examples: add a librados/hello_world program

This is a simple program with lots of explanatory comments people
can use as a model for using librados.

Signed-off-by: Greg Farnum <greg@inktank.com>

3 files changed, 311 insertions, 0 deletions

diff --git a/examples/librados/Makefile b/examples/librados/Makefile
new file mode 100644
index 00000000000..a40051facd5
--- /dev/null
+++ b/examples/librados/Makefile
@@ -0,0 +1,5 @@
+all: hello_world.cc
+	g++ -g -c hello_world.cc -o hello_world.o
+	g++ -g hello_world.o -lrados -o librados_hello_world
+clean:
+	rm hello_world.o librados_hello_world
\ No newline at end of file
diff --git a/examples/librados/hello_world.cc b/examples/librados/hello_world.cc
new file mode 100644
index 00000000000..0e6a7acb182
--- /dev/null
+++ b/examples/librados/hello_world.cc
@@ -0,0 +1,296 @@
+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
+// vim: ts=8 sw=2 smarttab
+/*
+ * Ceph - scalable distributed file system
+ *
+ * This is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License version 2.1, as published by the Free Software
+ * Foundation.  See file COPYING.
+ * Copyright 2013 Inktank
+ */
+
+// install the librados-dev package to get this
+#include <rados/librados.hpp>
+#include <iostream>
+#include <string>
+
+int main(int argc, const char **argv)
+{
+  int ret = 0;
+
+  // we will use all of these below
+  const char *pool_name = "hello_world_pool";
+  std::string hello("hello world!");
+  std::string object_name("hello_object");
+  librados::IoCtx io_ctx;
+
+  // first, we create a Rados object and initialize it
+  librados::Rados rados;
+  {
+    ret = rados.init("admin"); // just use the client.admin keyring
+    if (ret < 0) { // let's handle any error that might have come back
+      std::cerr << "couldn't initialize rados! error " << ret << std::endl;
+      ret = EXIT_FAILURE;
+      goto out;
+    } else {
+      std::cout << "we just set up a rados cluster object" << std::endl;
+    }
+  }
+
+  /*
+   * Now we need to get the rados object its config info. It can
+   * parse argv for us to find the id, monitors, etc, so let's just
+   * use that.
+   */
+  {
+    ret = rados.conf_parse_argv(argc, argv);
+    if (ret < 0) {
+      // This really can't happen, but we need to check to be a good citizen.
+      std::cerr << "failed to parse config options! error " << ret << std::endl;
+      ret = EXIT_FAILURE;
+      goto out;
+    } else {
+      std::cout << "we just parsed our config options" << std::endl;
+      // We also want to apply the config file if the user specified
+      // one, and conf_parse_argv won't do that for us.
+      for (int i = 0; i < argc; ++i) {
+	if ((strcmp(argv[i], "-c") == 0) || (strcmp(argv[i], "--conf") == 0)) {
+	  ret = rados.conf_read_file(argv[i+1]);
+	  if (ret < 0) {
+	    // This could fail if the config file is malformed, but it'd be hard.
+	    std::cerr << "failed to parse config file " << argv[i+1]
+	              << "! error" << ret << std::endl;
+	    ret = EXIT_FAILURE;
+	    goto out;
+	  }
+	  break;
+	}
+      }
+    }
+  }
+
+  /*
+   * next, we actually connect to the cluster
+   */
+  {
+    ret = rados.connect();
+    if (ret < 0) {
+      std::cerr << "couldn't connect to cluster! error " << ret << std::endl;
+      ret = EXIT_FAILURE;
+      goto out;
+    } else {
+      std::cout << "we just connected to the rados cluster" << std::endl;
+    }
+  }
+
+  /*
+   * let's create our own pool instead of scribbling over real data.
+   * Note that this command creates pools with default PG counts specified
+   * by the monitors, which may not be appropriate for real use -- it's fine
+   * for testing, though.
