{"id":148,"date":"2017-07-17T23:24:30","date_gmt":"2017-07-17T15:24:30","guid":{"rendered":"http:\/\/www.myway5.com\/?p=148"},"modified":"2023-07-05T22:01:18","modified_gmt":"2023-07-05T14:01:18","slug":"leveldb-intro","status":"publish","type":"post","link":"https:\/\/www.myway5.com\/index.php\/2017\/07\/17\/leveldb-intro\/","title":{"rendered":"leveldb\u6587\u6863\u7ffb\u8bd1(1)-leveldb\u6982\u89c8"},"content":{"rendered":"<p>\u8fd9\u662fleveldb\u6e90\u4ee3\u7801\u5305\u4e2d\u63d0\u4f9b\u7684 readme.md\uff0cindex.md \u7684\u90e8\u5206\u7ffb\u8bd1\u6587\u6863\uff0c\u4e0d\u662f\u9010\u5b57\u9010\u53e5\u7ffb\u8bd1\uff0c\u53ea\u662f\u628a\u81ea\u5df1\u611f\u5174\u8da3\u7684\u90e8\u5206\u7ffb\u8bd1\u51fa\u6765\u4e86\uff0c\u4f5c\u4e3a\u770b\u4ee3\u7801\u4e4b\u524d\u7684\u51c6\u5907\u5de5\u4f5c\u3002<\/p>\n<h2>1.\u516c\u5171\u63a5\u53e3<\/h2>\n<p>include\u6587\u4ef6\u5939\u4e0b\u9762\u7684\u662f\u516c\u5171\u63a5\u53e3\uff0c\u6240\u6709\u4f7f\u7528\u8005\u5e94\u8be5\u4ece\u8fd9\u91cc\u7684\u5934\u6587\u4ef6\u76f4\u63a5\u8c03\u7528\u3002<\/p>\n<ul>\n<li>include\/db.h:\u6570\u636e\u5e93\u7684\u4e3b\u63a5\u53e3:\u4ece\u8fd9\u91cc\u5f00\u59cb<\/li>\n<li>include\/options.h:\u63a7\u5236\u6574\u4e2a\u6570\u636e\u7684\u884c\u4e3a\uff0c\u4e5f\u63a7\u5236\u6570\u636e\u5e93\u7684\u8bfb\u5199\uff0c\u53ef\u4ee5\u770b\u505a\u662f\u914d\u7f6e\u6587\u4ef6\u3002<\/li>\n<li>include\/comparator.h:\u7528\u6237\u81ea\u5b9a\u4e49\u7684\u6bd4\u8f83\u51fd\u6570\u7684\u62bd\u8c61\u3002\u5982\u679c\u60f3\u8981\u4f7f\u7528\u57fa\u4e8e\u5b57\u8282\u7684\u65b9\u6cd5\u5bf9\u952e\u5927\u5c0f\u8fdb\u884c\u6bd4\u8f83\uff0c\u53ef\u4ee5\u4f7f\u7528\u9ed8\u8ba4\u7684\u6bd4\u8f83\u5668\uff0c\u7528\u6237\u4e5f\u53ef\u4ee5\u81ea\u5df1\u5b9e\u73b0\u6bd4\u8f83\u5668\u6765\u6309\u81ea\u5df1\u7684\u60f3\u6cd5\u5bf9\u5b58\u50a8\u8fdb\u884c\u6392\u5e8f\u3002<\/li>\n<li>include\/iterator.h:\u904d\u5386\u6570\u636e\u7684\u63a5\u53e3\u3002\u53ef\u4ee5\u4eceDB\u5bf9\u8c61\u4e2d\u83b7\u53d6\u8fd9\u6837\u4e00\u4e2a\u8fed\u4ee3\u5668\u3002<\/li>\n<li>include\/write_batch.h:\u5bf9\u6570\u636e\u5e93\u539f\u5b50\u7684\u8fdb\u884c\u591a\u4e2a\u66f4\u65b0\u64cd\u4f5c\u7684\u63a5\u53e3\u3002<\/li>\n<li>include\/slice.h:\u4e00\u4e2a\u7b80\u5355\u7684\u6a21\u5757\uff0c\u7528\u6765\u7ef4\u62a4\u4e00\u4e2a\u6307\u9488\u548c\u957f\u5ea6\uff0c\u6765\u8868\u793a\u4e00\u4e2a\u5b57\u8282\u6570\u7ec4\u3002<\/li>\n<li>include\/status.h:\u7528\u6765\u6c47\u62a5\u6210\u529f\u6216\u662f\u5176\u4ed6\u5404\u79cd\u5404\u6837\u7684\u9519\u8bef\u3002<\/li>\n<li>include\/env.h:\u64cd\u4f5c\u7cfb\u7edf\u73af\u5883\u7684\u62bd\u8c61\u3002\u8fd9\u4e2a\u63a5\u53e3\u7684posix\u5b9e\u73b0\u662f\u5728util\/env_posix.cc\u4e2d<\/li>\n<li>include\/table.h,include\/table_builder.h:\u4f4e\u5c42\u6b21\u7684\u6a21\u5757\uff0c\u7edd\u5927\u591a\u6570\u5ba2\u6237\u7aef\u4e0d\u4f1a\u4f7f\u7528\u8fd9\u4e2a\u63a5\u53e3\u3002<\/li>\n<\/ul>\n<h2>2.leveldb\u57fa\u672c\u4f7f\u7528<\/h2>\n<p>leveldb\u63d0\u4f9b\u6301\u4e45\u5316\u7684\u952e\u503c\u5b58\u50a8\u3002\u952e\u548c\u503c\u662f\u4efb\u610f\u7684\u5b57\u8282\u6570\u7ec4\u3002\u952e\u503c\u5bf9\u7684\u5b58\u50a8\u662f\u6309\u7167\u952e\u7684\u987a\u5e8f\u5b58\u50a8\u7684\uff0c\u5e76\u4e14\u8fd9\u4e2a\u987a\u5e8f\u662f\u6309\u7167\u7528\u6237\u81ea\u5b9a\u4e49\u7684\u6392\u5e8f\u51fd\u6570\u6765\u7684\u3002<\/p>\n<h3>2.1 \u6253\u5f00\u4e00\u4e2a\u6570\u7ec4\u5e93<\/h3>\n<p>leveldb\u5b58\u50a8\u662f\u4f7f\u7528\u6587\u4ef6\u7cfb\u7edf\u4e2d\u7684\u6587\u4ef6\u5939\u3002\u6240\u6709\u6570\u636e\u5e93\u5185\u5bb9\u90fd\u4f1a\u5b58\u50a8\u5728\u8fd9\u4e2a\u6587\u4ef6\u5939\u91cc\u3002\u4e0b\u9762\u662f\u4e00\u4e2a\u6253\u5f00\u6570\u636e\u5e93\u7684\u4f8b\u5b50\uff0c\u5e76\u4e14\u5982\u679c\u6570\u636e\u5e93\u4e0d\u5b58\u5728\u4f1a\u81ea\u52a8\u521b\u5efa:<\/p>\n<pre><code class=\"line-numbers\">#include &lt;cassert&gt;\n#include \"leveldb\/db.h\"\n\nleveldb::DB* db;\nleveldb::Options options;\noptions.create_if_missing = true;\nleveldb::Status status = leveldb::DB::Open(options, \"\/tmp\/testdb\", &amp;db);\nassert(status.ok());\n<\/code><\/pre>\n<p>\u5982\u679c\u5e0c\u671b\u5728\u6570\u636e\u5e93\u5df2\u7ecf\u5b58\u5728\u65f6\u62a5\u9519\uff0c\u5728<code>leveldb::DB::Open<\/code>\u524d\u8c03\u7528:<\/p>\n<pre><code class=\"line-numbers\">options.error_if_exists = true;\n<\/code><\/pre>\n<h3>2.2 \u72b6\u6001<\/h3>\n<p>\u4e0a\u9762\u7684\u4ee3\u7801\u4e2d\u4f7f\u7528\u4e86leveldb::Status\u7c7b\u578b\uff0cleveldb\u4e2d\u5927\u591a\u6570\u51fd\u6570\u90fd\u4f1a\u8fd4\u56de\u8fd9\u4e2a\u503c\u3002\u4f60\u53ef\u4ee5\u68c0\u67e5\u8fd9\u4e2a\u503c\u662f\u4e0d\u662f<code>ok<\/code>,\u6216\u8005\u6253\u5370\u51fa\u76f8\u5173\u7684\u9519\u8bef\u3002<\/p>\n<pre><code class=\"line-numbers\">leveldb::Status s = ....;\nif (!s.ok()) cerr &lt;&lt; s.ToString() &lt;&lt; endl;\n<\/code><\/pre>\n<h3>2.3 \u5173\u95ed\u6570\u636e\u5e93<\/h3>\n<p>\u5982\u679c\u6570\u636e\u5e93\u64cd\u4f5c\u7ed3\u675f\u540e\uff0c\u5220\u9664\u6570\u636e\u5e93\u5bf9\u8c61\u5373\u53ef\u5173\u95ed<\/p>\n<pre><code class=\"line-numbers\">.... open the db as described above ...