Application APIs

Owned by John Ousterhout
Last updated: Jan 21, 2014Version comment

API documentation

For information on the APIs available to client applications, consult the RAMCloud doxygen docs, which are generated from the RAMCloud source code. There are two ways you can access the doxygen docs:

  • Generate the latest version from your RAMCloud sources by typing make doc in the RAMCloud directory. Then open the file docs/doxygen/index.html in a Web browser.
  • Use the precompiled doxygen docs available from our Web site.

The doxygen documentation covers the entire source base, most of which is internal RAMCloud classes that are not useful to application writers.  The primary API documentation for client applications is that for the class RAMCloud::RamCloud (click on the Classes tab at the top of the main page, then search down for the link to RAMCloud::RamCloud). There is one method in this class for each RPC supported by RAMCloud. These methods invoke RPCs synchronously: for example, the createTable method will not return until the operation has completed and a response has been received from the coordinator.

Invoking RPCs asynchronously

You can also invoke any of the RAMCloud RPCs asynchronously, and it is relatively straightforward to manage multiple simultaneous outstanding RPCs.

  • There is a separate class corresponding to each RPC, whose name derives naturally from the name of the corresponding method in RAMCloud::RamCloud. For example, there is a class RAMCloud::CreateTableRpc corresponding to the createTable method.
  • To invoke an RPC asynchronously, construct the corresponding Rpc object such as RAMCloud::CreateTableRpc; the constructor arguments are the same as those for the synchronous method such as createTable. Constructing the Rpc object initiates the RPC, but does not wait for it to complete.
  • Objects such as RAMCloud::CreateTableRpcsupport the following methods:
    • wait: waits synchronously for the RPC to complete
    • isReady: tests whether the RPC has completed, without blocking
    • cancel: aborts the RPC (deleting the object also has this effect)
  • Note: RAMCloud RPCs are implemented using a polling approach. This means that asynchronous RPCs will not make progress or complete unless you occasionally invoke the RAMCloud polling mechanism. If you invoke a synchronous RPC, or if you call the wait method on an asynchronous Rpc object, the RAMCloud poller will automatically be invoked. However, if your program only calls isReady to test for completion, then you must also occasionally invoke the RAMCloud poller. If you have created a RamCloud object named rc, the following statement will invoke the poller:
         rc.clientContext->dispatch->poll();
  • In addition, you must call the isReady method on an Rpc object from time to time to ensure that the RPC "makes progress". Certain actions, such as retrying RPCs after server failures,  are invoked only from within isReady.
  • Once isReady has returned true for an RPC, you should call wait to complete the RPC. At this point, wait is guaranteed not to block. There are two reasons for calling wait. First, this is the only way to get any results returned by the RPC. Second, if the RPC completed with an error condition, this is the only way to find out about that error: wait may throw exceptions, but isReady never throws any exceptions.