResultMatch is returned and DDLocateMsg's forward is false, then the DDLocateMsg will not forward to the GUID object. If DDQueryResultNoMatch, then the DDLocateMsg will not forward to the GUID object. In DDQueryResultMatch, there is an option to stop the DDLocateMsg or not before getting to the root.
  • The main job of DDQuery is to implement the query method that return DDQueryResultMatch or DDQueryResultNoMatch under different circumstances.The job of DDQueryState is to memorize what have been done by DDQuery. DDQueryState is pass on with DDLocateMsg. Therefore, DDQueryState is available on every query that is on the way.On each query, the query can insert different information into the DDQueryState so that the query will know the past. For step 2, the HelloWorldQueryState just need to maintain a set of servers GUID that has been queried. If the size of the set = 3, then query will return DDQueryResultMatch with stop = true. and the DDLocateMsg will be stopped. More detail in the next section.
  • If DDLocateMsg made its way all the way to the root without getting a positive query result(positive query result means DDQueryResultMatch), then DDLocateFailure will be sent back to where DDLocateMsg originates from.
    
• For Step 2, there are only 3 files that needed to be modified
  • HelloWorldQueryState
  • HelloWorldQuery
  • HelloWorldClient
    
• HelloWorldQueryState
  • HelloWorldQueryState implements DDQueryState.
  • constructor take in one input: number of servers that needed to be located.
  • use a set data structure to keep track of the servers that have been located by DDLocateMsg.
  • 3 methods:
    • boolean add(SecureHash src) - add the server's location into the set data structure
    • int numberOfRequestServers() - return the number of the input from the constructor
    • int size() - the current size of the set data structue
• HelloWorldQuery
  • Only need to modified the query method
  • bp_guid in the query method is the pointer to one of the servers
  • Changes in the method:
    • add the server bp_guid to the HelloWorldQueryState
    • if GUID is not in the HelloWorldQueryStatebefore, return
      DDQueryResultMatch(m, helloWorldQueryState.size() == helloWorldQueryState.numberOfRequestServers())
    • Otherwise, return DDQueryResultNoMatch
• HelloWorldClient
  • When instantiating a HelloWorldLocateMsg, include HelloWorldQueryState, instead of null in step1, as one of the input for instantiating HelloWorldLocateMsg.
  • make sure it exits after receiving HelloWorldRouteMsg from 3 different servers
    
• Examples:
  • HelloWorldQuery: ostore.archive.RequestFragQuery
  • HelloWorldQueryState: ostore.archive.RequestFragQueryStat

Experiment file setting

• add another field NumOfRequestServers to HelloWorldClient2.stg
• add 10 servers and call them HelloWorldServers0 - HelloWorldServers9
• add 1 client

Click here to go to HelloWorld Exercise step3.