\n.... do something with db ....\ndelete db;\n<\/code><\/pre>\n<h3>2.4 \u8bfb\u5199<\/h3>\n<p>\u6570\u636e\u5e93\u63d0\u4f9b\u4e86Put\u3001Delete\u3001Get\u65b9\u6cd5\u6765\u4fee\u6539\/\u67e5\u8be2\u6570\u636e\u5e93\u3002\u4f8b\u5982\uff0c\u4e0b\u9762\u7684\u4ee3\u7801\u7528\u6765\u5c06key1\u7684\u503c\u79fb\u52a8\u5230key2\u4e2d\u3002<\/p>\n<pre><code class=\"line-numbers\">std::string value;\nleveldb::Status s = db-&gt;Get(leveldb::ReadOptions(), key1, &amp;value);\nif (s.ok()) s = db-&gt;Put(leveldb::WriteOptions(), key2, value);\nif (s.ok()) s = db-&gt;Delete(leveldb::WriteOptions(), key1);\n<\/code><\/pre>\n<h3>2.5 \u539f\u5b50\u66f4\u65b0<\/h3>\n<p>\u4e0a\u9762\u7684\u4f8b\u5b50\u4e2d\uff0c\u5982\u679c\u5728Put\u5b8ckey2\u7684\u503c\uff0cDelete key1\u7684\u503c\u524d\uff0c\u8fdb\u7a0b\u5d29\u6e83\u4e86\uff0c\u540c\u4e00\u4e2a\u503c\u5c31\u4f1a\u88ab\u5b58\u50a8\u5728\u591a\u4e2akey\u4e0b\u3002\u4f7f\u7528<code>WriteBatch<\/code>\u7c7b\u53ef\u4ee5\u539f\u5b50\u7684\u6267\u884c\u591a\u4e2a\u66f4\u65b0\u64cd\u4f5c:<\/p>\n<pre><code class=\"line-numbers\">#include \"leveldb\/write_batch.h\"\n....\nstd::string value;\nleveldb::Status s = db-&gt;Get(leveldb::ReadOptions(), key1, &amp;value);\nif (s.ok()) {\n  leveldb::WriteBatch batch;\n  batch.Delete(key1);\n  batch.Put(key2, value);\n  s = db-&gt;Write(leveldb::WriteOptions(), &amp;batch);\n}\n<\/code><\/pre>\n<p><code>WriteBatch<\/code>\u5b58\u50a8\u4e86\u4e00\u7cfb\u5217\u7684\u5bf9\u6570\u636e\u5e93\u7684\u7f16\u8f91\u64cd\u4f5c\uff0c\u8fd9\u4e9b\u64cd\u4f5c\u4f1a\u88ab\u987a\u5e8f\u6267\u884c\u3002\u6ce8\u610f\u5230\u6211\u4eec\u5148\u6267\u884c\u4e86<code>Delete<\/code>,\u518d\u6267\u884c<code>Put<\/code>,\u8fd9\u6837\u5f53key1\u548ckey2\u76f8\u540c\u65f6\uff0c\u6211\u4eec\u4e0d\u4f1a\u9519\u8bef\u7684\u5220\u9664\u6240\u6709\u7684\u503c\u3002<br \/>\n<code>WirteBatch<\/code>\u4e0d\u4ec5\u4ec5\u6709\u539f\u5b50\u64cd\u4f5c\u7684\u7279\u70b9\uff0c\u8fd8\u53ef\u4ee5\u7528\u6765\u52a0\u901f\u5927\u91cf\u7684\u66f4\u65b0\u64cd\u4f5c\u3002\u539f\u7406\u662f\u5c06\u5927\u91cf\u7684\u5355\u72ec\u7684\u53d8\u5316\u653e\u5165\u540c\u4e00\u4e2a\u6279\u64cd\u4f5c\u4e2d\u3002<\/p>\n<h3>2.6 \u540c\u6b65\u5199\u5165<\/h3>\n<p>\u9ed8\u8ba4\u60c5\u51b5\u4e0b,\u5bf9leveldb\u7684\u5199\u5165\u90fd\u662f\u5f02\u6b65\u7684:\u5b83\u5728\u5c06\u8fdb\u7a0b\u4e2d\u7684\u5199\u64cd\u4f5c\u63d0\u4ea4\u7ed9\u64cd\u4f5c\u7cfb\u7edf\u540e\u8fd4\u56de\u3002\u6570\u636e\u4ece\u64cd\u4f5c\u7cfb\u7edf\u5185\u5b58\u8f6c\u79fb\u5230\u6301\u4e45\u5316\u5b58\u50a8\u8fd9\u4e2a\u8fc7\u7a0b\u662f\u5f02\u6b65\u7684\uff08\u4e5f\u5c31\u662f\u548c\u8fd9\u91cc\u4e0d\u4f1a\u7b49\u6301\u4e45\u5316\u4e4b\u540e\u624d\u8fd4\u56de\uff09\u3002\u4f7f\u7528\u540c\u6b65\u5199\u5165\uff0c\u53ef\u4ee5\u8ba9\u67d0\u4e9b\u7279\u5b9a\u7684\u5199\u5165\u7b49\u5230\u6570\u636e\u88ab\u5199\u5165\u6301\u4e45\u5316\u5b58\u50a8\u540e\u624d\u8fd4\u56de\u3002\uff08\u5728Posix\u7cfb\u7edf\u4e2d\uff0c\u8fd9\u4e2a\u901a\u8fc7\u8c03\u7528 <code>fsync(....)<\/code> \u6216 <code>fdatasync(....)<\/code> \u6216 <code>msync(...., MS_SYNC)<\/code> \u6765\u5b9e\u73b0\u3002<\/p>\n<pre><code class=\"line-numbers\">leveldb::WriteOptions write_options;\nwrite_options.sync = true;\ndb-&gt;Put(write_options, ....);\n<\/code><\/pre>\n<p>\u5f02\u6b65\u5199\u5165\u901a\u5e38\u60c5\u51b5\u4e0b\u662f\u5343\u767e\u500d\u7684\u5feb\u4e8e\u540c\u6b65\u5199\u5165\u3002\u5f02\u6b65\u5199\u5165\u7684\u7f3a\u70b9\u662f\u5982\u679c\u673a\u5668\u5d29\u6e83\u53ef\u80fd\u4f1a\u9020\u6210\u6700\u540e\u7684\u4e00\u4e9b\u66f4\u65b0\u4e22\u5931\u3002\u6ce8\u610f\u4ec5\u4ec5\u662f\u5199\u8fdb\u7a0b\u7684\u5d29\u6e83\u4e0d\u4f1a\u9020\u6210\u4efb\u4f55\u6570\u636e\u4e22\u5931\uff0c\u5373\u4f7f\u6ca1\u6709\u4f7f\u7528\u540c\u6b65\u5199\u3002\u56e0\u4e3a\u5199\u8fdb\u7a0b\u53ea\u8d1f\u8d23\u5c06\u5199\u64cd\u4f5c\u63d0\u4ea4\u7ed9\u64cd\u4f5c\u7cfb\u7edf\uff0c\u63d0\u4ea4\u5b8c\u5c31\u610f\u5473\u7740\u5199\u5165\u5df2\u7ecf\u7ed3\u675f\uff08\u5373\u4f7f\u8fd9\u4e2a\u65f