+   */
+  {
+    ret = rados.pool_create(pool_name);
+    if (ret < 0) {
+      std::cerr << "couldn't create pool! error " << ret << std::endl;
+      return EXIT_FAILURE;
+    } else {
+      std::cout << "we just created a new pool named " << pool_name << std::endl;
+    }
+  }
+
+  /*
+   * create an "IoCtx" which is used to do IO to a pool
+   */
+  {
+    ret = rados.ioctx_create(pool_name, io_ctx);
+    if (ret < 0) {
+      std::cerr << "couldn't set up ioctx! error " << ret << std::endl;
+      ret = EXIT_FAILURE;
+      goto out;
+    } else {
+      std::cout << "we just created an ioctx for our pool" << std::endl;
+    }
+  }
+
+  /*
+   * now let's do some IO to the pool! We'll write "hello world!" to a
+   * new object.
+   */
+  {
+    /*
+     * "bufferlist"s are Ceph's native transfer type, and are carefully
+     * designed to be efficient about copying. You can fill them
+     * up from a lot of different data types, but strings or c strings
+     * are often convenient. Just make sure not to deallocate the memory
+     * until the bufferlist goes out of scope and any requests using it
+     * have been finished!
+     */
+    librados::bufferlist bl;
+    bl.append(hello);
+
+    /*
+     * now that we have the data to write, let's send it to an object.
+     * We'll use the asynchronous interface for simplicity.
+     */
+    ret = io_ctx.write_full(object_name, bl);
+    if (ret < 0) {
+      std::cerr << "couldn't write object! error " << ret << std::endl;
+      ret = EXIT_FAILURE;
+      goto out;
+    } else {
+      std::cout << "we just wrote new object " << object_name
+	        << ", with contents\n" << hello << std::endl;
+    }
+  }
+
+  /*
+   * now let's read that object back! Just for fun, we'll do it using
+   * async IO instead of synchronous. (This would be more useful if we
+   * wanted to send off multiple reads at once; see
+   * http://ceph.com/docs/master/rados/api/librados/#asychronous-io )
+   */
+  {
+    librados::bufferlist read_buf;
+    int read_len = 4194304; // this is way more than we need
+    // allocate the completion from librados
+    librados::AioCompletion *read_completion = librados::Rados::aio_create_completion();
+    // send off the request.
+    ret = io_ctx.aio_read(object_name, read_completion, &read_buf, read_len, 0);
+    if (ret < 0) {
+      std::cerr << "couldn't start read object! error " << ret << std::endl;
+      ret = EXIT_FAILURE;
+      goto out;
+    }
+    // wait for the request to complete, and check that it succeeded.
+    read_completion->wait_for_complete();
+    ret = read_completion->get_return_value();
+    if (ret < 0) {
+      std::cerr << "couldn't read object! error " << ret << std::endl;
+      ret = EXIT_FAILURE;
+      goto out;
+    } else {
+      std::cout << "we read our object " << object_name
+	  << ", and got back " << ret << " bytes with contents\n"
+	  << read_buf.c_str() << std::endl;
+    }
+  }
+
+  /*
+   * We can also use xattrs that go alongside the object.
+   */
+  {
+    librados::bufferlist version_bl;
+    version_bl.append('1');
+    ret = io_ctx.setxattr(object_name, "version", version_bl);
+    if (ret < 0) {
+      std::cerr << "failed to set xattr version entry! error "
+		<< ret << std::endl;
+      ret = EXIT_FAILURE;
+      goto out;
+    } else {
+      std::cout << "we set the xattr 'version' on our object!" << std::endl;
+    }
+  }
+
+  /*
+   * And if we want to be really cool, we can do multiple things in a single
+   * atomic operation. For instance, we can update the contents of our object
+   * and set the version at the same time.