6\u5019\u6ca1\u6709\u771f\u6b63\u7684\u5199\u5165\u5230\u6301\u4e45\u5316\u5b58\u50a8\u4e2d\uff09\u3002<\/p>\n<p>\u5f02\u6b65\u5199\u5927\u591a\u6570\u65f6\u5019\u90fd\u80fd\u88ab\u5b89\u5168\u4f7f\u7528\u3002\u6bd4\u5982\uff0c\u5f53\u8f7d\u5165\u5927\u91cf\u6570\u636e\u5230\u6570\u636e\u5e93\u4e2d\uff0c\u5982\u679c\u51fa\u73b0\u5d29\u6e83\u4f60\u53ef\u4ee5\u91cd\u65b0\u5f00\u59cb\u8fd9\u4e2a\u8f7d\u5165\u64cd\u4f5c\u3002\u4e5f\u53ef\u4ee5\u4f7f\u7528\u6df7\u5408\u5199\u5165\u6a21\u5f0f\uff0c\u4e5f\u5c31\u662f\u6bcf\u591a\u5c11\u4e2a\u5f02\u6b65\u5199\u4e4b\u540e\u4f7f\u7528\u4e00\u6b21\u540c\u6b65\u5199\u3002\u5982\u679c\u5d29\u6e83\u4e86\uff0c\u53ef\u4ee5\u4ece\u4e0a\u4e00\u6b21\u540c\u6b65\u5199\u5f00\u59cb\uff08\u540c\u6b65\u5199\u4f1a\u4ea7\u751f\u4e00\u4e2a\u6807\u8bb0\uff0c\u6240\u4ee5\u53ef\u4ee5\u4ece\u4e0a\u4e00\u6b21\u540c\u6b65\u5199\u5f00\u59cb\uff09\u3002<\/p>\n<p><code>WriteBatch<\/code>\u5bf9\u4e8e\u5f02\u6b65\u5199\u63d0\u4f9b\u4e86\u591a\u4e2a\u9009\u62e9\u3002\u591a\u4e2a\u66f4\u65b0\u64cd\u4f5c\u4e5f\u88ab\u653e\u5728\u540c\u4e00\u4e2a\u6279\u64cd\u4f5c\u4e2d\uff0c\u7136\u540e\u4f7f\u7528\u4e00\u4e2a\u540c\u6b65\u5199\u88ab\u4e00\u8d77\u6267\u884c\uff08<code>write_options.sync<\/code>\u8bbe\u7f6e\u4e3atrue\uff09\u3002\u540c\u6b65\u5199\u4ea7\u751f\u7684\u989d\u5916\u6027\u80fd\u6d88\u8017\u88ab\u5747\u644a\u5728\u6240\u6709\u7684\u5199\u64cd\u4f5c\u4e0a\u3002<\/p>\n<h3>2.7 \u5e76\u53d1<\/h3>\n<p>\u6570\u636e\u5e93\u5728\u540c\u4e00\u65f6\u95f4\u53ea\u80fd\u7531\u4e00\u4e2a\u8fdb\u7a0b\u6253\u5f00\u3002leveldb\u4f7f\u7528\u4e86\u64cd\u4f5c\u7cfb\u7edf\u7684\u9501\u6765\u9632\u6b62\u88ab\u9519\u8bef\u4f7f\u7528\u3002\u5728\u4e00\u4e2a\u5355\u8fdb\u7a0b\u4e2d\uff0c\u540c\u4e00\u4e2a<code>leveldb::DB<\/code>\u5bf9\u8c61\u53ef\u4ee5\u88ab\u591a\u4e2a\u5e76\u53d1\u7ebf\u7a0b\u5b89\u5168\u7684\u5171\u4eab\u3002\u4e0d\u540c\u7684\u7ebf\u7a0b\u4e4b\u95f4\u5728\u5bf9\u540c\u4e00\u4e2a\u6570\u636e\u5e93\u5bf9\u8c61\u5199\u5165\u6216\u8005\u904d\u5386\u6216\u8005\u8c03\u7528Get\u65f6\uff0c\u4e0d\u9700\u8981\u989d\u5916\u7684\u540c\u6b65\u64cd\u4f5c\uff08leveldb\u7684\u5b9e\u73b0\u4f1a\u81ea\u52a8\u505a\u8fd9\u4e2a\u5fc5\u8981\u7684\u540c\u6b65\u64cd\u4f5c\u3002\u7136\u800c\u5176\u4ed6\u5bf9\u8c61(\u6bd4\u5982Iterator\u548c<code>WriteBatch<\/code>)\u53ef\u80fd\u9700\u8981\u989d\u5916\u7684\u540c\u6b65\u3002\u5982\u679c\u4e24\u4e2a\u7ebf\u7a0b\u5171\u4eab\u4e86\u8fd9\u6837\u7684\u5bf9\u8c61\uff0c\u5b83\u4eec\u5fc5\u987b\u4f7f\u7528\u5b83\u4eec\u81ea\u5df1\u7684\u9501\u534f\u8bae\u6765\u4fdd\u62a4\u5bf9\u8c61\u7684\u4f7f\u7528\u3002\u66f4\u591a\u7684\u7ec6\u8282\u53ef\u4ee5\u5728\u516c\u5171\u5934\u6587\u4ef6\u4e2d\u770b\u5230\u3002<\/p>\n<h3>2.8 \u8fed\u4ee3\u5668(Iteration)<\/h3>\n<p>\u4e0b\u9762\u7684\u4f8b\u5b50\u5c55\u793a\u4e86\u5982\u4f55\u6253\u5370\u4e00\u4e2a\u6570\u636e\u5e93\u4e2d\u6240\u6709\u7684\u952e\u503c\u5bf9\u3002<\/p>\n<pre><code class=\"line-numbers\">leveldb::Iterator* it = db-&gt;NewIterator(leveldb::ReadOptions());\nfor (it-&gt;SeekToFirst(); it-&gt;Valid(); it-&gt;Next()) {\n  cout &lt;&lt; it-&gt;key().ToString() &lt;&lt; \": \"  &lt;&lt; it-&gt;value().ToString() &lt;&lt; endl;\n}\nassert(it-&gt;status().ok());  \/\/ Check for any errors found during the scan\ndelete it;\n<\/code><\/pre>\n<p>\u4e0b\u9762\u7684\u4f8b\u5b50\u5c55\u793a\u4e86\u5982\u4f55\u663e\u793a\u4e00\u4e2a\u7279\u5b9a\u8303\u56f4\u7684\u6570\u636e<br \/>\n[start,limit):<\/p>\n<pre><code class=\"line-numbers\">for (it-&gt;Seek(start);\n   it-&gt;Valid() &amp;&amp; it-&gt;key().ToString() &lt; limit;\n   it-&gt;Next()) {\n  ....\n}\n<\/code><\/pre>\n<p>\u4f60\u4e5f\u53ef\u4ee5\u5012\u5e8f\u904d\u5386\u6240\u6709\u7684\u6570\u636e\uff08\u5012\u5e8f\u6bd4\u6b63\u5e8f\u53ef\u80fd\u8981\u6162)<\/p>\n<pre><code class=\"line-numbers\">for (it-&gt;SeekToLast(); it-&gt;Valid(); it-&gt;Prev()) {\n  ....\n}\n<\/code><\/pre>\n<h3>2.9 \u5feb\u7167(Snapshots)<\/h3>\n<p>\u5feb\u7167\u63d0\u4f9b\u4e86\u952e\u503c\u5b58\u50a8\u6240\u6709\u72b6\u6001\u4e00\u81f4\u7684\u53ea\u8bfb\u89c6\u56fe\u3002<code>ReadOptions::snapshot<\/code>\u53ef\u80fd\u662fnon-NULL\u7684\u503c\uff0c\u6765\u8868\u793a\u4e00\u4e2a\u8bfb\u64cd\u4f5c\u5e94\u8be5\u6267\u884c\u5728\u7279\u5b9a\u7684DB\u72b6\u6001\u7248\u672c\u4e0a\u3002\u5982\u679c <code>ReadOptions::snapshot<\/code>\u662fNULL,\u8bfb\u64cd\u4f5c\u53ef\u80fd\u6267\u884c\u5728\u5f53\u524d\u72b6\u6001\u7684\u9690\u5f0f\u5feb\u7167\u4e0a\u3002<\/p>\n<p>\u5feb\u7167\u7684\u521b\u5efa:<code>DB::GetSnapshot()<\/code><\/p>\n<pre><code class=\"line-numbers\">leveldb::ReadOptions options;\noptions.snapshot = db-&gt;GetSnapshot();\n.... apply some updates to db ....\nleveldb::Iterator* iter = db-&gt;NewIterator(options);\n.... read using iter to view the state when the snapshot was created ....\ndelete iter;\ndb-&gt;ReleaseSnapshot(options.snapshot);\n<\/code><\/pre>\n<p>\u5f53\u4e00\u4e2a\u5feb\u7167\u4e0d\u9700\u8981\u7684\u65f6\u5019\uff0c\u4f7f\u7528<code>DB::ReleaseSnapshot<\/code>\u63a5\u53e3\u6765\u91ca\u653e\u3002<\/p>\n<h3>2.10 \u7247\uff08Slice\uff09<\/h3>\n<pre><code>*Slice\u4e0d\u77e5\u9053\u7ffb\u8bd1\u6210\u4ec0\u4e48*\n<\/code><\/pre>\n<p><code>it-&gt;key()<\/code>\u548c<code>it-&gt;value()<\/code>\u7684\u8fd4\u56de\u503c\u90fd\u8c03\u7528\u4e86<code>leveldb::Slice<\/code>\u7c7b\u578b\u7684\u5b9e\u4f8b\u3002Slice\u662f\u4e00\u4e2a\u7b80\u5355\u7684\u7ed3\u6784\uff1a\u5305\u542b\u5b57\u8282\u6570\u7ec4\u7684\u957f\u5ea6\u548c\u6307\u9488\u3002\u8fd4\u56de\u4e00\u4e2aSlice\u6bd4\u8fd4\u56de<code>std::string<\/code>\u662f\u4e00\u4e2a\u66f4\u8f7b\u91cf\u5212\u7b97\u7684\u9009\u62e9,\u56e0\u4e3a\u6211\u4eec\u4e0d\u9700\u8981\u590d\u5236\u6f5c\u5728\u7684\u5f88\u5927\u7684\u952e\u548c\u503c\uff08\u8fd4\u56de\u7684\u662f\u6307\u9488\u548c\u957f\u5ea6\uff09\u3002\u989d\u5916\u7684\uff0cleveldb\u7684\u65b9\u6cd5\u4e0d\u8fd4\u56denull-terminated\uff08\\0\u7ed3\u5c3e\uff09\u7684 C-style \u5b57\u7b26\u4e32\uff0c\u56e0\u4e3aleveldb\u7684\u952e\u548c\u503c\u662f\u5141\u8bb8\u5b58\u50a8<code>'\\0'<\/code>\u5b57\u8282\u7684\u3002<\/p>\n<p>Slice\u548cC++\u5b57\u7b26\u4e32\u4ee5\u53caC-style\u5b57\u7b26\u4e32\u4e4b\u95f4\u53ef\u4ee5\u4e92\u76f8\u8f6c\u6362\u3002<\/p>\n<pre><code class=\"line-numbers\">leveldb::Slice s1 = \"hello\";\n\nstd::string str(\"world\");\nleveldb::Slice s2 = str;\n<\/code><\/pre>\n<pre><code class=\"line-numbers\">std::string str = s1.ToString();\nassert(str == std::string(\"hello\"));\n<\/code><\/pre>\n<p>\u4f7f\u7528Slice\u65f6\u8981\u5c0f\u5fc3\uff0c\u56e0\u4e3aSlice\u53d6\u51b3\u4e8e\u8c03\u7528\u8005\u6765\u4fdd\u8bc1Slice\u4e2d\u5b58\u50a8\u7684\u6570\u636e\u7684\u751f\u547d\u5468\u671f\u3002\u4e0b\u9762\u5c31\u662f\u4e00\u6bb5\u6709\u95ee\u9898\u7684\u4ee3\u7801\u7684\u793a\u4f8b:<\/p>\n<pre><code class=\"line-numbers\">leveldb::Slice slice;\nif (....) {\n  std::string str = ....;\n  slice = str;\n}\nUse(slice);\n<\/code><\/pre>\n<p>\u5f53\u6267\u884c\u5230if\u5916\u9762\u7684\u65f6\u5019\uff0cstr\u88ab\u9500\u6bc1\uff0cslice\u4e2d\u7684\u5b58\u50a8\u4e5f\u5c31\u6d88\u5931\u4e86(\u6ce8:\u8fd9\u662f\u56e0\u4e3aslice\u4e2d\u5b58\u50a8\u7684\u662f\u6307\u9488\u548c\u957f\u5ea6)\u3002<\/p>\n<h3>2.11 \u6bd4\u8f83\u5668\uff08Comparators\uff09<\/h3>\n<p>\u524d\u9762\u7684\u4f8b\u5b50\u4f7f\u7528\u7684\u90fd\u662f\u9ed8\u8ba4\u7684\u6392\u5e8f\u51fd\u6570\u6765\u5bf9\u952e\u505a\u6392\u5e8f\uff0c\u662f\u5bf9\u5b57\u8282\u505a\u5b57\u5178\u6392\u5e8f\u7684\u3002\u5f53\u6253\u5f00\u6570\u636e\u5e93\u7684\u65f6\u5019\uff0c\u53ef\u4ee5\u4f7f\u7528\u81ea\u5b9a\u4e49\u7684\u6bd4\u8f83\u5668\u6765\u6392\u5e8f\u3002\u4f8b\u5982\uff0c\u5047\u8bbe\u6bcf\u4e2a\u6570\u636e\u7684\u952e\u90fd\u7531\u4e24\u4e2a\u6570\u5b57\u7ec4\u6210\uff0c\u6211\u4eec\u5e94\u8be5\u4f7f\u7528\u7b2c\u4e00\u4e2a\u6570\u5b57\u6765\u6392\u5e8f\uff0c\u4f7f\u7528\u7b2c\u4e8c\u4e2a\u6570\u5b57\u6765\u6253\u7834\u8054\u7cfb\u3002\u9996\u5148\uff0c\u5b9a\u4e49\u4e00\u4e2a\u5408\u9002\u7684<code>leveldb::Comparator<\/code>\u7684\u5b50\u7c7b\u3002<\/p>\n<pre><code class=\"line-numbers\">class TwoPartComparator : public leveldb::Comparator {\n public:\n  \/\/ Three-way comparison function:\n  \/\/   if a &lt; b: negative result\n  \/\/   if a &gt; b: positive result\n  \/\/   else: zero result\n  int Compare(const leveldb::Slice&amp; a, const leveldb::Slice&amp; b) const {\n    int a1, a2, b1, b2;\n    ParseKey(a, &amp;a1, &amp;a2);\n    ParseKey(b, &amp;b1, &amp;b2);\n    if (a1 &lt; b1) return -1;\n    if (a1 &gt; b1) return +1;\n    if (a2 &lt; b2) return -1;\n    if (a2 &gt; b2) return +1;\n    return 0;\n  }\n\n  \/\/ Ignore the following methods for now:\n  const char* Name() const { return \"TwoPartComparator\"; }\n  void FindShortestSeparator(std::string*, const leveldb::Slice&amp;) const {}\n  void FindShortSuccessor(std::string*) const {}\n};\n<\/code><\/pre>\n<p>\u73b0\u5728\u4f7f\u7528\u8fd9\u4e2a\u6bd4\u8f83\u5668\u6765\u6253\u5f00\u6570\u636e\u5e93:<\/p>\n<pre><code class=\"line-numbers\">TwoPartComparator cmp;\nleveldb::DB* db;\nleveldb::Options options;\noptions.create_if_missing = true;\noptions.comparator = &amp;cmp;\nleveldb::Status status = leveldb::DB::Open(options, \"\/tmp\/testdb\", &amp;db);\n....