+   */
+  {
+    librados::bufferlist bl;
+    bl.append(hello);
+    bl.append("v2");
+    librados::ObjectWriteOperation write_op;
+    write_op.write_full(bl);
+    librados::bufferlist version_bl;
+    version_bl.append('2');
+    write_op.setxattr("version", version_bl);
+    ret = io_ctx.operate(object_name, &write_op);
+    if (ret < 0) {
+      std::cerr << "failed to do compound write! error " << ret << std::endl;
+      ret = EXIT_FAILURE;
+      goto out;
+    } else {
+      std::cout << "we overwrote our object " << object_name
+		<< " with contents\n" << bl.c_str() << std::endl;
+    }
+  }
+
+  /*
+   * And to be even cooler, we can make sure that the object looks the
+   * way we expect before doing the write! Notice how this attempt fails
+   * because the xattr differs.
+   */
+  {
+    librados::ObjectWriteOperation failed_write_op;
+    librados::bufferlist bl;
+    bl.append(hello);
+    bl.append("v2");
+    librados::ObjectWriteOperation write_op;
+    write_op.write_full(bl);
+    librados::bufferlist version_bl;
+    version_bl.append('2');
+    librados::bufferlist old_version_bl;
+    old_version_bl.append('1');
+    failed_write_op.cmpxattr("version", LIBRADOS_CMPXATTR_OP_EQ, old_version_bl);
+    failed_write_op.write_full(bl);
+    failed_write_op.setxattr("version", version_bl);
+    ret = io_ctx.operate(object_name, &failed_write_op);
+    if (ret < 0) {
+      std::cout << "we just failed a write because the xattr wasn't as specified"
+		<< std::endl;
+    } else {
+      std::cerr << "we succeeded on writing despite an xattr comparison mismatch!"
+		<< std::endl;
+      ret = EXIT_FAILURE;
+      goto out;
+    }
+
+    /*
+     * Now let's do the update with the correct xattr values so it
+     * actually goes through
+     */
+    bl.clear();
+    bl.append(hello);
+    bl.append("v3");
+    old_version_bl.clear();
+    old_version_bl.append('2');
+    version_bl.clear();
+    version_bl.append('3');
+    librados::ObjectWriteOperation update_op;
+    update_op.cmpxattr("version", LIBRADOS_CMPXATTR_OP_EQ, old_version_bl);
+    update_op.write_full(bl);
+    update_op.setxattr("version", version_bl);
+    ret = io_ctx.operate(object_name, &update_op);
+    if (ret < 0) {
+      std::cerr << "failed to do a compound write update! error " << ret
+		<< std::endl;
+      ret = EXIT_FAILURE;
+      goto out;
+    } else {
+      std::cout << "we overwrote our object " << object_name
+	        << " following an xattr test with contents\n" << bl.c_str()
+	        << std::endl;
+    }
+  }
+
+  ret = EXIT_SUCCESS;
+  out:
+  /*
+   * And now we're done, so let's remove our pool and then
+   * shut down the connection gracefully.
+   */
+  int delete_ret = rados.pool_delete(pool_name);
+  if (delete_ret < 0) {
+    // be careful not to
+    std::cerr << "We failed to delete our test pool!" << std::endl;
+    ret = EXIT_FAILURE;
+  }
+
+  rados.shutdown();
+
+  return ret;
+}
diff --git a/examples/librados/hello_world.readme b/examples/librados/hello_world.readme
new file mode 100644
index 00000000000..54f89ac1a7d
--- /dev/null
+++ b/examples/librados/hello_world.readme
@@ -0,0 +1,10 @@
+This simple librados program can be built by running "make" (and cleaned up
+with "make clean"), assuming you have librados-dev already installed.
+
+And executed using
+./librados_hello_world -c ../../src/ceph.conf
+(or whatever path to a ceph.conf is appropriate to you, or
+by explicitly specifying monitors, user id, and keys).
+
+It demonstrates using librados in a non-Ceph project and the code should
+be self-explanatory.
\ No newline at end of file