\n<\/code><\/pre>\n<h4>2.11.1 \u5411\u540e\u517c\u5bb9<\/h4>\n<p>\u6bd4\u8f83\u5668\u7684Name()\u65b9\u6cd5\u7684\u8fd4\u56de\u503c\u5728\u6570\u636e\u5e93\u88ab\u521b\u5efa\u65f6\u5c31\u5df2\u7ecf\u8054\u7cfb\u8d77\u6765\u4e86\uff0c\u5728\u4e4b\u540e\u7684\u6bcf\u6b21\u6570\u636e\u5e93\u6253\u5f00\u65f6\u90fd\u4f1a\u88ab\u68c0\u67e5\u3002\u5982\u679cname\u6539\u53d8\u4e86\uff0c<code>leveldb::DB::Open<\/code>\u8c03\u7528\u65f6\u4f1a\u5931\u8d25\u3002\u56e0\u6b64\uff0c\u5f53\u4e14\u4ec5\u5f53\u65b0\u7684\u952e\u683c\u5f0f\u548c\u6bd4\u8f83\u51fd\u6570\u548c\u5df2\u7ecf\u5b58\u5728\u7684\u6570\u636e\u5e93\u4e0d\u517c\u5bb9\u65f6\uff0c\u624d\u4f1a\u6539\u53d8name\u7684\u503c\uff0c\u8fd9\u65f6\u4e0d\u5f97\u4e0d\u4e22\u5f03\u6240\u6709\u5df2\u7ecf\u5b58\u5728\u7684\u6570\u636e\u3002<\/p>\n<p>\u5f53\u7136\u4f60\u4e5f\u53ef\u4ee5\u901a\u8fc7\u4e00\u4e9b\u63d0\u524d\u7684\u8ba1\u5212\u6765\u9010\u6b65\u7684\u62d3\u5c55\u4f60\u7684\u952e\u683c\u5f0f\u3002\u4f8b\u5982\uff0c\u4f60\u53ef\u4ee5\u5b58\u50a8\u4e00\u4e2a\u7248\u672c\u53f7\u5728\u6bcf\u4e00\u4e2a\u952e\u7684\u672b\u5c3e\uff08\u4e00\u4e2a\u5b57\u8282\u5c31\u591f\u7528\u4e86\uff09\u3002\u5f53\u4f60\u60f3\u8981\u6539\u53d8\u5230\u65b0\u7684\u952e\u683c\u5f0f\uff08\u4f8b\u5982\u6dfb\u52a0\u7b2c\u4e09\u4e2a\u90e8\u5206\u5230\u952e\u4e0a\uff09<br \/>\n &#8211; (a) \u4fdd\u6301\u540c\u6837\u7684\u6bd4\u8f83\u5668\u540d\u5b57<br \/>\n &#8211; (b) \u589e\u52a0\u65b0\u7684\u952e\u7684\u7248\u672c\u53f7<br \/>\n &#8211; (c) \u6539\u53d8\u6bd4\u8f83\u51fd\u6570\uff0c\u4f7f\u7528\u952e\u7684\u7248\u672c\u53f7\u6765\u51b3\u5b9a\u5982\u4f55\u89e3\u91ca\u8fd9\u4e9b\u952e<\/p>\n<h3>2.12 \u6027\u80fd<\/h3>\n<p>\u6027\u80fd\u53ef\u4ee5\u901a\u8fc7\u6539\u53d8\u914d\u7f6e\u6587\u4ef6\u6765\u8c03\u8282<br \/>\n<code>include\/leveldb\/options.h<\/code><\/p>\n<h3>2.13 \u5757\u5927\u5c0f<\/h3>\n<p>leveldb\u5c06\u76f8\u90bb\u7684\u952e\u5408\u6210\u4e00\u7ec4\u5b58\u50a8\u5728\u540c\u4e00\u4e2a\u5757\u4e2d\uff0c\u8fd9\u6837\u7684\u5757\u662f\u4ece\u6301\u4e45\u5316\u5b58\u50a8\u548c\u5185\u5b58\u95f4\u8f6c\u79fb\u7684\u6700\u5c0f\u5355\u5143\u3002\u9ed8\u8ba4\u7684\u5757\u5927\u5c0f\u662f\u5927\u7ea64096\u4e2a\u672a\u538b\u7f29\u7684\u5b57\u8282\u3002\u5982\u679c\u5e94\u7528\u7a0b\u5e8f\u603b\u662f\u5728\u6570\u636e\u5e93\u4e2d\u505a\u5927\u91cf\u7684\u626b\u63cf\uff0c\u53ef\u4ee5\u8003\u8651\u9002\u5f53\u589e\u52a0\u8fd9\u4e2a\u503c\u7684\u5927\u5c0f\u3002\u5e94\u7528\u7a0b\u5e8f\u505a\u5927\u91cf\u7684\u5f88\u5c0f\u7684\u503c\u7684\u70b9\u8bfb\u53d6\uff0c\u53ef\u4ee5\u8003\u8651\u4f7f\u7528\u5c0f\u4e00\u70b9\u7684\u5757\u5927\u5c0f\uff08\u5982\u679c\u8fd9\u4e9b\u8bbe\u7f6e\u786e\u5b9e\u63d0\u5347\u4e86\u6027\u80fd\uff09\u3002\u5982\u679c\u5757\u5927\u5c0f\u5c0f\u4e8e1kilobyte,\u6216\u8005\u5927\u4e8e\u51e0\u4e2amegabytes\u662f\u6ca1\u6709\u592a\u591a\u597d\u5904\u7684\u3002\u6ce8\u610f\u5bf9\u4e8e\u66f4\u5927\u7684\u5757\u5927\u5c0f\u4f7f\u7528\u538b\u7f29\u4f1a\u66f4\u6709\u6548\u7387\u3002<\/p>\n<h3>2.14 \u538b\u7f29<\/h3>\n<p>\u6bcf\u4e00\u4e2a\u6570\u636e\u5757\u5728\u5199\u5165\u6301\u4e45\u5316\u5b58\u50a8\u524d\u4f1a\u5355\u72ec\u7684\u88ab\u538b\u7f29\uff0c\u538b\u7f29\u662f\u9ed8\u8ba4\u7684\uff0c\u56e0\u4e3a\u9ed8\u8ba4\u7684\u538b\u7f29\u51fd\u6570\u662f\u975e\u5e38\u5feb\u7684\u3002\u5bf9\u4e8e\u4e0d\u53ef\u538b\u7f29\u7684\u6570\u636e\u4f1a\u81ea\u52a8\u7981\u7528\u3002\u5728\u6781\u5c11\u6570\u7684\u4f8b\u5b50\u4e2d\uff0c\u7a0b\u5e8f\u53ef\u80fd\u4f1a\u5e0c\u671b\u5b8c\u5168\u7981\u7528\u538b\u7f29\uff0c\u4ec5\u4ec5\u5f53\u6027\u80fd\u6d4b\u8bd5\u663e\u793a\u6027\u80fd\u63d0\u5347\u4e86\u624d\u63a8\u8350\u8fd9\u4e48\u505a\u3002<\/p>\n<pre><code class=\"line-numbers\">leveldb::Options options;\noptions.compression = leveldb::kNoCompression;\n.... leveldb::DB::Open(options, name, ....) .....\n<\/code><\/pre>\n<h3>2.15 \u7f13\u5b58<\/h3>\n<p>\u6570\u636e\u5e93\u7684\u5185\u5bb9\u88ab\u5b58\u50a8\u5728\u6587\u4ef6\u7cfb\u7edf\u7684\u6587\u4ef6\u96c6\u5408\u4e2d\u3002\u6bcf\u4e00\u4e2a\u6587\u4ef6\u5b58\u50a8\u4e86\u4e00\u7cfb\u5217\u7684\u538b\u7f29\u7684\u6570\u636e\u5757\u3002\u5982\u679coptions.cache\u662fnon-NULL\u7684\uff0c\u7ecf\u5e38\u4f7f\u7528\u7684\u672a\u538b\u7f29\u7684\u6570\u636e\u5757\u5185\u5bb9\u5c06\u4f1a\u88ab\u7f13\u5b58\u3002<\/p>\n<pre><code class=\"line-numbers\">#include \"leveldb\/cache.h\"\n\nleveldb::Options options;\noptions.cache = leveldb::NewLRUCache(100 * 1048576);  \/\/ 100MB cache\nleveldb::DB* db;\nleveldb::DB::Open(options, name, &amp;db);\n.... use the db ....\ndelete db\ndelete options.cache;\n<\/code><\/pre>\n<p>\u6ce8\u610f\u7f13\u5b58\u4fdd\u5b58\u4e86\u672a\u538b\u7f29\u7684\u6570\u636e\uff0c\u56e0\u6b64\u6839\u636e\u7a0b\u5e8f\u7ea7\u7684\u6570\u636e\u91cf\u6765\u8bbe\u7f6e\u7f13\u5b58\u7684\u5927\u5c0f\uff0c\u4f7f\u7528\u538b\u7f29\u7684\u6570\u636e\u4e0d\u4f1a\u6709\u4efb\u4f55\u7684\u51cf\u5c11\u3002\uff08\u538b\u7f29\u7684\u6570\u636e\u5757\u7684\u5b58\u50a8\u7559\u7ed9\u64cd\u4f5c\u7cfb\u7edf\u7f13\u51b2\u533a\u7f13\u5b58\uff0c\u6216\u8005\u5ba2\u6237\u7aef\u7684\u5176\u4ed6\u81ea\u5b9a\u4e49\u73af\u5883\u53d8\u91cf\u6765\u5b9e\u73b0\uff09<\/p>\n<p>\u5f53\u6267\u884c\u5927\u91cf\u7684\u8bfb\u53d6\u64cd\u4f5c\u65f6\uff0c\u5e94\u7528\u7a0b\u5e8f\u6700\u597d\u7981\u7528\u7f13\u5b58\uff0c\u8fd9\u6837\u5927\u91cf\u7684\u6570\u636e\u8bfb\u53d6\u4e0d\u4f1a\u5bfc\u81f4\u7f13\u5b58\u5185\u5bb9\u5927\u91cf\u88ab\u6539\u53d8\u3002per-iterator\u9009\u9879\u7528\u6765\u5b9e\u73b0\u8fd9\u4e2a:<\/p>\n<pre><code class=\"line-numbers\">leveldb::ReadOptions options;\noptions.fill_cache = false;\nleveldb::Iterator* it = db-&gt;NewIterator(options);\nfor (it-&gt;SeekToFirst(); it-&gt;Valid(); it-&gt;Next()) {\n  ....\n}\n<\/code><\/pre>\n<h4>2.15.1 \u952e\u5c42\uff08Key Layout\uff09<\/h4>\n<p>\u6ce8\u610f\u786c\u76d8\u4e2d\u8f6c\u79fb\u548c\u7f13\u5b58\u7684\u5355\u5143\u662f\u4e00\u4e2a\u6570\u636e\u5757\u3002\u76f8\u90bb\u7684\u952e\uff08\u6839\u636e\u6570\u636e\u5e93\u6392\u5e8f\u987a\u5e8f\uff09\u5c06\u88ab\u653e\u5728\u540c\u4e00\u4e2a\u6570\u636e\u5757\u4e2d\u3002\u56e0\u6b64\u5e94\u7528\u7a0b\u5e8f\u901a\u8fc7\u5c06\u6240\u6709\u7684\u952e\u4f9d\u6b21\u76f8\u90bb\u7684\u6392\u653e\uff0c\u5c06\u4e0d\u5e38\u8bbf\u95ee\u7684\u503c\u5185\u5bb9\uff08\u901a\u8fc7key\u6765\u7d22\u5f15\uff09\u653e\u5728\u53e6\u5916\u4e00\u4e2a\u533a\u57df\u53ef\u4ee5\u63d0\u5347\u7a0b\u5e8f\u6027\u80fd\u3002<\/p>\n<p>\u4e3e\u4f8b\u6765\u8bf4\uff0c\u5047\u8bbe\u6211\u4eec\u6b63\u5728\u5728leveldb\u4e0a\u5b9e\u73b0\u4e00\u4e2a\u7b80\u5355\u7684\u6587\u4ef6\u7cfb\u7edf\uff0c\u4e0b\u9762\u7684\u5165\u53e3\u7c7b\u578b\u6211\u4eec\u53ef\u80fd\u5e0c\u671b\u5b58\u50a8\u3002<\/p>\n<pre><code>filename -&gt; permission-bits, length, list of file_block_ids\nfile_block_id -&gt; data\n<\/code><\/pre>\n<p>\u6211\u4eec\u53ef\u80fd\u60f3\u8981\u8bbe\u7f6efilename\u7684\u952e\u524d\u7f00\u4e3a\u4e00\u4e2a\u5b57\u6bcd(&#8216;\/&#8217;)\uff0c<code>file_block_id<\/code>\u7684\u952e\u524d\u7f00\u4e3a\u53e6\u4e00\u4e2a\u5b57\u6bcd(&#8216;0&#8217;)\uff0c\u56e0\u6b64\u626b\u63cf\u8fd9\u4e9b\u5143\u6570\u636e\u4e0d\u4f1a\u5f3a\u5236\u6211\u4eec\u53d6\u51fa\u548c\u7f13\u5b58\u5927\u91cf\u7684\u6587\u4ef6\u5185\u5bb9\u3002<\/p>\n<h3>2.16 \u8fc7\u6ee4\u5668<\/h3>\n<p>\u56e0\u4e3aleveldb\u7684\u6570\u636e\u5728\u78c1\u76d8\u4e0a\u6709\u7ed3\u6784\u5730\u5b58\u653e\uff0c\u4e00\u4e2a\u7b80\u5355\u7684<code>Get()<\/code>\u53ef\u80fd\u8c03\u7528\u591a\u4e2a\u4ece\u78c1\u76d8\u7684\u8bfb\u53d6\u3002\u9009\u9879 <code>FilterPolicy<\/code>\u673a\u5236\u53ef\u4ee5\u88ab\u7528\u6765\u51cf\u5c11\u78c1\u76d8\u8bfb\u53d6\u7684\u6b21\u6570\u3002<\/p>\n<pre><code class=\"line-numbers\">leveldb::Options options;\noptions.filter_policy = NewBloomFilterPolicy(10);\nleveldb::DB* db;\nleveldb::DB::Open(options, \"\/tmp\/testdb\", &amp;db);\n.... use the database ....\ndelete db;\ndelete options.filter_policy;\n<\/code><\/pre>\n<p>\u4e0a\u9762\u7684\u4ee3\u7801\u5c06\u4e00\u4e2a\u57fa\u4e8e\u8fc7\u6ee4\u7b56\u7565\u7684\u5e03\u9686\u8fc7\u6ee4\u5668(Bloom filter)\u548c\u6570\u636e\u5e93\u8054\u7cfb\u8d77\u6765\u3002\u5e03\u9686\u8fc7\u6ee4\u5668\u7684\u57fa\u7840\u8fc7\u6ee4\u4f9d\u8d56\u4e8e\u4fdd\u5b58\u6bcf\u4e2a\u952e\u7684\u4e00\u4e9b\u4f4d\u7684\u6570\u636e\u5728\u5185\u5b58\u4e2d\uff08\u5728\u8fd9\u4e2a\u4f8b\u5b50\u4e2d\u6bcf\u4e2a\u952e\u670910\u4e2abit)\u3002\u8fd9\u4e2a\u8fc7\u6ee4\u5668\u5c06\u4f1a\u51cf\u5c11Get()\u5e26\u6765\u7684\u4e0d\u5fc5\u8981\u7684\u78c1\u76d8\u8bfb\u53d6\u7684\u6b21\u6570\u5927\u6982\u4e3a100\u3002\u589e\u52a0\u4fdd\u5b58\u7684\u6bcf\u4e2a\u952e\u7684bit\u6570\u4f1a\u66f4\u660e\u663e\u7684\u51cf\u5c11\u8bfb\u53d6\u6b21\u6570\uff0c\u4f46\u662f\u8fd9\u662f\u5728\u66f4\u591a\u7684\u5185\u5b58\u6d88\u8017\u7684\u4ee3\u4ef7\u4e0a\u5f97\u6765\u7684\uff08\u6ce8\uff1a\u5178\u578b\u7684\u4ee5\u7a7a\u95f4\u6362\u65f6\u95f4\uff09\u3002\u6211\u4eec\u5efa\u8bae\u5e94\u7528\u7a0b\u5e8f\u5de5\u4f5c\u65f6\u8fd8\u5269\u5f88\u591a\u7684\u5185\u5b58\uff0c\u5e76\u4e14\u4f1a\u505a\u5927\u91cf\u7684\u968f\u673a\u8bfb\u53d6\u65f6\uff0c\u8bbe\u7f6e\u4e00\u4e2a\u8fc7\u6ee4\u7b56\u7565\u3002<\/p>\n<p>(\u6ce8\uff1a\u5e03\u9686\u8fc7\u6ee4\u5668\u662f\u4e00\u4e2a\u9488\u5bf9\u4e8e\u5927\u6570\u636e\u91cf\u7684\u53bb\u91cd\u7b97\u6cd5\uff0c\u5b83\u4f1a\u901a\u8fc7\u591a\u4e2ahash\u5c06\u4e00\u4e2a\u5b57\u8282\u6570\u7ec4\u6620\u5c04\u5230\u4e00\u4e2a\u5f88\u5927\u7684bit\u5411\u91cf\u4e2d\u7684k\u4e2a\u4f4d\u7f6e\uff0c\u901a\u8fc7\u68c0\u67e5\u4e00\u4e2a\u5b57\u8282\u6570\u7ec4hash\u540e\u7684k\u4e2a\u4f4d\u7f6e\u5bf9\u5e94\u7684bit\u5411\u91cf\u662f\u5426\u90fd\u4e3a1\uff0c\u5982\u679c\u4e0d\u662f\u5219\u8fd9\u4e2a\u5b57\u8282\u6570\u7ec4\u6ca1\u6709\u51fa\u73b0\u8fc7\uff0c\u5426\u5219\u51fa\u73b0\u8fc7\uff08\u4f1a\u6709\u5c11\u91cf\u7684\u8bef\u62a5\uff09\u3002\u4f7f\u7528\u5728\u8fd9\u91cc\uff0c\u662f\u4e3a\u4e86\u5728\u8bfb\u53d6\u524d\u68c0\u67e5\u8fd9\u4e2a\u952e\u662f\u5426\u5b58\u5728\uff0c\u5982\u679c\u4e0d\u5b58\u5728\u5219\u907f\u514d\u4e86\u4ee5\u6b64\u5168\u6570\u636e\u5e93\u67e5\u8be2\uff0c\u4ece\u800c\u5927\u5927\u7684\u63d0\u5347\u7684\u6027\u80fd\uff09<\/p>\n<p>\u5982\u679c\u4f60\u4f7f\u7528\u4e00\u4e2a\u81ea\u5b9a\u4e49\u7684\u6bd4\u8f83\u5668\uff0c\u4f60\u5e94\u8be5\u4fdd\u8bc1\u4f60\u6b63\u5728\u4f7f\u7528\u7684\u8fc7\u6ee4\u7b56\u7565\u662f\u548c\u4f60\u6bd4\u8f83\u5668\u517c\u5bb9\u7684\u3002\u4f8b\u5982\uff0c\u60f3\u8c61\u4e00\u4e2a\u6bd4\u8f83\u5668\u6bd4\u8f83\u952e\u65f6\u5ffd\u7565\u4e86\u672b\u5c3e\u7684\u7a7a\u95f4\uff0c<code>NewBloomFilterPolicy<\/code>\u4e0d\u80fd\u548c\u8fd9\u6837\u7684\u6bd4\u8f83\u5668\u4e00\u8d77\u4f7f\u7528\u3002\u76f8\u53cd\u7684\uff0c\u5e94\u7528\u7a0b\u5e8f\u9700\u8981\u63d0\u4f9b\u4e00\u4e2a\u81ea\u5b9a\u4e49\u7684\u8fc7\u6ee4\u7b56\u7565\u540c\u6837\u5ffd\u7565\u8fd9\u4e9b\u672b\u5c3e\u7684\u7a7a\u95f4\u3002\u6bd4\u5982\uff1a<\/p>\n<pre><code class=\"line-numbers\">class CustomFilterPolicy : public leveldb::FilterPolicy {\n private:\n  FilterPolicy* builtin_policy_;\n\n public:\n  CustomFilterPolicy() : builtin_policy_(NewBloomFilterPolicy(10)) {}\n  ~CustomFilterPolicy() { delete builtin_policy_; }\n\n  const char* Name() const { return \"IgnoreTrailingSpacesFilter\"; }\n\n  void CreateFilter(const Slice* keys, int n, std::string* dst) const {\n    \/\/ Use builtin bloom filter code after removing trailing spaces\n    std::vector&lt;Slice&gt; trimmed(n);\n    for (int i = 0; i &lt; n; i++) {\n      trimmed[i] = RemoveTrailingSpaces(keys[i]);\n    }\n    return builtin_policy_-&gt;CreateFilter(&amp;trimmed[i], n, dst);\n  }\n};\n<\/code><\/pre>\n<p>\u6709\u4e00\u4e9b\u5e94\u7528\u7a0b\u5e8f\u53ef\u80fd\u63d0\u4f9b\u5176\u4ed6\u8fc7\u6ee4\u7b56\u7565\u4e0d\u4f7f\u7528\u5e03\u9686\u8fc7\u6ee4\u5668\uff0c\u800c\u662f\u4f7f\u7528\u5176\u4ed6\u7684\u673a\u5236\u6765\u5904\u7406\u952e\u7684\u96c6\u5408\u3002\u7ec6\u8282\u770b<code>leveldb\/filter_policy.h<\/code><\/p>\n<h3>2.17 \u6821\u9a8c\uff08Checksums)<\/h3>\n<p>leveldb\u4f1a\u6821\u9a8c\u6240\u6709\u5b58\u50a8\u5728\u6587\u4ef6\u7cfb\u7edf\u4e2d\u7684\u6570\u636e\u3002\u8fd9\u91cc\u6709\u4e24\u4e2a\u4e0d\u76f8\u5173\u7684\u63a7\u5236\u90e8\u5206\u63d0\u4f9b\u79ef\u6781\u7684\u6821\u9a8c\uff1a<\/p>\n<p><code>ReadOptions::verify_checksums<\/code>\u53ef\u88ab\u8bbe\u7f6e\u4e3atrue\uff0c\u5f3a\u5236\u6821\u9a8c\u4ece\u6587\u4ef6\u7cfb\u7edf\u4e00\u6b21\u8bfb\u53d6\u7684\u6240\u6709\u6570\u636e\u3002\u9ed8\u8ba4\u60c5\u51b5\u662f\u4e0d\u4f1a\u505a\u8fd9\u4e2a\u9a8c\u8bc1\u7684\u3002<\/p>\n<p><code>Options::paranoid_checks<\/code>\u5728\u6253\u5f00\u6570\u636e\u5e93\u4e4b\u524d\u53ef\u88ab\u8bbe\u7f6e\u4e3atrue\uff0c\u6765\u4f7f\u5f97\u6570\u636e\u5e93\u4e00\u65e6\u68c0\u67e5\u5230\u5185\u90e8\u9519\u8bef\u7acb\u9a6c\u62a5\u9519\u3002\u8fd9\u53d6\u51b3\u4e8e\u6570\u636e\u5e93\u7684\u54ea\u4e00\u90e8\u5206\u51fa\u9519\uff0c\u5f53\u6570\u636e\u5e93\u88ab\u6253\u5f00\u65f6\u8fd8\u662f\u5176\u4ed6\u540e\u6765\u7684\u64cd\u4f5c\u65f6\uff0c\u8fd9\u4e2a\u9519\u8bef\u4f1a\u88ab\u8b66\u544a\u3002\u9ed8\u8ba4\u60c5\u51b5\u4e0b\uff0c\u504f\u6267\u68c0\u67e5\u662f\u5173\u95ed\u7684\uff0c\u56e0\u6b64\u5373\u4f7f\u6709\u65f6\u5019\u6301\u4e45\u5316\u5b58\u50a8\u5df2\u7ecf\u51fa\u9519\u4e86\uff0c\u6570\u636e\u5e93\u4f9d\u7136\u53ef\u4ee5\u88ab\u4f7f\u7528\u3002<\/p>\n<p>\u5982\u679c\u4e00\u4e2a\u6570\u636e\u5e93\u51fa\u9519\u4e86\uff08\u53ef\u80fd\u5b83\u5728\u504f\u6267\u68c0\u67e5\u5f00\u542f\u65f6\u4e0d\u80fd\u6253\u5f00\uff09\uff0c <code>leveldb::RepairDB<\/code>\u65b9\u6cd5\u4f1a\u5c3d\u53ef\u80fd\u7684\u4fee\u590d\u6570\u636e\u5e93\u7684\u6570\u636e\u3002<\/p>\n<h3>2.18 \u8fd1\u4f3c\u5927\u5c0f<\/h3>\n<p><code>GetApproximateSizes<\/code>\u65b9\u6cd5\u53ef\u4ee5\u7528\u6765\u901a\u8fc7\u4e00\u4e2a\u6216\u591a\u4e2a\u952e\u8303\u56f4\u6765\u83b7\u53d6\u6587\u4ef6\u7cfb\u7edf\u5df2\u7528\u7684\u7a7a\u95f4\u7684\u8fd1\u4f3c\u5b57\u8282\u6570<\/p>\n<pre><code class=\"line-numbers\">leveldb::Range ranges[2];\nranges[0] = leveldb::Range(\"a\", \"c\");\nranges[1] = leveldb::Range(\"x\", \"z\");\nuint64_t sizes[2];\nleveldb::Status s = db-&gt;GetApproximateSizes(ranges, 2, sizes);\n<\/code><\/pre>\n<p>\u524d\u9762\u7684\u4ee3\u7801\u8c03\u7528\u5c06\u4f1a\u8bbe\u7f6e<code>sizes[0]<\/code>\u4e3a\u952e\u8303\u56f4\u5728[a..c)\u4e4b\u95f4\u7684\u6570\u636e\u4f7f\u7528\u7684\u6587\u4ef6\u7cfb\u7edf\u5b58\u50a8\u7a7a\u95f4\u7684\u5927\u5c0f\uff0c\u8bbe\u7f6e<code>sizes[1]<\/code>\u4e3a\u952e\u8303\u56f4\u5728[x..z)\u4e4b\u95f4\u7684\u6570\u636e\u4f7f\u7528\u7684\u6587\u4ef6\u7cfb\u7edf\u5b58\u50a8\u7a7a\u95f4\u7684\u5927\u5c0f\u3002<\/p>\n<h3>2.19 \u73af\u5883<\/h3>\n<p>\u6240\u6709\u7684\u6587\u4ef6\u64cd\u4f5c\uff08\u4ee5\u53ca\u5176\u4ed6\u7684\u7cfb\u7edf\u8c03\u7528\uff09\u90fd\u5728<code>leveldb:Env<\/code>\u4e2d\u5b9e\u73b0\u4e86\u3002\u590d\u6742\u7684\u5ba2\u6237\u7aef\u53ef\u80fd\u5e0c\u671b\u63d0\u4f9b\u5b83\u4eec\u81ea\u5df1\u7684Env\u7684\u5b9e\u73b0\u6765\u83b7\u5f97\u66f4\u597d\u7684\u63a7\u5236\u3002\u4f8b\u5982\uff0c\u4e00\u4e2a\u5e94\u7528\u7a0b\u5e8f\u53ef\u80fd\u4f1a\u5728\u6587\u4ef6IO\u4e2d\u5f15\u5165\u4eba\u4e3a\u5ef6\u8fdf\uff0c\u9650\u5236leveldb\u5bf9\u7cfb\u7edf\u5176\u4ed6\u6d3b\u52a8\u7684\u5f71\u54cd\u3002<\/p>\n<pre><code class=\"line-numbers\">class SlowEnv : public leveldb::Env {\n  .... implementation of the Env interface ....\n};\n\nSlowEnv env;\nleveldb::Options options;\noptions.env = &amp;env;\nStatus s = leveldb::DB::Open(options, ....);\n<\/code><\/pre>\n<h3>2.20 \u79fb\u690d(Porting)<\/h3>\n<p>leveldb\u53ef\u80fd\u901a\u8fc7\u63d0\u4f9b\u88ab<code>leveldb\/port\/port.h<\/code>\u5bfc\u51fa\u7684\u7c7b\u578b\/\u65b9\u6cd5\/\u51fd\u6570\/\u7684\u5e73\u53f0\u76f8\u5173\u7684\u5b9e\u73b0\uff0c\u6765\u79fb\u690d\u5230\u65b0\u7684\u5e73\u53f0\u3002\u8be6\u7ec6\u5185\u5bb9\u53c2\u8003<code>leveldb\/port\/port_example.h<\/code><\/p>\n<p>\u53e6\u5916\u7684\uff0c\u65b0\u7684\u5e73\u53f0\u53ef\u80fd\u9700\u8981\u65b0\u7684\u9ed8\u8ba4<code>leveldb::Env<\/code>\u7684\u5b9e\u73b0\u3002\u8be6\u7ec6\u5185\u5bb9\u53c2\u8003<code>leveldb\/util\/env_posix.h<\/code><\/p>\n<h3>2.21 \u5176\u4ed6\u4fe1\u606f<\/h3>\n<p>\u5176\u4ed6\u4fe1\u606f\u53ef\u4ee5\u53c2\u8003\u5176\u4ed6\u7684\u6587\u6863\u6587\u4ef6<br \/>\n1. impl.md<br \/>\n2. table_format.md<br \/>\n3. log_format.md<\/p>\n","protected":false},"excerpt":{"rendered":"<p>\u8fd9\u662fleveldb\u6e90\u4ee3\u7801\u5305\u4e2d\u63d0\u4f9b\u7684 readme.md\uff0cindex.md \u7684\u90e8\u5206\u7ffb\u8bd1\u6587\u6863\uff0c\u4e0d\u662f\u9010\u5b57\u9010\u53e5\u7ffb\u8bd1\uff0c\u53ea &hellip; <a href=\"https:\/\/www.myway5.com\/index.php\/2017\/07\/17\/leveldb-intro\/\" class=\"more-link\">\u7ee7\u7eed\u9605\u8bfb<span class=\"screen-reader-text\">leveldb\u6587\u6863\u7ffb\u8bd1(1)-leveldb\u6982\u89c8<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4,23],"tags":[24],"class_list":["post-148","post","type-post","status-publish","format-standard","hentry","category-cplusplus","category-leveldb","tag-leveldb"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v25.4 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>leveldb\u6587\u6863\u7ffb\u8bd1(1)-leveldb\u6982\u89c8 - \u4e00\u53ea\u5b89\u9759\u7684\u732b<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.myway5.com\/index.php\/2017\/07\/17\/leveldb-intro\/\" \/>\n<meta property=\"og:locale\" content=\"zh_CN\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"leveldb\u6587\u6863\u7ffb\u8bd1(1)-leveldb\u6982\u89c8 - \u4e00\u53ea\u5b89\u9759\u7684\u732b\" \/>\n<meta property=\"og:description\" content=\"\u8fd9\u662fleveldb\u6e90\u4ee3\u7801\u5305\u4e2d\u63d0\u4f9b\u7684 readme.md\uff0cindex.md \u7684\u90e8\u5206\u7ffb\u8bd1\u6587\u6863\uff0c\u4e0d\u662f\u9010\u5b57\u9010\u53e5\u7ffb\u8bd1\uff0c\u53ea &hellip; \u7ee7\u7eed\u9605\u8bfbleveldb\u6587\u6863\u7ffb\u8bd1(1)-leveldb\u6982\u89c8\" \/>\n<meta property=\"og:url\" content=\"https:\/\/www.myway5.com\/index.php\/2017\/07\/17\/leveldb-intro\/\" \/>\n<meta property=\"og:site_name\" content=\"\u4e00\u53ea\u5b89\u9759\u7684\u732b\" \/>\n<meta property=\"article:published_time\" 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